David H. Trinklein
Horticulture State Specialist
Division of Plant Sciences
Raised-bed gardening is a popular technique for growing plants in Missouri. Vegetables, fruits, flowers, trees and shrubs may be grown in raised beds, which can be attractive as well as useful (Figure 1).
Raised beds are not only functional, they can also be attractive.
Advantages of raised-bed gardening
- Better drainage
Growing plants in raised beds is a logical choice for gardeners with heavy, poorly drained soils. Raised beds permit plant roots to develop in soil held above water-logged or compacted zones. This soil environment is much better for root growth. As beds are built up, compost or other forms of organic matter may be incorporated, further improving soil structure, drainage and nutrient-holding capacity.
- Higher yields
Better root growth from improved soils leads to higher yields for food crops and lusher growth of ornamental plantings. Also, intensive planting in raised beds means more plants can be grown in a smaller area than with conventional row-cropping techniques. No space is wasted between rows.
- Expanded growing season
Better drainage speeds soil warming and allows earlier spring planting. In wet seasons, soil dries faster, so planting can proceed between rains.
- Easy maintenance
Because plants grow above the level of walkways, less stooping is required for weeding, watering and other chores. Intensively planted raised beds provide dense foliage cover, shading out much weed growth.
- Using difficult sites
Raised beds make gardening possible on sites where growing plants would otherwise be impossible. Rooftop gardens and raised beds on top of solid rock are examples. Terraced raised beds turn hillsides into productive growing areas while reducing soil erosion potential.
Types of raised beds
Temporary raised beds work well for many backyard vegetable gardeners. As the soil is tilled, it is loosened. If tillage equipment and foot traffic are kept off tilled beds, the loosened soil remains slightly raised above surrounding pathways. Adding compost or other organic matter to the bed raises it even higher (Figure 2).
The main advantage of temporary raised beds is their simplicity. No expense is involved in constructing framework to contain the soil. Temporary beds are less labor intensive to make than permanent beds. However, temporary beds flatten over the course of a growing season and require reconstruction the next year. Also, because a temporary bed has no wall, the soil may erode from the top of the bed into walkways or down hillsides.
Permanent raised beds are more satisfactory for most situations. In the landscape, planting berms may be constructed by hauling in topsoil to create traffic barriers as well as visual interest. When planted and mulched, berms need no edging to keep the soil in place. Walled raised beds may be used in the landscape or for vegetable gardens. Besides controlling erosion better than temporary beds, walled beds permit deep soil amendment.
Although constructing walls for raised beds takes labor and has an initial cost, the finished product should last for many years.
Temporary raised beds can be created by digging soil in a 3- to 4-foot wide bed. Digging loosens the soil and keeps it higher than surrounding ground if there is no foot or equipment traffic.
The choice of framework to use for walls depends on the availability and cost of the construction material and how you want the final product to look in the landscape. Treated landscape timbers and used railroad ties are popular materials. Naturally rot-resistant lumber, such as redwood or cedar, may also be used. Other possibilities include concrete blocks, bricks and stones, or synthetic lumber made of recycled plastic. A group of half barrels can make a convenient raised bed for use on a patio (Figure 3). For a consistent look, match materials to those used elsewhere in the landscape.
Generally, wood-based products are less expensive than stone or masonry materials. However, resourceful gardeners may be able to find used bricks, concrete blocks or other materials at little or no cost.
Concern has been raised about the safety of using treated lumber in food gardens. Pressure-treated lumber using CCA (chromated copper arsenate) as a preservative prompted this concern. CCA-treated wood was banned for residential use by the Environmental Protection Agency (EPA), and its production was phased out. Any remaining stock probably should not be used in raised beds for growing food crops.
ACQ (akaline copper quaternary) is an alternative preservative choice for pressure-treated lumber. Unlike CCA, it does not contain arsenic or chromium. It does, however, contain copper, which can leach into the soil from treated lumber. Although copper is an essential element for both plants and animals, excessive amounts can be harmful. A 2007 study of the safety of ACQ published in Human and Ecological Risk Assessment concluded that exposure to copper from contact with ACQ-treated wood is not expected to have adverse effects on the health of adults or children.
CA (copper azole) is another wood preservative based on the fungicidal properties of copper. Its toxicity risk should be similar to that of ACQ.
Creosote, which is used to treat railroad ties, may cause injury or death to plants that come into direct contact with it. The effect diminishes after a few years. Old, discarded ties do not injure plants (Figure 4). However, ties that are still oozing black, sticky creosote or have an intense smell may cause injury.
If you are uncertain about the safety of treated lumber, place a heavy plastic liner between the treated lumber and the soil used for growing plants to prevent direct contact of plant roots with the treated lumber. Be careful not to tear the plastic when tilling the bed.
A grouping of barrels makes a convenient herb garden on the patio. Make certain that drainage is provided.
Old railroad ties make a satisfactory raised bed if they are not oozing creosote, which could injure plants.
Raised beds take many forms, depending on the gardener's goals. Taming a hillside with terraces may require different bed dimensions than those used for flat-land vegetable gardens. On hillsides, follow the contour of the land and adjust the depth of beds according to the slope of the hill.
Typically, raised beds are laid out in a rectangular pattern. Level the area first to make a flat base for starting the building project.
Four feet is a convenient width for beds. At this width, the center of the bed is easily accessible from either side. Lumber is readily available in 4-foot-length multiples, minimizing the amount of sawing needed and waste produced in building the bed. If the bed is accessible from only one side, limit the width to 3 feet. Most gardeners find it uncomfortable to reach farther than 3 feet to tend a bed.
The length of a raised bed is not critical. It is only limited by the dimensions of the yard. However, break up long distances into shorter beds. To prevent soil compaction, foot traffic and garden equipment such as wheelbarrows should not be permitted to go through the raised beds. So for example, instead of building one long bed, break a 50-foot length into two 24-foot-long beds with a 2-foot walkway between to save steps when tending the garden.
The depth of a raised bed is a matter of personal preference. Most plants need at least a 6- to 12-inch rooting zone, but deeper would be better. With deep tillage, some of the rooting depth may come from soil at or below the existing grade. Beds built higher than 18 to 24 inches require retaining walls with foundations and supports, which are topics beyond the scope of this publication.
Stakes to hold raised-bed walls in place should be twice the height of the raised bed. Bury half the stake in firm ground. Leave half of the stake projecting above the ground as a support.
Hold landscape timbers and railroad ties in place with construction rebar, which is a steel reinforcing bar (Figure 5). Drill holes all the way through each layer every 4 feet, staying 6 to 8 inches in from the ends of timbers. Drive a length of construction rebar through the holes and into the ground below. Tie individual layers together by driving spikes from one layer into the next.
Use decay-resistant wooden stakes to hold dimensional lumber such as 2-by-8s in place. If placed on the inside of the board, the stakes will not be visible once the bed is filled with soil.
For a unique-looking raised bed, cut landscape timbers or posts to uniform 1- to 3-foot lengths. Set the posts vertically in the ground, half buried and half above ground.
For raised beds less than 2 feet tall, stones or cement blocks may be stacked on top of one another without mortar or footings (Figure 6). Carefully place irregularly shaped stones to enhance the stability of the wall. Offset seams and gaps from one layer to the next to help tie the wall together. You may use mortar for greater strength.
Make pathways between raised beds wide enough for easy access to beds. For foot traffic only, 1-foot-wide paths are adequate. Keep in mind, however, that plants at the border of raised beds will hang over the edge, cutting into the available walking space. To allow room for a wheelbarrow or garden cart, plan on 2- to 3-foot-wide paths. A space-conserving option is to make most paths narrow, with an occasional wider path for access with garden equipment.
Several additional design features increase the convenience of raised beds. Seating can be made on the edges of wooden raised beds by capping the walls with a 2-by-6- or 2-by-8-inch board. If you regularly use a rototiller for tilling the beds, ramps into the raised beds save heavy lifting. Hollow pipes attached to the inside wall and spaced regularly along raised beds double as support posts for spring and fall season-extending cold frames or summer trellises for vine crops.
To make a raised bed wheelchair accessible, construct walls about 2 feet high and limit the width of the bed to about 3 feet (Figure 7).
Stacked concrete blocks make a simple raised bed. For greater stability, offset the layers of blocks.
Raised beds can be made wheelchair accessible by making the walls about 2 feet tall and limiting the width of the bed to about 3 feet.
To double dig prior to establishing a raised bed, remove all the soil from the bed one spade's depth. Dig the next layer down, leaving the soil in place. Return the topsoil to the bed and thoroughly mix the layers. Double digging permits deeper rooting by plants growing in the bed.
Good-quality existing topsoil may be used in raised beds. However, the addition of organic matter to any soil will improve its physical and chemical makeup, thus making it more productive. Peat moss, compost and decomposed manures are good sources of organic matter.
To take full advantage of the deep rooting potential with raised beds, work up the base soil by rototilling or hand-digging before bringing in additional soil. Many gardeners double dig beds.
Double digging involves removing the topsoil the depth of a spade, setting the soil aside, then loosening the subsoil another spade's depth (Figure 8). Finally, the topsoil is returned with added amendments, such as compost, manure or fertilizers. This labor-intensive soil preparation method provides an excellent rooting zone for plants. However, less-intensive methods also permit satisfactory plant growth.
Avoid hauling in new layers of soil without mixing them into existing soil. Distinct layers of soil create barriers through which water will not readily penetrate and roots will not easily grow.
To double dig before establishing a raised bed, remove all the soil from the bed one spade's depth. Dig the next layer down, leaving the soil in place. Return the topsoil to the bed, and thoroughly mix the layers. Double digging permits deeper rooting by plants growing in the bed.
Maintenance of raised beds
Soil in raised beds warms faster and dries out more quickly than soil at ground level. In spring and fall, these traits are desirable. They are less desirable during the heat of summer.
Use of organic mulches, such as straw or hay, in vegetable gardens, or wood chips placed on landscape fabric weed barriers around ornamental plantings helps combat both problems. Soil temperatures are lower under organic mulches, less water is lost through evaporation, and weed growth is suppressed. Use irrigation to supplement natural rainfall during dry periods. Soaker hoses or drip irrigation may be placed directly on the bed. Overhead sprinklers can also be used, but because they wet foliage they are more likely to spread diseases.
At the end of the growing season, plant residue in a vegetable garden can be tilled into the soil, adding organic matter. Additional compost may be added before the next planting. Over time, the soil may become improved enough so little additional tillage will be necessary.
Fertilization of plants grown in raised beds is similar to that of plants grown conventionally. For most crops, a complete fertilizer such as 10-10-10 applied at the rate of 1 to 2 pounds per 100 square feet is satisfactory. Organic fertilizers and manures may also be used. For more specific fertilizer suggestions, rely on recommendations based on soil tests.
Denny Schrock, Department of Horticulture