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Home , Garden, Gardening, Square foot gardening

THE SERIES W 346-E BACKYARD

THE TENNESSEE

BUILDING AND USING RAISED BEDS

Natalie Bumgarner, Assistant Professor and UT Extension Residential and Consumer Specialist Department of Sciences

Vegetable production is increasingly Productive vegetable generally popular for Tennessee residents. SITE CHALLENGES require 6 to 8 hours of full per Growing vegetables at home FOR HOME VEGETA- day while a few leafy crops (lettuce, provides financial and nutritional BLE spinach) may be grown with 4 to 6 benefits through the bounty of hours of sunlight. The best method of estimating sunlight is to observe a fresh harvest, and the activity SITE CHARACTERISTICS — potential sites throughout the day and enhances personal health and LIGHT, ACCESSIBILITY AND track sunlight hours. Nearby can well-being. However, a basic WATER block light, compete with crops for understanding of soils, site selection water, or even produce compounds and maintenance is required Many who are interested in vegetable (specifically black walnut) that are before backyard growers can take may not have suitable land toxic to some . It would likely full advantage of the benefits of to plant a traditional garden. Some live be preferable in the long run to build home production. To meet in apartments or condominiums where a raised bed and use amended soil they have no access to soil, while others these needs, this series of fact in an area with good light but poor may have small lots or restrictions on sheets has been prepared by UT soil than to choose a site with good planting in their residential areas. In soil quality but more shade than is Extension to inform home these situations, raised beds can provide desirable. Additionally, a nearby, high- and propel them to success in a great alternative. However, other site quality water source will enhance the residential vegetable production. considerations are still important. productivity of your garden and reduce maintenance time.

BACKYARD VEGETABLES | THE TENNESSEE VEGETABLE GARDEN - Building and Using Raised Beds 1 Figure 1. Raised beds can be appropriate for a variety of crops and sites.

SOIL CHARACTERISTICS OPTIONS FOR SITES WITH space on on and . POOR QUALITY OR DEGRADED Raised beds and containers enable Soil that is easily workable and allows gardeners who may not be able to water to soak in (infiltrate) and drain NATIVE SOIL handle the equipment or the labor of easily is best. In some residential Some home gardeners enjoy the traditional gardening to stay active and areas, there may be a scarcity of high- process of improving the soil on their continue to produce edible crops. quality soil. Common issues include site over time. Soil can be brought to removal or compaction during the site or cover crops and organic POTENTIAL DRAWBACKS OF neighborhood development and soil matter can be used to improve soil RAISED BEDS that is high in clay or rocks or drains characteristics (see W 346-G). But poorly. Be aware of any previous uses many gardeners may prefer to have • Mechanical tillage equipment is or structures on the site that may have alternatives to provide faster return on difficult to use in raised beds. added heavy metals, debris or other their efforts. undesirable materials to the soil. • Bed must be carefully constructed Raised beds are one of the most to withstand the weight of substrate The home has several common options to grow residential plants and management over time. choices if soil conditions are not ideal, food and crops (Figure 1). They • Close spacing can reduce air flow and but the first step is completing a soil can be constructed to take advantage test as discussed in UT Extension increase the opportunity for disease of the best lighting and water access infection or shading. publication W 346-A “The Tennessee and allow garden size to be tailored to Vegetable Garden — Site Selection family needs. Raised beds allow more • The growing substrate will drain faster and Soil Testing.” This test can help control over substrate (a term used and require more frequent watering. determine the pH and nutrient levels of here to refer to the soil and/or mineral the soil as well as the organic matter and organic material that the plants are • Large vining plants like many squash content. Over time, organic matter grown in), drain more easily, and warm and pumpkin selections are generally additions, applications, and up faster in the spring than traditional not well-suited to raised beds. pH management can improve the in-ground gardens. Containers also workability and productivity of can be used by gardeners with limited native soil.

BACKYARD VEGETABLES | THE TENNESSEE VEGETABLE GARDEN - Building and Using Raised Beds 2 Figure 2. A raised bed that accommodates standing or seated gardeners.

Gardeners can line raised beds with only 4 inches to 6 inches in depth. INTRODUCTION plastic to reduce leaching concerns or Taller and deeper rooted crops — such TO RAISED BED chose cedar or redwood lumber that is as tomatoes, peppers, okra and corn more resistant to decay. Additionally, — often require deeper beds for root GARDENING boards made from recycled plastic exploration and stability. A smaller used for decking are a more expensive substrate volume will retain lower RAISED BED MATERIALS AND but long-term option for constructing amounts of water and nutrients. Since CONFIGURATION raised beds. It is important to select raised beds drain more rapidly than a material that will allow construction nearby level soil, deeper beds with Temporary raised beds using soil of a stable bed that will support itself, more substrate can decrease watering can be constructed without side the growing substrate and plants. frequency. An additional consideration support, although these are mostly Taller and longer beds will contain in bed construction is the ability to used in traditional gardens. A range more substrate and weight and require accommodate those who may not be of materials can be used to construct more substantial construction, support able to work at ground level (Figure 2). permanent raised beds including and bracing. Small beds may be wood, stone, brick, block or composite constructed of 1-inch-wide lumber, but RAISED BED INSTALLATION materials. Prior to December 2003, it is more common to use lumber that is AND CONSTRUCTION chromated copper arsenate (CCA) at least 2 inches wide. was used as a preservative in lumber If the site has usable soil, there may be for residential uses. So, use caution A common bed width is 4 feet if no concern with leaving the bottom of because this older treated lumber as accessed from both sides, and 2 to 3 the bed open and letting the roots of well as railroad crossties can contain feet if accessed from one side. Beds plants grow into native soil. However, metals that can leach into garden soil. are generally constructed 6 inches to hand dig or till to combine the raised Each gardener should select and use 12 inches in height but can be deeper. bed substrate with the upper layer of materials he or she is comfortable with Shallow-rooted crops — such as lettuce, native soil to avoid creating an abrupt for a home vegetable production. See spinach, kale and other leafy crops gradient between amended and native additional resources listed below for — may be produced in beds that are soil. If native soil is of poor quality or more details on using treated materials.

BACKYARD VEGETABLES | THE TENNESSEE VEGETABLE GARDEN - Building and Using Raised Beds 3 the garden is on a non-soil surface, a CROP SELECTION Warm-season crops wooden or porous ground cloth bottom Warm-season crops are damaged or The two most important considerations can be installed under the raised bed. killed by frost and should be planted in selecting crops for raised bed include Newspapers or cardboard also can be after the threat of spring frosts and seasons best suited for growth and used as a barrier to prevent or freezes are passed (see W 346-B space needed for production. The most plant growth from underlying soil and “The Tennessee Vegetable Garden — common way to classify vegetables sod, but they will be less effective if Garden Planning, Plant Preparation is according to their temperature weed pressure is severe. and Planting”). These crops produce requirements. optimally under warmer temperatures and also tend to have a deeper root Cool-season crops system that is better able to withstand IMPORTANT Cool-season crops are more productive drier, hotter conditions that are typical CONSIDERATIONS and of higher quality when they of Tennessee summers. are grown and mature under cooler IN RAISED BED spring and fall season temperatures The second important consideration GARDENING (approximately 60-75 F). They tend in selecting crops is space. Often to be shallower rooted and more beginning gardeners make the mistake of planting too closely, which creates GROWING SUBSTRATE sensitive to periods of low moisture. Many cool-season crops can be well- competition for water, light and If the native soil is of high quality, suited to raised beds because they nutrients. Even if these factors are not raised beds can be filled with topsoil can be closely spaced and sequentially limiting, close spacing can reduce air from the site. If constructing a taller planted. Raised beds, because of movement and increase disease risk. (12 inches to 24 inches) raised bed, their potential for higher spring soil The chart below provides estimates of be cautious of reduced temperatures and excellent drainage, spacing for vegetable crops in typical when using only native soil. Often can often be planted earlier than in- raised beds. An increasing number soilless mixes are used as a substrate ground gardens. Earlier planted cool- of space efficient cultivars are being to increase drainage, aeration and season crops are more likely to mature introduced. One of the benefits of optimize nutrition. Soilless mixes under preferred cooler temperatures. raised beds is that no walking space is usually consist of moss, , The opportunity to plant earlier is needed, so often or plants are vermiculite and perlite. A common mix especially useful in many parts of placed in a grid rather than in rows. is equal volumes (not weight) of 1) peat Tennessee where spring temperatures moss, 2) compost and 3) vermiculite may quickly rise to levels that are (it is best not to use older supplies of higher than ideal for cool-season crops. vermiculite that may be on site as they may contain asbestos).

Purchased components also can be Cool-season vegetables Warm-season vegetables mixed with high-quality native soil to decrease costs. A common mixture Beet, broccoli, cabbage, cauliflower, carrot, Bush, pole, and lima beans, muskmelon, would be equal volumes of 1) garden kale, collard, kohlrabi, and head lettuce, sweet corn, cucumber, eggplant, okra, pea soil, 2) peat moss or compost, and 3) mustard, onion, English and snap pea, Irish (field, southern), pepper, pumpkin, Malabar vermiculite or perlite. Keep in mind that potatoes, , spinach, Swiss chard, spinach, squash, summer and winter, sweet testing is important when preparing turnip potato, , watermelon substrate for a raised bed to determine if lime or fertilizer is required. If Table 1. Vegetable crops categorized by broad temperature requirements. compost is used in the substrate, it can provide plant nutrients but should still 2 to 4 inches between 6 to 8 inches 12 to 18 inches between More than 18 be tested to account for variation. Use seeds or plants between seeds or seeds or plants inches between caution when introducing any material plants plants into your substrate (soil, compost) that could add weed seeds. If compost Snap bean, beet, , Swiss chard, Cabbage, collard, broccoli, Winter squash, carrot, , pak choy, small cauliflower, eggplant, pep- watermelon, heating is insufficient, weed seeds can onion, pea, turnips, head lettuce, per, compact or determinate many tomatoes still be viable. garlic, spinach, leaf strawberry, kale tomato, compact cucumber lettuce and summer squash

Table 2. Vegetable crops categorized by typical spacing in traditional raised beds.

BACKYARD VEGETABLES | THE TENNESSEE VEGETABLE GARDEN - Building and Using Raised Beds 4 CROP MANAGEMENT Fertilization in raised beds can be they can help conserve soil moisture similar to managing nutrients in and reduce watering needs. Water and fertilization traditional gardens (see W 346-C “The Tennessee Vegetable Garden Raised beds often have better drainage — Managing Plant Nutrition). If It is best to cycle different types of and aeration than traditional gardens, using compost in preparing growing crops in raised beds to reduce the but usually will dry out faster due substrate (as recommended in the buildup of pests and diseases. It is to elevation and greater substrate method), be common to group crops in terms porosity. Close spacing in raised beds sure to consider any nutrients that of fruiting (tomatoes, peppers, also may mean plant roots do not have are contained in the compost when eggplants), vining (cucumber, squash, as much substrate volume from which calculating fertilizer needs. to draw water. So, water management muskmelon, watermelon), Brassica is especially important in raised beds. crops (broccoli, cabbage, cauliflower, Weeding and mulching kale, radish, turnip), and root crops Typically garden vegetables require One of the greatest advantages of (beets, carrots). These different types 1 inch to 1.5 inches or more of water raised beds is that the smaller growing of crops have varied nutrient needs and per week. Raised beds would require area and close plant spacing can sometimes different pest and diseases at least this much moisture, and likely reduce time spent weeding compared complexes, so rotating them through more during warm or hot weather. to traditional gardens. Care in selecting your beds can increase production and Overhead watering can be used, but a substrate without weed seeds can reduce disease risks (see W 316 “Home do not allow to remain wet also prevent problems before Vegetable Garden Disease Control” for overnight because of disease risk. It they start. additional information on rotation). is especially important in the close spacing of raised beds to keep leaves Mulching also can be an excellent way Support and training as dry as possible to prevent infection to reduce weed issues (see W 346-D). and spread of diseases. Natural — such as compost or Providing support for vegetable plants straw — can be a good way to improve can improve their ability to intercept involving drip tape or soaker the growing substrate over time while light to support photosynthesis and hoses is often preferable to overhead reducing weed pressures. Black plastic make sugars needed for leaf and irrigation because it keeps leaves dry mulches block light to prevent weed growth. It also can improve air and efficiently delivers water to the growth and increase soil temperatures, movement and reduce disease risk. In substrate and roots. especially in the spring, by absorbing addition to these benefits, raised bed also allows more thorough watering solar radiation and transferring heat to gardeners should consider training because water can be provided slowly the soil beneath. Most plastic mulches and supporting their crops because of and has the opportunity to spread block moisture, so drip irrigation is space efficiency (see Figure 1). evenly through the substrate (see required under the . Woven black Vertical supports can consist of W 346-D “The Tennessee Vegetable or white ground covers are a mulch trellises, stakes, cages, twine or Garden — Plant Management option that is permeable to water and a combination (see W 346-D). It Practices”). air and can be reused. Both natural is common to plant vining crops and plastic mulches reduce the rate (cucumbers, winter squash, pole beans, of evaporative water loss from soil, so

Require support for space efficient Could benefit from support Do not need support use in raised beds

Vining cucumber, pole bean, tomato, Bush bean, compact cucumber and winter Lettuce, beet, spinach, cabbage, broccoli, watermelon, muskmelon, winter squash, squash cultivars, pepper, eggplant, okra, cauliflower, kale, radish, carrot, Swiss chard, snap and shell pea compact pea bush summer squash

Table 3. Vegetable crops categorized by their need for vertical support in raised beds.

1 plant/2 ft2 1 plant/ft2 2 plants/ft2 4 plants/ft2 8-9 plants/ft2 16 plants/ft2

Muskmelon, vining Broccoli, cabbage, Cucumber Swiss chard, corn, Large beets, peas, Small beets, squash (bush types cauliflower, eggplant, lettuce, basil, parsley, beans, spinach carrots, onion, 9 ft2) okra, pepper, vine potato, strawberry radish, chives tomatoes, cilantro

Table 4. Spacing for crops in the SFG method (All New Square Foot Gardening, 2nd Ed. Bartholomew, 2013)

BACKYARD VEGETABLES | THE TENNESSEE VEGETABLE GARDEN - Building and Using Raised Beds 5 Figure 3. This image illustrates the use of agricultural fabrics both floating on the top of a raised bed crop and supported by hoops to form a low tunnel. (Photo from UT Extension Publication PB 1578 “Tennesee Master Gardener Handbook.”)

etc.) on the north side of the raised are some specifics. The SFG method is through. These covers can be simple bed so that a can be installed based on raised beds with dividers that to install because they do not need a that will not shade the shorter crops. create 1-square-foot plots within the support system. However, for some Tomatoes, peppers and eggplants are larger raised bed. A common size is a crops with sensitive growing points, more commonly supported individually 4-by-4-foot bed that creates 16 square like tomatoes and peppers, floating row using stakes or cages. They also can foot units. Common crops are planted covers can cause abrasion under be attached to twine and suspended at set densities within each square foot windy conditions. from an overhead support. Tomatoes, unit for clarity and productivity especially indeterminate crops that (Table 4). There are a range of row cover weights continue to grow both from the that differentially alter temperatures. primary growing point and side shoots, SFG typically uses a mix of equal Greater temperature increase will may be more manageable if some side volumes of peat moss, compost and occur during sunny days as the covers shoots are removed (see W 346-D). vermiculite in a 6-inch-deep raised bed. trap sunlight. The lightest covers Due to compost in the substrate, SFG may provide 1-2 F temperature buffer gardeners are encouraged to rely on overnight while the heaviest covers the nutrients from the compost, which can provide 4-8 F protection. Heavier PRACTICES THAT is added at the end of each cropping covers reduce light much more than cycle to resupply nutrients. the lighter covers. While the light CAN COMPLEMENT covers can allow around 90 percent RAISED BED SEASON EXTENSION of ambient light to reach the plants, the heaviest covers will block over 50 PRODUCTION Season extension methods combine percent. These heavy covers often are well with raised beds. Several removed in the morning and replaced techniques can be used to provide SQUARE FOOT GARDENING in the late afternoon to provide increased temperatures and longer nighttime temperature protection while (SFG) growing seasons for vegetable garden still enabling light to reach the crops crops, and two of the most common during the day. Square foot gardening, a concept first are described below (see also introduced by Mel Bartholomew, has W 346-F). Trapping heat can be detrimental helped introduce many to small-scale under warm and sunny conditions. So, home gardening. While the principles Floating row covers, or agricultural season extension methods are typically of square foot gardening are similar fabrics, form a lightweight cover that used in the spring, fall and winter and to other raised bed methods, there allows light, air and moisture to pass are removed when warm conditions

BACKYARD VEGETABLES | THE TENNESSEE VEGETABLE GARDEN - Building and Using Raised Beds 6 persist. They also block pollinating of polyethylene are often perforated , so vine crops like pumpkins, or vertical slits are made to allow air squash, muskmelons and watermelons movement and prevent overheating must have the covers removed when in bright sunlight. The downside of flowering begins. An added benefit is polyethylene low tunnels is that they that the covers also can protect plants do not allow to reach the crops, so from some damage if installed irrigation will probably be necessary. early enough and tightly. Low tunnels made of floating row cover material are permeable to water, Low tunnels can be made of so irrigation needs will depend on agricultural fabrics used for floating precipitation. row covers or clear polyethylene plastic (often a lighter version of the plastic that would cover a ). Either ADDITIONAL RESOURCES material is stretched over wire hoops to Garden use of treated lumber. Penn State Extension publication. form miniature typically http://pubs.cas.psu.edu/freepubs/pdfs/uc173.pdf 2 to 3 feet tall (see W 346-F). Like floating row covers, they are generally Small spaces gardens. University of Kentucky. applied in the spring and fall and http://www2.ca.uky.edu/enri/gardening/smspacegardening.pdf removed during the warmest part of the growing season. Low tunnels made EPA information on CCA treated wood. http://www.epa.gov/ingredients-used-pesticide-products/chromated -copper-arsenate-cca

AG.TENNESSEE.EDU

W 346-E 6/16 16-0143

Programs in and natural resources, 4-H youth development, family and consumer sciences, and resource development. University of Tennessee Institute of Agriculture, U.S. Department of Agriculture and county governments cooperating. UT Extension provides equal opportunities in programs and employment.

BACKYARD VEGETABLES | THE TENNESSEE VEGETABLE GARDEN - Building and Using Raised Beds 7