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RecommendedUrban Trees:

A CornellCampus Walk

Cornell UniversityIthaca, New York

Text: Michelle Buckstrup and Nina L. BassukLayout and Design: Michelle BuckstrupIllustrations: Olivia DiRenzoMap: Angelica Hammer

Tree Walk Map onInside Back Cover

AcknowledgmentsFor their thoughtful assistance, we thank George Hawver, Don Rakow,Marvin Adleman, Christine Doell, Kristine Flahive, Robert Mower,Peter Trowbridge, and Dan Otis.

This all-season campus tree walk highlights trees that can withstandtough urban situations. Please note that some trees in this walk willinevitably disappear as a result of winter storm damage, construction,and other circumstances. Most of the walk is wheelchair accessible, withshort detours sometimes necessary.

This tree walk and other publications related to recommendedurban trees are available on-line at the Cornell University UrbanHorticulture Institute web site:

http://www.hort.cornell.edu/department/faculty/bassuk/uhi/

©2003 Urban Horticulture Institute

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The reputation for natural beauty of the Cornell campus is rightly celebrated.Campus trees strategically planted in the quads and around buildingscontribute to that reputation. But although campus trees appear to be in anidyllic setting, many of them are under as much stress as trees in downtownIthaca. For trees, the “urban environment” is not just “downtown.” It includesresidential streets, parking lot islands, entrances to shopping malls —anywhere trees face a host of stresses brought on by people and theiractivities.

Trees in urban settings like the Cornell campus endure drought, poor drainage,heat, vehicle exhaust, foot and vehicular traffic over their roots, limited rootingspace and the spray of deicing salts. As a medium for growing healthy plants,urban soils range from barely adequate to totally unacceptable. They maycontain construction debris or be contaminated by chemical leaching. The pHof urban soils is almost always above the desirable range due toleaching from limestone-based construction materialssuch as concrete. Often urban trees areplanted in windy,unprotected spots and mustabsorb heat radiating frombuildings. It’s no wonderurban trees don’t get asbig as their wildrelatives... life in the cityis not easy!

Your journey begins atthe Turkish filbert(Corylus colurna, 1)tree in the MinnsGarden on Tower Road. Look for the symmetrical, pyramid-shaped tree in thenorthwest corner of the garden. Notice the attractive scaly bark and the widebranch angles. Branches with wider angles are less likely to have includedbark, which weakens branch attachment, so they are better able to withstandsnow loads without breaking.

(1) Corylus colurna, Turkish Filbert

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Turkish filbert has great potential as a street tree and has been planted onIthaca streets in recent years. It is slow growing but once established, ittolerates drought, heat, and a wide range of soil pH. With the exception offungal blight, which can be serious, it has few insect and disease problems.When other, less drought-resistant trees show leaf scorch, Turkish filbertfoliage stays a deep, attractive green. The tiny nuts are edible, if you canwrestle them away from the squirrels and bluejays.

Leave the Minns Garden through the nearby exit and head north on thesidewalk that runs along the west side of the Plant Science building. Fourcornelian cherry dogwoods (Cornus mas, 2) line the building. This species isadaptable to a variety of soil types and pH, and is pest-free.It flowers in very early spring when few other trees put on ashow and it has gorgeous, edible red fruits in late summer.

Climb the stairs, pass a purple-leaved tree, and turn righton the asphalt sidewalk. The leggy young tree on theright is hardy rubber tree (Eucommia ulmoides, 3).Although it is not used commercially for rubber, latexexudes from its torn leaves. Hardy rubber tree canwithstand drought and high soil pH, has few pestproblems, and its dark green leaves stay attractive allsummer. Close to the ground level, notice themechanical damage to the trunk. Such injuries oftenreduce a tree’s vigor.

Why care about urban trees?Urban green space quite simply makes our cities livable. City trees:

• contribute to air purification and oxygen regeneration• provide cooling in summer and wind abatement in winter• reduce storm water runoff and allow groundwater to recharge• reduce noise, buffer against unsightly views, and slow traffic• increase property values, urban beauty, and neighborhood pride• provide habitat for wildlife

(3) Eucommiaulmoides,

Hardy RubberTree

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About UHIThe Urban Horticulture Institute (UHI) at Cornell, founded in 1980 byDr. Nina Bassuk, focuses on research related to street trees. The goals of UHIresearch are:

• to select, evaluate, and propagate superior trees that withstand such urbanstresses as soil compaction, periodic flooding, drought, and salinity;

• to develop improved technologies for assessing and ameliorating sitelimitations to improve plant growth and development; and

• to develop improved transplanting technologies to ensure successfulestablishment of trees in the urban environment.

UHI works closely with the city of Ithaca and the Cornell PlantationsArboretum to conduct experiments and to apply research findings to real-lifesituations. For more information, see the UHI web site: <http://www.hort.cornell.edu/department/faculty/bassuk/uhi>

Continue past some yewshrubs and benches tothe front entrance toPlant Science. The largetrees that flank theentrance are littleleaflindens (Tilia cordata,4). Given its distributionin the wild on limestonesoils, it’s not surprisingthat this European nativetolerates high soil pH. Ashade tree since ancient times, littleleaf linden is fast growing and easy totransplant. Although it can tolerate intermittent drought, prolonged droughtmay lead to leaf scorch. Also, littleleaf linden is sensitive to salt.

Just beyond the second linden is a single black gum, or tupelo (Nyssasylvatica, 5) planted in the early 1970s. Tupelo’s versatility as an urban tree

(4) Tilia cordata, Littleleaf Linden

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(5) Nyssa sylvatica, Tupelo

(6) Cercidiphyllum japonicum,Katsura Tree

comes from its diverse native origins — from drymountainous sites to wet swampland. It isrelatively pest free and often hasspectacular fall color. Much ado hasbeen made about the difficulty oftransplanting tupelo. If transplantedyoung and watered regularly thefirst year, it should be fine. Onething to note is that soil pH above6.0 may depress tupelo’s growth.The tree you see here is small forits 30-plus years. Perhapsleaching from the building andsidewalk and resultant high soilpH has stunted the tree’s growth.

Follow the sidewalk around to the left. With your back to Mann Library, lookout over the length of the Ag Quad; it looks quite different than it did in thenot-so-distant past. Until the early 1970s, a double row of American elms

towered over the quad, running east to west.They formed a canopy that gave the quad —and many other parts of campus — a decidedlyformal character. Students today have difficultyvisualizing the stunning visual impact and senseof sanctuary that these grand trees provided.The scourge that took the elms was Dutch elmdisease. American elms were so widely plantedas street trees that when the disease pathogenwas introduced, it found many hosts on whichto multiply. Monoculture, the planting of somany of one species, was the culprit.

Moving past the silver “Mann Library”lettering, you’ll come to a multistem katsuratree (Cercidiphyllum japonicum, 6). Hailing

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from China and Japan, katsura makes a good street tree due to its lack ofmajor pest problems and its ability to tolerate a range of pH levels. Its onlylimitation: it does not tolerate drought. In the fall when the leaves turn colorand start to drop, you may notice a sugary smell. It’s caused by maltosevolatizing from the leaves as they die.

Continue left on the sidewalk. To one side of the Warren Hall entrance is asmooth-barked multistem yellowwood (Cladrastis kentukea, 7). Calledyellowwood because of the color of the heartwood, this medium-sized tree isnative to sites underlain by limestone and thus is adapted to a high soil pH.Yellowwood is fast growing, and it has attractive white flowers in spring andnoteworthy yellow fall color. It has few insect or disease problems. Notice,however, evidence of one of the failings of yellowwood: branch splitting dueto narrow crotch angles and weak wood (Cladrastis means “brittle branch”).You can see where a fairly large branch split off and left a wound.

From the yellowwood,head in a diagonaldirection away fromWarren Hall andtowards the evergreensin the distance. On yourright are two large tuliptrees (Liriodendrontulipifera, 8). Tulip treeis a fast growing,relatively pest-free, pH-tolerant choice forurban use. However,because of its massivesize (to 70 to 90 feet inheight), it needs ampleroom both above andbelow ground.

(7) Cladrastis kentukea, Yellowwood

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On the other side of the pathfrom the tulip trees is a singlewhite ash, (Fraxinusamericana ‘Autumn Purple,’9). Generally, white asharen’t as tough as green ash(Fraxinus pennsylvanica), butthey have more ornamentalvalue. White ash can tolerateintermittent flooding and highsoil pH. The cultivar ‘AutumnPurple’ is known for its goodform and attractive reddish-purple fall color.

Past the tulip trees you willcome into a grove of oaks:

three red and three black. You can tell red and black oak apart by looking attheir buds — red oak buds tend to appear rounded when viewed head on,

Start with the RootsTo understand the best planting and management practices for urban trees, knowwhere and how roots grow. Most of us grew up with the idea that roots mirrorthe crown of the tree, penetrating as far down as the tree grows tall. Now weknow that this is emphatically not the case. Shade tree roots are found primarilyin the top 12 inches of soil, and the tiny absorbing roots responsible for most ofthe tree’s intake of water and nutrients are in the top several inches. In terms ofhorizontal root spread, roots not only grow beyond the dripline, there may be ahigher percentage of them beyond the dripline than within it. This is why foursquare feet of tree protection zone around a trunk during construction is oftenwoefully inadequate. Finally, know that 90–95% of tree roots are routinelyremoved in the nursery at the time of harvest. What results when trees are thenplanted in new environments, with only about 5% of their former root mass, istransplant shock.

(8) Liriodendron tulipifera, Tulip Tree

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(9) Fraxinus americana,White Ash

while black oak buds have amore angular appearance.Also, black oak leavesgenerally are glossier than redoak leaves.

Northern red oaks (Quercusrubra, 10) are workhorsesamong street trees. You willsee many of them on theCornell campus because redoaks are fairly easy totransplant, are fast growersamong the oaks, and cantolerate salt and pollutionalong city streets. They canwithstand moderately alkalinesoil pH, and they cope wellwith dry sites once they are established. If red oaks decline, they do so slowlyand gracefully, giving tree care experts plenty of time to figure out what to do.

Next on the right are thornlesshoneylocusts, Gleditsia triacanthosvar. inermis (11). The first is thecultivar ‘Majestic’; the second is'Skyline.’ Cultivars of honeylocustare chosen for such qualities asinsect resistance, compact form,attractive fall color, and reducedfruiting. Honeylocusts aremembers of the pea family andbear leathery, seven- to eight-inchpods that do not decomposereadily.

(10) Quercus rubra, Northern Red Oak

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Honeylocust is a tough tree. Because ofits versatility and ability to withstandurban conditions, the honeylocust hasbeen widely planted, frequently in spotsthat American elms had occupied.Because of the overplanting ofhoneylocust, we are now seeing avariety of serious pests on it:honeylocust plant bug, spider mites,borers, webworms . . . history repeatsitself. Honeylocust may be tough, butno tree species can toleratemonoculture. The lesson here isDIVERSITY!

Head toward the hemlock. You’ll see two trees on the right, near a fire hydrant.The first tree on your right is Hesse European ash (Fraxinus excelsior ‘Hessei,’12), a cultivar that has simple leaves. In its native Europe, it thrives onlimestone soils; this makes it a good choice for high pH soils. Hesse ash alsotolerates intermittent flooding. This large-maturing tree is valued for its darkgreen, lustrous leaves. For reasons unknown,this particular specimen has notperformed well.

Just on the other side of the firehydrant is a tree with someinteresting folklore behind it.The seeds of Kentucky coffeetree (Gymnocladus dioicus,13) were roasted and groundup by early settlers as a coffeesubstitute (the roasting wascritical — raw seeds arepoisonous!). Female trees

(11) Gleditsia triacanthos var. inermis,Thornless Honeylocust

(12) Fraxinus excelsior'Hessei,' Hesse Ash

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Assess the SituationSite assessment is the critical step-by-step siteevaluation process that leads to appropriate treeselection. Site assessment guidelines andchecklist in the publication Urban Trees: SiteAssessment, Selection for Stress Tolerance,Planting are available on the UHI web site:<http://www.hort.cornell.edu/department/faculty/bassuk/uhi/pubs.html>. Siteassessment includes determining cold hardinesszone, soil pH, degree of soil compaction,available below- and above-ground space, anddrainage patterns.

(13) Gymnocladus dioicus,Kentucky Coffee Tree

have large, attractive fruitpods that persist intowinter. Coffee tree lendsitself to urban use due to itstolerance of soil alkalinity,salt, and drought. It hasnotably few pest problems.A fruitless male cultivar ofcoffee tree called‘Espresso’ is available inthe trade, for those whofind the pods a nuisance.

Backtrack a little now, justfar enough to head downthe straight path to Caldwell Hall. You’ll pass a honeylocust on your left; thetree farther down on the right is basswood or American linden (Tilia americana,14). The leaves of basswood are noticeably larger than those of the littleleaflindens on the quad. Basswood lendsitself to urban use because it tolerateshigh soil pH and it is easy to transplant.

One of the most gorgeous crabappletrees on campus is straight ahead, bythe entrance to Caldwell Hall. It isMalus ‘Snowdrift’ (15), named for itsattractive blanket of white flowers inspring. Crabapples are small-staturetrees that are ideal for the urbanlandscape because they can toleratedrought, high pH, and salt, and theyare easy to transplant. Hundreds ofcrabapple cultivars are available inthe trade, and several dozen are

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recommended for urban use (see<http://www.hort.cornell.edu/department/faculty/bassuk/uhi/chart1.html> for a list).

Not far from the crabapple, acrossthe sidewalk, is a trio of treeswhose leaves have silveryundersides. These are silverlindens (Tilia tomentosa, 16).Silver linden tolerates heat anddrought better than littleleaf lindenand is resistant to Japanese beetlefeeding. A helpful ID feature isthat stems of silver linden tend tobe more pubescent or “fuzzier”than those of other lindens.

Continue on the sidewalk headingwest. At the corners of theComputing and CommunicationsCenter (CCC) are small trees withattractive bark, Japanese tree lilac(Syringa reticulata, 17). Japanesetree lilac is a desirable tree forurban sites because, unlike itsshrubby common-lilac counterpart,it resists powdery mildew andborers. It doesn’t mind alkalinesoil or drought, it transplantsreadily, and it matures at under 30feet.

Head south, away from the CCC.Six scholar trees (Styphnolobiumjaponicum, 18) are scattered near

(14) Tilia americana, Basswood

(16) Tilia tomentosa, Silver Linden

(15) Malus sp.,Crabapple

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The Trouble with SaltMost successful urban trees tolerate deicing salts, rock salts frequentlycomprised of sodium and chloride ions (NaCl). As snow is moved off streets andsidewalks and onto the tree lawn, deicing salts accumulate in the soil. Damageoccurs in a number of ways: salt competes with tree roots to soak up availablewater in the soil, it damages soil structure by preventing the formation ofaggregates, and it interferes with the absorption of other nutrients. The marginalor tip leaf scorch that you see on some trees is caused by the uptake of chlorideions into the plant’s vascular system. Chloride ions accumulate in toxic levels inthe newest plant growth, killing leaf tissue at the tips and along margins.

What to do about salt? Tolerant species experience less damage from theseeffects; they include white ash, honeylocust, and white, bur and red oak.However, they too can be damaged with heavy salt concentrations. Other deicingagents such as sand, light gravel, or cinders can be used in place of NaCl.

(17) Syringa reticulata,Japanese Tree Lilac

(18) Styphnolobium japonicum,Scholar Tree

the main walkway that leads into the Kennedy-Roberts arch. Scholar tree is the last of the largeornamental trees in this region to flower. Trees bearcreamy white flower spikes in late summerfollowed by intriguing seedpods that look likebeans, but with a twist. Scholar tree is performingextremely well on the Cornell campus and inIthaca. It is tolerant of drought, salt, and high soilpH.

This is a good place to scan the quad one lasttime. Do you notice what’s missing? Besides thetwo clumps of hemlocks, there are no coniferson the quad. Pines, spruces, and other conifersgenerally do not tolerate urban conditions. Theyrequire moist soils and cannot tolerate salt.Because their leaves remain through winter,evergreen conifers lose some water to

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transpiration at a time when rootscannot draw water out of the soil. Thismakes evergreens especially susceptibleto desiccation. Deicing salts in the soilonly compound the problem.

Walk through the arch between Kennedyand Roberts Hall. Cross the road to see arow of four small trees alongside MalottHall. They are Washington hawthorns(Crataegus phaenopyrum, 19). Thisplanting is a great example of “righttree, right place.” A simple siteassessment reveals that there is only 15feet of space between the building and the sidewalk. Overhead eaves cast apartial rain shadow over the site, and the slope tends to be dry. Given the

surrounding lime-basedbuilding materials andconcrete, the soil pH islikely to be high. Clearly,the site calls for a drought-tolerant, pH-adaptablesmall tree. Washingtonhawthorn fits the bill. Asidefrom being suited to thissite, it has attractive whiteflowers in spring and redfruits in fall that persist intowinter. One warning,though — this is not a treefor playgrounds: it has one-to three-inch thorns.

Backtrack a bit and followthe sidewalk around the

B&B vs. Bare RootBalled and burlapped (B&B) transplanting isfrequently regarded as essential for trees.However, bare root trees have manyadvantages over B&B: they are lighter,cheaper, less expensive to ship, and theyrequire no special equipment to move or plant,allowing more community involvement.Furthermore, UHI found that bare root treeshave 200% more root mass than B&B trees.For these reasons, the city of Ithaca plantsvirtually all of its trees bare root. A hydrogelroot dip at the nursery helps preventdesiccation of the bare roots between harvestand planting. For more information about bareroot planting, see Creating the Urban Forestat <http://www.hort.cornell.edu/department/faculty/bassuk/uhi/pubs.html>.

(19) Crataegusphaenopyrum,

Washington Hawthorn

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(21) Quercus robur,English Oak

(22) Phellodendron amurense,Amur Corktree

north side of Malott Hall. Head for the brick-pillared entrance to the Big RedBarn. Pass the Barn on your right and follow the gravel path as it veers left.The first tree on the left is Crimean linden (Tilia euchlora, 20). It resembleslittleleaf linden, but the leaves are a bit larger and the new stems are yellow-green. Crimean linden is more drought- and aphid-resistant than littleleaflinden and tolerates high soil pH.

Head back toward the Big Red Barn andveer to the left on the gravel path, whichchanges to parking lot, then to an asphaltsidewalk. Just past the Barn on the right,beside the asphalt sidewalk, you’ll see alarge English oak (Quercus robur, 21)with bluish-green leaves. English oak isone of the few oaks that can toleratealkaline soils without showing leafchlorosis, or yellowing. Although itshould be given plenty of space to lookits best, English oak can cope with a somewhat morerestricted rooting volume and is drought tolerant. Near the end of the growingseason you can expect the leaves to become gray with powdery mildew, butthat is the only noteworthy pest of English oak.

Continue past the fence and turn left.Walk down to East Avenue, cross it, andtake a right. Head north along EastAvenue past red-brick Lincoln Hall untilyou come to a path between Lincoln andRand halls that curves off to the left.Along this short section of path is aEuropean beech on the left with itscharacteristic smooth elephantine bark.Behind it is Amur cork tree(Phellodendron amurense, 22). If youpress your fingers into the bark, you’ll

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see how cork tree got itscommon name. Cork tree isrelatively pest-free andtolerates alkaline soil andintermittent drought. Note thedesirable wide crotch angles.This tree does less well inrestricted root areas than someother tree species in this walk;thus, it is best used in urbanparks where it has plenty ofrooting volume.

The curving path leads downsteps, across a service road,and into the Arts Quad. If youlook up, you’ll see the whitedome of Sibley Hall; followthe sidewalk that hugs Sibleyand take a right at the end ofthe building. These six younglacebark elms (Ulmusparvifolia, 23) could somedaybe among the most dramatictrees on campus. The mottledgray, green, orange, andbrown bark gives the tree itscommon name. Lacebark elmtolerates the gamut of urbanconditions. It is a large tree,maturing at 50 to 70 feet.

Get back on the sidewalkheading west, then veer off tothe left where the sidewalk

The Tree-Turf ConflictTrees and turf don’t get along. When youngtrees are planted in grass, turf canoutcompete fine tree roots for water andnutrients. Some turf grasses actually exudechemicals that inhibit tree root growth.Conversely, when turf is planted undermature trees, shade and root competitionmake it harder to establish grass. To avoidthis basic incompatibility, trees shouldalways be provided a generous expanse ofmulched, grass-free area. This area shouldextend at least as far as the dripline of thetree. The mulch should be two to four inchesdeep (not more than four inches, as oxygendeprivation to the roots results), and shouldbe pulled back from around the trunk.Besides holding down weeds and conservingmoisture, mulch keeps the lawn mowers andstring trimmers away from the trunk, whereone accidental wound can do seriousdamage.

(23) Ulmus parvifolia, Lacebark Elm

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descends. At White Hall, take the diagonal paththat crosses the quad. Just after the fire hydranton your left, look on your right for shingle oak(Quercus imbricaria, 24), a pyramid-shaped,wide-spreading tree with long, unlobed, strap-shaped leaves. Both the common name and thespecific epithet imbricaria (“overlapping”) referto the use of the wood for shingles, a practicebegun by pioneers. Shingle oak is a medium tolarge tree that does well on sites where it hasample room for its wide-spreading lowerbranches. It can tolerate drought, and pH up to7.5. Its leaves persist through winter, functioningas a windbreak.

Continue along the diagonal path into the oak grove, one of the most beautifulspots on campus and a great place to pause and take a rest. The grove includesa mixture of mature black, white, and red oaks, with a few hickories sprinkledin. Leaving the oak grove, take a right onto the zelkova-lined sidewalk(Zelkova serrata, 25) running north-south. Zelkova is drought, salt, and pHtolerant, and has a beautiful vase shape reminiscient of the American elms.However, it has one major flaw. Notice how narrow the branch angles are;splitting and crotch cankers result. If you look into the trees, you will seecables running from trunk to trunk to prevent further splitting.

At the “Goldwin Smith Hall” lettering,turn right and head down the diagonalsidewalk toward the clock tower. A fewpaces past the clock tower, turn left.Lining the left side of the sidewalk areflowering cherry (Prunus ‘Accolade,’ 26)interplanted with elms. Members of theproblematic Rose family, many floweringcherries tend to be riddled with insect anddisease problems, and they are often

(24) Quercus imbricaria,Shingle Oak

(25) Zelkova serrata,Japanese Zelkova

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short-lived. ‘Accolade’ is more vigorousand has better resistance to pests than otherflowering cherries. It’s also a rapid growerand sports attractive bark and very earlysemidouble pink flowers.

Wind your way behind Sage Chapel,where, in the triangle of lawn behind DayHall, there are three trees with star-shapedleaves, sweet gum (Liquidambarstyraciflua, 27). Hardy to zone 5b, they areat the edge of their cold hardinesson the Cornell campus. Notfor use under utility wires,Liquidambar can grow to bevery large, 70 to 100 feet,and its roots need amplespace. Apart from beautifulfoliage that turns a varietyof striking colors in fall, itis valued for its toleranceof intermittent flooding,moderate drought and forthe absence of major pestproblems. One warning, though— don’t walk barefoot anywherenear it in the fall. The seeds areborne in round, woody fruits thatare painfully sharp.

Moving around to the other sideof Sage Chapel, you’ll find aseries of four multistem riverbirches (Betula nigra ‘Heritage,’28). This cultivar is the nursery

(26) Prunus 'Accolade,'Flowering Cherry

(27) Liquidambar styraciflua,Sweetgum

(28) Betula nigra 'Heritage,'River Birch

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industry’s answer to white barkbirches that inevitably succumbto bronze birch borer. Althoughnot a long-lived street tree (inpart because it suffers from irondeficiency at high pH), it is oftenworth using for the beauty andyear-round interest it imparts.The exfoliating, salmon-pinkbark is noteworthy.

Go down the steps to your leftand head downhill on Ho Plaza.The double row of trees in theplaza are London plane trees,Platanus x acerifolia‘Bloodgood,’ 29). London planeis often selected for city usebecause it is very fast-growingand durable. It is a cross betweenOriental plane tree and Americansycamore; its common namecomes from its extensive use inLondon as a dependable urbantree. ‘Bloodgood’ is resistant tothe fungal disease anthracnose,but overplanting has made itvulnerable to other pests. Leaf,bark, and fruit litter can also bean annoyance.

The trees outside the stone plazawall have a much more favorablesoil environment. Growing inlawn instead of concrete are two

The pH FactorUrban trees are selected partly on thebasis of their tolerance of high soil pH.Soil pH is a measure of how acidic oralkaline (sour or sweet) the soil is. Acidsoil has a higher concentration ofhydrogen ions (H+), whereas alkalinesoil has more hydroxide ions (OH

_ ).

Their relative concentration determinesthe nature of chemical reactions in thesoil, especially whether nutrients can bemade available to the roots. For thatreason, when soil pH is out of thedesirable range for a particular plant,nutrient uptake is limited and the plantexhibits symptoms of deficiency. Mostplants prefer a soil pH of 6.0-7.5. Toidentify potential street trees that canhandle the high pH of the concrete-ladenenvironment, the UHI looks for treesthat grow in limestone-based soils intheir native habitat.

(29) Platanus x acerifolia 'Bloodgood,'London Plane Tree

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rows of callery pears (Pyruscalleryana ‘Aristocrat,’ 30). The‘Bradford’ callery pear is notoriousfor its narrow branch angles, whichmake it vulnerable to splittingunder snow and ice, but ‘Aristocrat’pear has a more open branchstructure and is less prone tosplitting. It tolerates drought,alkaline soils, and salt. ‘Aristocrat’ isalso favored for profuse flowersearly in spring and for leaves thatpersist into the fall and early winter.

Just past the Gannett center on theright are three pin oaks (Quercuspalustris, 31). If you get close enough you’ll see that “pin” refers to the short,pinlike side twigs or spurs. The specific epithet palustris means “swampy”since native pin oak is found primarily on wet sites. Despite manygood attributes, pin oak is not recommendedfor urban use unless the soils arenaturally acidic. On alkaline soil itdevelops unsightly iron chlorosis, orleaf yellowing.

Continue straight ahead acrossCampus Road and head downCentral Avenue past Anabel Taylorand Myron Taylor halls. Continueacross the Central Avenue bridge tothe Performing Arts Center. Situatedwhere three roads meet is an islandplanted with five multistem greenash (Fraxinus pennsylvanica, 32);cross over to the green ash island.

(30) Pyrus calleryana 'Aristocrat,' Callery Pear

(31) Quercus palustris,Pin Oak

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Car exhaust, heat, pedestrian traffic, and limited soil volume conspire to makethis an incredibly inhospitable place for trees.

Look down at the surface — how will water get to the roots of these trees?The hardscape material on this site, although visually nondescript, wasactually chosen based on careful research. It’s known that any water thatinfiltrates a site like this does sothrough the joints between thepavers. When these joints aresealed shut, precious little waterreaches the tree roots fromrainfall. The key is to provide ajointing material that is permeableto water, yet has the structuralstrength necessary to keep thepavers level and in place. Here, thematerial used is asphalt mastic, acoarse sand mixed with bituminsfrom asphalt. The bitumins coatand bind the other materials, yetinclude air space for water toinfiltrate and percolate down to theroots.

Green ash is one of the toughest,most adaptable street treesavailable in the trade. In its native habitat, it is found in both wet and dryenvironments, so it can tolerate both drought and intermittent flooding. Greenash also tolerates salt and high soil pH, and it is relatively pest-resistant. Thedrawbacks? It is not the most attractive species, and it is overused.

Head back into campus on the asphalt path that leads into the woods and to afoot bridge. After you cross the bridge, veer left, cross the road, and head forthe mixed planting on the east side of Snee Hall (33). This landscape was

(32) Fraxinus pennsylvanica, Green Ash

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Give Me SpaceThe conventional city tree pit is 4 foot x 4 foot and surrounded by concrete.Ever wonder if that’s enough space for tree roots? Inadequate soil volume isone of the main reasons why city trees last, on average, only seven years.

UHI has developed a step-by-step methodology to calculate how much soilis needed for a given-size tree. The following is a shortcut version of thatmethodology that can be used to approximate adequate soil volume in areaswith rainfall levels similar to those of Ithaca.

1. For an existing tree, measure the distance from the tree’s main trunk tothe dripline (the outermost edge of the canopy). Or, consult a reference bookto find the optimum mature spread of the tree you are considering. Estimatethat your tree will reach 75% of the optimum, then take half of that number.This is your radius, r.

2. Calculate 3.14 x r2 to get the crown projection, the area of the circle

within the dripline of the tree.

3. Crown projection is measured in square feet. For every square footof crown projection, provide 2 cubic feet of soil.

Example: You read that common chokecherry ( Prunus virginiana) spreads18 to 25 feet at maturity. You can expect that in most city sites the lowerfigure, 18 feet, is more realistic. Then:

1. The radius would be 9 feet.

2. The crown projection would be (3.14)(92) = 254 square feet. For everysquare foot of crown projection, provide 2 cubic feet of soil. Thus,254 x 2 = 508 cubic feet of soil volume is needed.

3. Since roots are rarely deeper than 3 feet, use 3 feet as your depthdimension (unless you know your planting site will be shallower).Thus, 508/3 = 169, and the area of usable soil in your planter or sitemust be at least 169 square feet (equivalent to a planting site that’s 13feet wide, 13 feet long, and 3 feet deep).

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designed to look like an extension of thenative vegetation of Cascadilla Gorge.As it turns out, the garden includesmany recommended urban trees.

The three small trees near the entrancewith shredding bark in long, rectangularstrips are American hophornbeam(Ostrya virginiana). Native to easternNorth America, it often grows in thewoodland understory and thus istolerant of some shade. Hophornbeamtolerates some urban stresses but cannotwithstand the most difficult sites;it is especially sensitive to salt.Use hophornbeam in moderatelychallenging situations such asurban parks and greenways. It willreward you with stress toleranceand interesting bark, leaves, andfruits.

Young dawn redwoods(Metasequoia glyptostroboides) areunmistakably and consistentlycone-shaped. Look for one here,intermingled with the shorterhophornbeams — it has feathery,flattened, needlelike leaves. Ifdawn redwood strikes you asprimitive-looking, it should. Dawnredwood has been around for more than 50million years. It’s known as a “living fossil” since fossil evidence of it wasdiscovered before the first trees were sighted. Since its discovery in China inthe 1940s, it has been widely propagated. Given its longevity as a species it is

(33) Ostrya virginiana,American Hophornbeam

(33) Metasequoia glyptostroboides,Dawn Redwood

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no surprise that it tolerates urbanstress. Since it is low branching, itneeds to be “limbed up” for streettree use; otherwise, its neat conicalhabit eliminates the need for pruning.Dawn redwood prefers moist,slightly acid soils.

Just beyond the dawn redwood, onthe same side, are two red maples(Acer rubrum) with smooth, silverygray bark, red new growth, andabundant red buds. Like green ash,red maple has evolved in both wetsites and dry sites. Among EastCoast trees, red maple has one of thelargest north-south ranges. It is alarge tree with year-roundornamental interest and is superior tosilver maple in its wood strength.Many cultivars have beenselected for superior fallcolor, columnar form, orflood tolerance. Take care,however, to give red mapleboth ample space andmoderate to acidic soils.On alkaline soils,manganese uptake islimited and red mapleleaves become chlorotic, oryellowed.

(33) Acer rubrum, Red Maple

Go Native?Like the word “organic,” the phrase“native plant” does not have auniversally accepted definition. Howlong must a plant have been here tobe native — 200 years, sinceColumbus arrived, since the time ofthe dinosaurs?

One argument in favor of nativesover exotics holds that trees “from”this region are the most suited togrow here. However, since the urbanenvironment has so many featuresthat have been changed by humans,trees native to the surroundingcountryside will not necessarilyflourish in the city. With any urbantree, the results may be disappointingif the native hasn’t been carefullymatched to the site.

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Leave this unique spot and head back outthe main entrance. Cross the road to thecorridor that is flanked by pines and has atrio of honeylocust at its entrance. Headnorth through the corridor, which is plantedwith a mixture of trees, shrubs, andgroundcovers, including a tupelo, basswood,and several serviceberry trees (Amelanchiersp., 34). One serviceberry grows close to thestairs on the right. This small tree is oftenused in multistem form. It is an appealingmember of the Rose family that should beused where soils are acidic or only moderatelyalkaline. Ornamental features include early whiteflowers and attractive bark, fruit, and fall color.

Up the stairs and to yourright is the Engineering

Quad; make your waydiagonally across it.At the intersection of Campus Road and EastAvenue are two trees of great interest, a mature

ginkgo (Ginkgo biloba, 35) at the corner ofPhillips Hall, and a mature bald cypress(Taxodium distichum, 36) south of historicSage Hall. Ginkgo, like dawn redwood, hasancient origins, having existed on the planet formore than 150 million years. It is an all-aroundlow-maintenance, tough urban tree. It thrives insome of the tiniest tree lawns in Ithaca. Thistough species has one notorious disadvantage, thememorably bad smell of the fleshy seeds (oftencalled “fruits,” but botanically, they are seeds) of

(34) Amelanchier sp., Serviceberry

(35) Ginkgo biloba, Ginkgo

(36) Taxodium distichum,Bald Cypress

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(37) Quercus bicolor,Swamp White Oak

(38) Quercus macrocarpa, Bur Oak

female trees. For that reason, male cultivars area must.

Bald cypress is so named because it is so late toleaf out — it often remains “bald” until earlysummer. When you think of bald cypress,swampy sites come to mind. Since it originatesfrom such places, it can manage wet soils andintermittent flooding. However, bald cypressleaves can become chlorotic in soil pH above7.5. If high pH is not an issue, it is a great choiceas a street tree since it is relatively pest andmaintenance free (just allow plenty of room — asyou can see, it gets big).

A third mature tree is found in analcove of Statler Hall, a spectacularswamp white oak (Quercus bicolor,37) that probably predates theCornell campus. Swamp white oakwas named the “Urban Tree of theYear” in 1998 by the Society ofMunicipal Arborists. It is a nativethat can tolerate both dry andpoorly drained sites; it is relativelypest-free, but doesn’t tolerate soilpH much above the neutral range.

Head further up East Avenue, turnright on Tower Road, and cross overto the north side of Tower. Halfwayup the block, look to your left to seea young bur oak (Quercusmacrocarpa, 38). It is close to thesidewalk, in front of a small stand

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All in the MixNew sidewalks and roads are a thorn in the cityforester's side. Meanwhile, trees can upheavesidewalks and aggravate the city engineer. Must it beso? One of UHI's research-based introductions isstructural soil, a mix of crushed stone, clay loam, anda polymer called hydrogel that keeps the stone andsoil evenly mixed. Structural soil allows tree roots topenetrate under sidewalks and roads without heavingpavement. Its uniform stone satisfies loadrequirements set by municipal engineers. The mix hasbeen used sucessfully on campus, in Ithaca, and inNew York City. For more information, see the UHIweb site: <http://www.hort.cornell.edu/department/faculty/bassuk/uhi/ssoils/sld001.htm>.

of hemlocks and a dedication plaque.Like English oak, bur oak toleratesalkaline soils. It also withstands bothdrought and intermittent flooding. Inwinter the ornamental value of itscorky bark is especially apparent. Likemost oaks, bur oak can become a giantin height and spread. Because theleaves of this specimen are not strictlycharacteristic of bur oak, it isspeculated to be a hybrid between buroak and white oak.

Further up Tower Road, flanking thesouth entrance to Malott Hall, are three multistem musclewood, or ironwood(Carpinus caroliniana, 39). Note the fluted wood of the trunk that gives thistree its common name. Musclewood is a small tree that tolerates occasionallywet soils. It can be grown as a single- or multistem tree. You can find thisnative in the understory of our local woodlands.

Cross over GardenAvenue and head upTower Road backtoward the MinnsGarden. The red oaktrees lining TowerRoad have aninteresting history. Theoaks on the left andright sides of the roadwere planted at thesame time. Notice howmuch bigger the treeson the left are — theyare grown in generous

(39) Carpinus caroliniana,Musclewood

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swaths of lawn. By contrast, the trees on the right, starting across from theMinns Garden, contend with the indignity of parking lots on both sides ofthem. Root damage from parking lot construction and soil compaction fromcars has stunted these once-vigorous red oaks. The trees on the left are livingthe good life, but those on the right show what happens when urban stresstakes its toll. u

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Notes

1. Corylus colurna, Turkish Filbert 2. Cornus mas, Cornelian Cherry Dogwood 3. Eucommia ulmoides, Hardy Rubber Tree 4. Tilia cordata, Littleleaf Linden 5. Nyssa sylvatica, Tupelo 6. Cercidiphyllum japonicum, Katsura Tree 7. Cladrastis kentukea, Yellowwood 8. Liriodendron tulipifera, Tulip Tree 9. Fraxinus americana 'Autumn Purple,' White Ash10. Quercus rubra, Northern Red Oak11. Gleditsia triacanthos var. inermis, Thornless Honeylocust12. Fraxinus excelsior 'Hessei,' Hesse European Ash13. Gymnocladus dioicus, Kentucky Coffee Tree14. Tilia americana, Basswood15. Malus sp., Crabapple16. Tilia tomentosa, Silver Linden17. Syringa reticulata, Japanese Tree Lilac18. Styphnolobium japonicum 'Halka,' Scholar Tree19. Crataegus phaenopyrum, Washington Hawthorn20. Tilia euchlora, Crimean Linden21. Quercus robur, English Oak22. Phellodendron amurense, Cork Tree23. Ulmus parvifolia, Lacebark Elm24. Quercus imbricaria, Shingle Oak25. Zelkova serrata, Japanese Zelkova26. Prunus 'Accolade,' Flowering Cherry27. Liquidambar styraciflua, Sweet Gum28. Betula nigra 'Heritage,' River Birch29. Platanus x acerifolia ‘Bloodgood,’ London Plane Tree30. Pyrus calleryana 'Aristocrat,' Callery Pear31. Quercus palustris, Pin Oak32. Fraxinus pennsylvanica, Green Ash33. Ostrya virginiana, American Hophornbeam Metasequoia glyptostroboides, Dawn Redwood Acer rubrum, Red Maple34. Amelanchier sp., Serviceberry35. Ginkgo biloba, Ginkgo36. Taxodium distichum, Bald Cypress37. Quercus bicolor, Swamp White Oak38. Quercus macrocarpa, Bur Oak39. Carpinus caroliniana, Musclewood

Tree Key