Taking an environmentally sensitive approach to pest management


Missouri Environment & Garden


David Trinklein
University of Missouri
Plant Science & Technology
(573) 882-9631

Raised Bed Gardening

David Trinklein
University of Missouri
(573) 882-9631

Published: July 2, 2015

Raised-Bed Gardening
With the memory of water-logged gardens from abundant spring rains fresh on our minds, now is a good time to consider raised-bed gardening.The latter is a popular technique for growing flowers, vegetables, fruits, trees and shrubs in Missouri. Raised bed gardens can be attractive, productive and functional.

A primary advantage of raised-bed gardening is improved drainage.This makes raised beds a logical choice for gardeners who must contend with heavy, poorly-drained soils.Raised beds permit plant roots to develop in soil located above water-logged or compacted areas.This elevated soil is much better for root growth. As the soil in beds are amended, compost or other forms of organic matter frequently are incorporated.This practice further improves soil structure, drainage and nutrient-holding ability.

Raised beds can be temporary or permanent in nature.Temporary raised beds work well for many backyard vegetable gardeners and consist of forming raised mounds or berms from loose soil as it is tilled.When planted and mulched, temporary raised beds need no edging to keep the soil in place.However, they may need to be reconstructed from time-to-time.
Permanent raised beds are more satisfactory for most situations. Besides controlling erosion better than temporary beds, walled beds permit deep soil amendment.Although the initial cost of construction is higher, most gardeners prefer raised beds with walls because of their permanence.

The choice of framework to use for raised beds with 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 decay-resistant lumber, such as redwood or cedar, may also be used. Other possibilities include concrete blocks, bricks and stones, or synthetic lumber made from recycled plastic. A group of half barrels can make a convenient raised bed for use on a patio. For a consistent look, match materials used to construct a raised bed 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 recently was banned for residential use by the Environmental Protection Agency, and its production was phased out. Any remaining stock probably should not be used to construct 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.However, ties that are still oozing black, sticky creosote or have an intense odor may cause injury.

Gardeners who are uncertain about the safety of treated lumber should consider placing a heavy plastic liner between the treated lumber and the soil used for growing plants. This will prevent direct contact of plant roots with the treated lumber. Be careful not to tear or puncture the plastic when preparing the bed.

Raised beds take many forms and sizes. Typically, raised beds are rectangular in shape.Four feet is a convenient width for beds. At this width, the center of the bed is easily accessible from either side. If the bed is accessible from only one side, limit the width to three feet. Most gardeners find it uncomfortable to reach farther than three 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, for the sake of convenience, consider breaking up long distances into several shorter beds. For example, instead of building one long bed 50 feet in length, construct two 24-foot-long beds with a two-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 6- to 12-inches of soil to develop their roots.However, 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 article.

Whatever the depth, raised beds constructed from wood must be held in place.Treated wooden stakes or concrete rebar are good choices to accomplish the latter.In either case make sure sufficient reinforcement is provided to keep raised beds from falling apart.

For raised beds less than two feet tall, stones or cement blocks may be stacked on top of one another without mortar or footings. 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. Mortar may be used for greater strength.

Make pathways between raised beds wide enough for easy access to beds. For foot traffic only, paths one foot in width 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 paths two or three feet in width. A space-conserving option is to make most paths narrow, with an occasional wider path for access with garden equipment.

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.

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 avoid the formation of soil layers, gardeners often 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. Next, the topsoil removed 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.

Some advocates of intensive “square foot gardening” recommend filling raised beds with a mixture of 1/3 topsoil, 1/3 well-decomposed organic matter and 1/3 haydite or other large aggregate amendments.Mixtures such as the previous are highly porous and encourage the development of extensive root systems.However, because of their porosity, they require more frequent watering.Added expense also is a concern.

Given the amount of time, labor and expense often required to build a raised bed, their management should not be taken lightly.Good management begins with plant spacing which is closer in a raised bed than in a conventional garden. The goal is to use as much of the available area as possible, without overly crowding plants. This is accomplished by spacing plants closely together in a staggered (or diamond) pattern. By the time the plants mature, 80 percent or more of the surface area should be covered with plants.

Another way to increase the productivity of raised bed vegetable gardens is through succession planting. The latter can be defined as planting a second crop in the area vacated when the original crop is harvested. The same crop may be repeated or a different crop may be planted, depending on the date and/or food preferences of the gardener. Succession planting extends the supply of produce from the garden late into the year and helps to make optimum use of valuable garden space by growing two crops instead of one in a given area.

Plant nutrition, irrigation and pest control also are important in raised-bed gardening because of the value of the space involved.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 one to two 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.

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 encourage diseases. Organic mulches, such as straw or hay, in vegetable raised beds, or wood chips placed on landscape fabric weed barriers around ornamental bed reduce the amount of water lost through evaporation. Weed growth also is suppressed.

Insect and disease control should follow integrated pest management principles. The latter begins with trying to exclude the pest, if at all possible. Thoroughly cleaning raised beds of plant material at the end of the growing season is the first step in pest management.If chemical controls are deemed necessary, early intervention using the most eco-friendly pesticide is best.

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REVISED: July 2, 2015