Planning a Raised Bed Garden

Raised beds are freestanding garden beds constructed several or more inches above the natural terrain. Texas gardeners are discovering that raised bed gardens can help solve many problems. In many areas of the state the soil is of poor quality. It may contain too much sand or clay, or be too alkaline for some plants to grow well. Soil that is poorly aerated because of compaction or poor drainage also may be a problem. Soil quality problems are often aggravated in urban and suburban settings, where topsoil and vegetation may be removed or the grade changed during construction.

Raised bed gardens improve the environment for plants by lifting their roots above poor soil. Soil in the beds can be amended to provide a better growing medium for plants, even those that would not naturally thrive there. Raised beds are less apt to be invaded by certain grasses and by tree roots. Soil in raised beds warms up earlier in the spring. Also, raised beds are easier to maintain.

Site Selection

The first step in planning a raised bed is deciding where it will be located. Site selection and plant selection go hand in hand. Many vegetables, ornamentals and herbs require a lot of sunlight; a bed for these plants should be located where it will receive full sun. If that is not possible, select a site that receives morning rather than afternoon sun. If only shady sites are available, try growing cool season vegetables that tolerate shade, such as broccoli, cabbage and lettuce. Some ornamental plants do best in partial shade also. In windy regions, place beds where they are protected from prevailing winds by fences, buildings or other structures. Beds should not be located in frost pockets or where air circulation is poor. Fungal diseases often develop where there is little air flow.

Drainage

A raised bed should drain well so that plants do not become water logged. Soil that remains very wet will deprive plant roots of air. Also, plant diseases develop more easily under wet conditions. Good drainage is especially important in vegetable beds. Both the soil and the location determine how well a raised bed will drain. If the bed contains clay soil, it should be amended with sand, organic matter or a coarse grade of perlite to improve drainage.

Do not locate a bed in a marshy area where it will sit in water. Construct landscape beds so that they slope about 2 percent (a 1/4 -inch drop per foot of horizontal distance) away from any structures, or away from the center of the bed. Sometimes it is necessary to install special drains, and this should be determined during the planning stage. Drain tiles or septic line tubing can be extended the length of the bed and through the walls at either end to create a drainage channel. Normally, one line every 4 to 6 feet is sufficient. Another method is to dig a trench in the desired direction of water flow (from the bed to a lower elevation), lay 3 to 4 inches of coarse stone in the trench, and then lay tiles made of clay, concrete or plastic in the center of the trench. Cover the trench with more coarse stone and then soil. The French drain, another alternative, is simply a narrow trench filled with coarse stone leading from a poorly drained area to a lower elevation.

Design

The design of a raised bed should blend with its surroundings. Beds may be formal or informal, depending on their shape and the kind of edging chosen. A rectangular bed edged with a low brick wall, and filled with yaupon or boxwood pruned into straight hedges or topiaries, has a formal look that might be appropriate in the front of a house. An irregularly shaped perennial border tucked behind a dry stone wall is less formal, but could be attractive almost anywhere in the landscape. A vegetable garden has an informal look that works best in private areas of the yard. The size of the bed should be kept in proportion to the space around it.

A raised bed does not have to be excessively deep to be effective. Eight to 12 inches is usually adequate. If drainage is a problem, or if the plants you are growing prefer drier soil, the bed could be taller and filled with a porous growing medium. Vegetable beds should be 12 to 18 inches deep. The material used to edge a raised bed should be stable, durable and attractive. It is the edging that gives the bed its ³look² within the landscape. It also establishes the outline of the bed and holds the soil in place. Edging may be as simple as metal strips, railroad ties or landscape timbers, or as intricate as a mortared brick or stone wall. A crested bed is one in which the soil is simply mounded from the edges of the bed to the center; it may or may not have an edging.

Metal edging comes in 8- to 10-foot lengths, is easy to install, and is convenient for edging curved beds. However, it can rust with time, and unless plantings overflow the bed or the edging is camouflaged with a more aesthestic material, it may not be as attractive as you would like. Ties and timbers can be laid singly or in layers and have a rustic appearance. Railroad ties treated with creosote do not pose any health problems because most of the creosote has leached away. There is some controversy about the use of treated landscape timbers, but studies have shown that any compounds that leach out are well within safe levels established by the EPA, both in growing media and in harvested produce. If you are concerned about using treated timbers, line the inside of the bed walls with polyethylene, roofing felt or similar materials to form a protective barrier. Stone walls make interesting beds, and can be constructed with cracks and openings for creative plantings.

However, stone can be expensive. Whichever edging material you choose, it should be strong enough to withstand being bumped into or ridden over by a riding lawn mower. It should be complementary to the rest of the landscape, and properly installed.

Irrigation

The design phase is the best time to decide how you will irrigate your raised bed. Hand watering may be simplest in many cases, but it can become tedious; the gardener must also know when and how much to water or plants will suffer.

With an automated sprinkler system beds can be watered on a regular basis with little effort, but sprinkler heads wet the foliage, which contributes to disease and salt damage. If the system is automatically timed it may come on whether or not there has been recent rainfall, and thus waste water. So, an automated system may be the most convenient for the gardener, but it is not necessarily best for the plants.

Irrigation systems that work well for woody plants and vegetables include microsprinkler, drip, trickle and soaker hose systems. These systems conserve water, can be installed under mulch, can be regulated to tailor flow rates to individual plant needs, and are less likely to wet foliage. However, they do have some disadvantages. Emitters are prone to clogging unless the water used is very clean, and if emitters are installed under mulch it is difficult to spot problems. Emitters are sensitive to elevation changes along the irrigation line and require pressure compensating lines. Also, lines are easily damaged by rodents and other wildlife.

If you choose a sprinkler system, decide whether the sprinkler heads will pop up or be set on risers, and how many sprinkler heads you will need. Be sure to consider the spray overlap, angle of spray and height of the sprinkler heads. Always design the system so that at least one additional riser per section can be added later. You may need this flexibility as your plantings mature. If you choose drip or trickle irrigation, determine the length of the hose and the number of emitters you will need. (Drip tape with 12-inch emitter spacing is best for vegetables.) Beds should be divided into watering zones according to the needs of the plants, the size of the system, the water pressure, and the volume of water available. Zone watering can be a manual or timed system. There is no "best" irrigation system for raised beds. Sometimes a combination of systems works best.