PNW 497

A Pacific Northwest Extension Publication

University of Idaho – Oregon State University – Washington State University

Introductionacific Northwest gardeners may find thegrowing season where they live too short to

grow certain vegetables. Air and soil tempera-tures may be lower than optimal for plantgrowth. Untimely frosts or snow, combined withlow humidity and excessive wind, make it neces-sary for the home gardener to use special prac-tices to get the best production. The techniquesoutlined in this publication will be essential togrow most vegetable crops.

To successfully grow vegetables in short seasonareas, you will need to become a weatherwatcher. Information is available on weatherradios, cable TV stations, and local newspapers.These should be monitored to anticipate adverseweather conditions such as local frosts. Equipyour garden with an accurate high and lowtemperature recording thermometer to chartdaily extremes. Obtain a number of high quality,dial type soil thermometers. Place the thermom-eter probes in your garden soil at rooting depth.The temperature of your soil will indicate whatseeds and plants can be planted, and how muchheat is available under plastic mulches and plantcovers.

In order to get started in vegetable production,determine the length of your frost-free growingseason. It can be as short as 21 days. Remember,the growing season may be 20 percent shorter orlonger than average, and some years are betterthan others. Don’t let conventional local wisdomdiscourage your gardening adventures but realizethat local experience is useful in defining what’spossible. (See EXP 494, “Spring and Fall FreezingTemperatures and Growing Seasons in Idaho”and CIS 844, “Frost Protection for the HomeGardener.”)

PChoose the warmest spot possible

for your garden.Choosing the best garden site is critical for suc-cess in vegetable gardening in areas with shortseasons and cool climates. Even a small change intemperature can make a difference during springand fall frosts. Garden site selection affects windexposure, temperature, and humidity. Windremoves heat radiating from the soil, dries outplants, and causes plants to close their stomates(tiny leaf openings) to conserve water, thusreducing growth potential. Humidity can raisethe temperature slightly by acting as a heat sinkaround plants.

The garden site should be in an area that receivesfull sun and provides protection from the windwindbreaks such as vegetation or buildings. Thesouth-facing side of a building or wall is ideal, asthe site will receive reflected light and heat. Agentle south or southeast slope with good airdrainage will warm early in the spring and willnot be a frost pocket. Avoid low-lying areaswhere cold air accumulates. Orient plant rowsnorth to south to maximize sun interception overthe entire leaf canopy. Light, sandy loam soilwarms faster than heavy clay soils. (See CIS 755,“Planning and Preparing the Vegetable GardenSite,” and EC 871, “Planning a Home or FarmVegetable Garden.”)

Prepare the soil carefully.Short season gardeners must have the best soilpossible in order to develop crops quickly. Wet,clay soil stays cold late into the spring. A light,sandy loam is ideal. Amending soil becomesimportant if it has too much clay or sand, or if itis compacted. Adding organic matter will loosen

Short SeasonVegetable Gardening

Jo Ann Robbins and Wm. Michael Colt

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clay and compacted soil, and increase waterretention and fertility in a gravelly, or sandy soil.

Tilling wet soil results in compaction and de-struction of structure texture. In the spring, letyour soil dry before tilling, or prepare yourgarden soils in the fall when conditions are dry.Organic matter, incorporated in the fall, such ascompost, will be available for breakdown whenspring temperatures rise. Fall soil preparationwill allow the short season gardener to plantearlier in the spring.

Beds raised 10 to 12 inches above the normal soillevel have many advantages in short seasonareas. Soil in raised beds dries out and warmsfaster in the spring, and drainage is betterthroughout the season, allowing for earlierplanting and better plant growth. Soil in raisedbeds with a southern exposure will warm morethan level soil (Figure 1). Add new soil, or amendnative soil, during the formation of the beds.Make beds 3 to 4 feet wide so you can work fromthe pathways on either side, eliminating compac-

plastic, with a small, insulating airspace betweenthe layers, will produce even higher soil tempera-tures.

Black plastic is not as effective as clear plastic inwarming the soil and results in higher tempera-tures only when in contact with the soil.

Wavelength selective plastic film, which allowsheat waves to penetrate the film, but keeps lightrays from entering, will also warm soil effectivelyin early spring. This type of material (marketedas infrared transmitting (IRT) plastic or underbrand initials Al-Or) is used from early springthrough the growing season. (See the section onpermanent mulches for more details in workingwith plastic film mulch.)

Foster optimum fertility.Vegetables planted in short season areas musthave adequate supplies of plant nutrients (espe-cially nitrogen) for optimal growth. Plants mustget off to a fast start and sustain continued goodgrowth to mature quickly. A soil test prior toplanting is useful to determine the pH of yoursoil and what levels of nitrogen, phosphorous,and potassium are needed for best growth.

Organic fertilizers require warm soils to promotemaximum release of nutrients. The warmer thesoil, the faster the microbes can break downorganic sources of nutrients to make them avail-able over a period of time. In colder soils, theseorganic nutrients may be available too slowly.Supplemental soluble inorganic fertilizer mayhelp promote the rapid growth needed.

Soluble fertilizers work well for plants in coldsoils because the nutrients are immediately

available. Dissolve these in water and apply oradd to soil in dry, granular form. Apply acomplete fertilizer high in phosphorous atplanting time either in a band adjacent to therow or as a water solution. Add nitro-gen later dry or as a water-applied

tion from foot traffic. (See EC 1228, “Planningand Preparing Your Vegetable Garden Site,” andNCFS 270, “Raised Bed Gardening.”)

Warm the soil early.To provide warm soil early in the spring, coverthe bare ground with clear polyethylene plasticfilm. The plastic retains the sun’s warmth andwill often raise the soil temperature 10 to 15degrees in a few days. A double layer of clear

South

South

Figure 1.

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fertilizer. Several applications of nitrogen overthe growing season, or fertilizing with a slowrelease material, will ensure a steady supply ofnutrients. (See CIS 922, “Fertilizing Gardens;” FG0050, “Fertilizer Guide: Vegetable and FlowerGardens, Except Irrigated Central Washington;”and FG 0052, “Fertilizer Guide: Home VegetableGardens, Irrigated Central Washington.”)

Choose cool seasonadapted vegetables.

Cool season vegetables seeds can germinate insoil that is 40˚F or cooler. They are also able togrow and mature when exposed to at cooler dayand night temperatures. Most and can resistsome frost and light freezes. Vegetables that willgerminate at 40˚F include: fava beans, beets,broccoli, brussels sprouts, Chinese cabbage,cabbage, carrots, cauliflower, kale, collards,kohlrabi, leeks, parsley, peas, radish, rutabaga,Swiss chard, celery, and turnips. Lettuce, onion,parsnip, and spinach will germinate at 35˚F.Direct planting of these crops is possible in mostgrowing areas. However, when the growingseason is short (less than 90 days) and cold soilspersist, season-extending cultural techniquesmay be necessary to mature even these crops.Techniques include soil warming, growing orpurchasing transplants, pre-germination of seeds,plastic mulches, and plant covers.

Of the cool season crops, beets, carrots, andonions will grow well in warmer temperatures,but the others will form seed stalks or producelower yields with inferior flavor when plantedduring warmer temperatures. Crops like Chinesecabbage and celery may also produce seed stalksif exposed to cool spring temperatures unlessvarieties resistant to flowering and seeding areused.

Crops requiring 50˚F or higher soil temperaturesfor seed germination will need to be startedindoors and transplanted after the soil and airtemperatures warm, or directly sowed at thattime. Examples of such crops are: corn, tomatoes,

green beans, dry beans, cucumbers, squash,pumpkin, eggplant, melons, cantaloupe, okra,and peppers. Season-extending cultural tech-niques will be necessary to realize a harvest ofsome of these crops in areas where the growingseason is less than about 120 days. These warmseason crops will need protection because theywill not withstand any frosts, and cold nightswill slow their growth. (See PNW 170, “Propagat-ing Plants from Seed.”)

Choose early maturing vegetablesand cultivars.

The use of early maturing crops is another im-portant factor for short season vegetable garden-ers. Cool season crops such as spinach, radishes,lettuce, and kohlrabi are generally able to reach astage where they can be harvested in less than 60days. Of those requiring 60 to 90 days, peas,broccoli, beets, carrots, onions, cabbage, andcauliflower can all be planted during fairly coolweather. With corn and bush beans, however, theshort season gardener will need to use season-extending cultural techniques. These techniqueswill become essential with crops requiring morethan 90 days for maturity such as most tomatoes,peppers, eggplant, melons, squashes, and pump-kins. Not only do these crops require a greaterlength of time to reach an acceptable harveststage, but they also must have warmer tempera-tures to grow.

Within each crop type, choice of cultivar iscritical for the short season gardener. Chooseearly cultivars with the shortest days fromplanting to maturity. Even a few days can meanthe difference between a harvestable crop or nocrop at all. For example, corn cultivars vary from53 to 112 days in the number of days to harvest.Choose cultivars for “northern gardens” or “coldclimates.” These grow and produce quickly. Lookfor number of days to maturity on the seedpacket or in the catalog. (See PNW 45, “Choosingand Using Western Vegetables.”)

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Buy your seeds from local garden centers or seedoutlets. In short season areas, they will carryseason-appropriate crops and cultivars. Be sureto read the length of growing season on eachpacket before purchase. Seed catalogs have awide choice of cultivars, and companies in thenorthern states and Canada are likely to havecultivars that are specific for short growingseason needs. Obtain sources from garden maga-zines, or from your nearest Cooperative Exten-sion office.

Start plants insideto get a jump on the season.

To produce some warm season crops in shortseason areas start vegetable plants indoors andtransplant them to the garden. You can purchasetransplants or grow your own. Growing yourown transplants allows you to choose amongmore cultivars, and you can provide healthyplants of the desired size for transplanting.Research show the older the transplant, the betterthey will resistance to cold weather. it will have.Younger plants will take abuse and poor growingconditions better, but are more susceptible tofrost damage.

Harden transplants by gradual exposure tooutdoor conditions for seven to ten days toslowly acclimate seedlings to the outdoors beforetransplanting. Gradually expose the plants to acooler environment with more light and lesswater. You can harden warm season vegetablesso they can better resist exposure to against full

sun, wind-whipping, and dry soil, but not tofrost. Transplant on a cloudy day or in theevening to lessen the shock. Do not set tall plantsany deeper than necessary. To avoid placing theroots in cold soil, lay them in shallow trencheswith leaves turned up above the soil. Peppersand tomatoes are particularly suited to thispractice. (See CIS 800, “Growing VegetableSeedlings for Transplanting.”)

Soak or pre-germinate seeds.Germination can be accelerated a few days bysoaking seeds of squash and other cucurbits inwater overnight and planting while still moist.This approach also avoids a cold sensitive stagein early seed germination. This method is not ashelpful for seed such as beans and peas thatswell and germinate rapidly.

There is an advantage from pre-germination ofthe seed of most vegetables. Do this by keepingseeds warm and damp in a wet paper or clothtowel. When the new growth is 1/4-inch long,plant the seeds without allowing them to dry. Adelay in planting, because of bad weather, maycause seedlings to grow too big to handle. If thereis a delay in planting, refrigerate the seedlings fora short time at a temperature above freezing. Thismethod can save up to two weeks for slowgerminating crops such as parsley and carrot.

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Use a permanent mulch, after thesoil has warmed.

Most mulches will conserve moisture, protect soilfrom erosion, and reduce weed growth. They canalso moderate soil temperatures and affect thenitrogen level in the surface layers of the soil.Short season gardeners will want to choose agrowing season mulch that maintains or in-creases soil temperatures, and does not reducethe nitrogen level in the soil.

Probably the best mulch is a clear plastic mulch,which, in the early spring, is the best soil warmer.Remove clear plastic mulch when you plant seedor transplants. Planting can also be accomplishedthrough holes and slits in the plastic. If the plasticis left in place, weeds may grow under it. Blackplastic mulch will prevent the growth of weeds,but does not warm the soil as much as clearplastic, especially if it is not in good contact withthe soil. Clear mulch can be replaced with blackplastic at the time of planting to get the benefitsof both.

Wavelength selective plastic film, which allowsheat waves to penetrate the soil, but keeps lightrays from entering, will both warm the soil andkeep weeds from growing. This type of mulchcan be left in place all summer.

Plastic film mulches serve as barriers to fruit rotand soilborne diseases by providing a soil freesurface for vegetables. They also reduce leaching

from rainfall and keep mobile fertilizer nutrientsfrom being lost from the root zone. If treated withcare, some thicker plastics can be used for morethan one growing season. Disposal of largeamounts of plastic can be a problem. A dripirrigation system installed over or under plasticmulch provides a very efficient and low mainte-nance method for growing vegetables.

Organic mulches generally moderate soil tem-perature. Most, if present on cold soil, will keepthe soil from warming as fast as bare soil, keep-ing soil cold longer into the spring. This is espe-cially true of light colored mulches, which reflectthe sun’s rays. Short season gardeners shouldonly apply dark colored mulches, such as com-post, after the soil has warmed. An application ofdark colored mulch to warm soil in the latesummer will help maintain a warmer soil laterinto the fall.

Organic mulches decompose on the soil surface,adding valuable plant nutrients to the soil. This isespecially true with materials high in nitrogensuch as vetch clippings. Very woody mulches,such as sawdust, when incorporated into the soil,will use soil nitrogen during decomposition,reducing the levels available to vegetable plants.To avoid this problem, do not incorporate woodyorganic mulches until after harvest. Addition ofextra nitrogen to the soil at that time will enhancebreakdown. It may be best to compost thesematerials prior to using them as mulch.

Clear fiberglass

Clear plastic

Figure 2.

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One disadvantage of any type of mulch to a shortseason gardener is the reduction of nighttimeheat radiated from warm soil into the surround-ing air. This radiated heat, during spring or fallfrosts, can be the difference between a frozenplant and one that is untouched by the cold.Placing a cover over a plant on a thickly mulchedarea may not afford much extra frost protection.(See CIS 837, “Mulches for the Home Landscapeand Garden,” and EC 1247, “Gardening withComposts, Mulches, and Row Covers.”)

Use covers or screensto protect plants.

Covering plants modifies the temperature, wind,and humidity. Air and soil temperatures arewarmer and the cover will conserve heat radi-ated from the soil during the night. A smallincrease in temperature has a large effect, be-cause many plant growth reactions double theirrate for every 18˚F. In short season areas, cover-ing plants in the spring and the fall extends thewarm season. Keeping covers in place during thewhole season may be necessary in particularlycool areas.

Some protectors are large enough to cover sev-eral plants or rows of plants, similar to having agreenhouses with no foundation over tendergarden crops (Figure 2). Cold frames have hingedcovers that can be removed or propped openduring the day. A light frame covered withplastic in an “A” shape (Figure 2) can be placedover one or several rows and left in place thewhole growing season. Plant tall crops in themiddle and low ones near the sides. A sheet offiberglass arched over a row makes a tunnelleaving the ends open for ventilation (Figure 2).Clear plastic stretched over hoops provides thesame effect. Floating row covers of spun polyes-ter material can also be used to cover vegetables.The light material rests directly on the plants.These materials also will reduce insect damageby excluding the pests.

Small protectors, such as hotcaps of partiallytransparent waxed paper, can be used for one ortwo plants (Figure 3). Empty plastic gallon jugswith the bottoms removed can be placed oversmall plants. Clear plastic fences or screensplaced around one or several plants give thesame effect, but the open top will need to becovered on cold nights. Small plant covers(cloches) made of panes of glass or plastic heldby a wire clamp have been used for many yearsin England. Old tires can provide wind protec-tion for single plants planted inside and willabsorb the sun’s warmth. When filled with water,the water will hold heat for release during thenight (Figure 3). Commercial plant covers withvertical tubes of water that store heat for releaseat night are ideal for individual plants.

Any covering method excludes bees, so uncoverplants periodically for pollination. All coveringsystems need to provide ventilation to preventoverheating during warm days. A single layer ofplastic or glass is a poor insulating material, socover with blankets, sheets, canvas, or burlap

6" x 6" wireWith plastic cover

Stake framewith plastic cover

Plastic jug withbottom removed

Figure 3.

Hot cap(cut-away view)

Tire

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sacks filled with straw to insulate the structureson cold nights.

Only the imagination and ingenuity of the gar-dener limit types and varieties of coveringmethods. For example, clear plastic stretchedover the top of vegetables planted in a furrowwill trap the sun’s heat in the air and soil of thetrench to speed the early growth (Figure 4). Later,slit the plastic over the row to prevent overheat-ing. You may need to pull the leaves of the plantsup through the slit.

Add heat to any of the above covering systemsby burying heat tape in the soil, extending thegrowing season even longer. Planting vegetablesinto soil covering decomposing compost ormanure piles and using any of the plant coversdiscussed will also provide extra heat for growth.

Water stores the sun’s heat well. A simple tech-nique is to half bury a gallon jug of water neareach plant. During sunny days, water will warmand release the heat to the soil and the air nearthe plant during cool nights (Figure 5). Water-filled plastic bags, metal cans, or containers canbe used in the same way. Heavy solid materialssuch as bricks, concrete blocks, and rocks willalso absorb the sun’s heat and release it later.

Include a storing device inside plant covers tostore solar heat for release during the night. Blackcontainers and materials will absorb more heatthan light-colored ones.

Windscreens and light reflectors used individu-ally or in combination are useful to keep windaway from plants and to concentrate the sun’slight and heat. An effective combination is avertical screen along each row on the side towardthe prevailing wind. If this is on the north side ofthe row, cover the screen with reflective materialsuch as aluminum foil to reflect the sun’s raystoward the crop. Shingles or boards set in theground near small, newly transplanted plantswill provide wind and sun protection. (See NCFS246, “Constructing Cold Frames and Hotbeds.”)

The techniques outlined in this publication canhelp the gardener meet the short season chal-lenge and grow a variety of healthy homegrownvegetables. Accept the limitations of the climate,choose crops and varieties carefully, and selectthe season-extending practices that fit your life-style.

About the authorsJoAnn Robbins is an Extension Educator andHorticulturist at UI’s Blaine County ExtensionOffice. Wm. Michael Colt, is Extension Horticul-turist at UI’s Parma R&E Center.

Clear plastic covers

Water jug

Figure 5.

Water in heavy plastic tubor old innertube

Stone circle

Figure 4.

Published December 1996. Reprinted November 2000 $2.00

Pacific Northwest Extension publications are jointly produced by the three Pacific Northwest states—Idaho, Oregon, andWashington. Similar crops, climate, and topography create a natural geographic unit that crosses state lines. Since 1949, thePNW program has published more than 400 titles. Joint writing, editing, and production have prevented duplication of effort,broadened the availability of faculty specialists, and substantially reduced costs for the participating states.

Published and distributed in furtherance of the Acts of Congress of May 8 and June 30, 1914, by the University of IdahoCooperative Extension System, the Oregon State University Extension Service, Washington State University CooperativeExtension, and the U.S. Department of Agriculture cooperating.

The three participating Extension services provide equal opportunity in education and employment on the basis of race, color,religion, national origin, gender, age, disability, or status as a Vietnam-era veteran as required by state and federal laws.The University of Idaho Cooperative Extension System, Oregon State University Extension Service, and Washington StateUniversity Cooperative Extension are Equal Opportunity Employers.