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Raised Bed Gardening

Raised beds are used to grow many vegetables, herbs, fruits, trees, shrubs and flowers. The benefits of raised beds include improved soil drainage, warmer soil, easier access, less soil compaction, longer growing season and higher yields in a limited space. With the sloped terrain in West Virginia, raised beds are an excellent way to use terraced hillsides for gardening. Most plants respond positively to being grown in raised beds with an abundance of loose soil, which improves root growth.

Generally speaking, there are two styles of raised beds that gardeners can create. One method of forming a raised bed is tilling and ridging the existing soil into a mound or hill. This type of raised bed is not permanent and is typically used for annual vegetables, such as tomatoes and squash, flowers and herbs grown on beds 6 to 10 inches high. Another form of raised bed is a framed bed, which is permanent and filled with soil or a soil-compost-sand blend to create a large volume of loose, friable soil (Figure 1). This type of bed is conducive to square-foot, intensive gardening and growth of many perennial and annual plants.

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Figure 1. Permanent raised beds can be used for growing an abundance of plants in a limited space. (Photo credit: L. Jett)

Permanent raised beds can be framed with many types of building materials, including wood boards, planks, concrete blocks, plastic boards and straw bales, to name a few. Any type of framing material, such as recycled plastic or metal can be used. The frame should be square and level across the width of the bed – this will make it easier to plant and harvest garden plants. A raised bed that is 10 to 18 inches in height is ideal. The length can be variable depending on the space available. The width of each raised bed should be based on how easily you can reach plants from either side, with typical widths ranging from 30 to 48 inches. If using treated wood, the bed should be lined with a landscape fabric material. The location of the raised bed should be in close proximity to water because a drip or micro-irrigation system may be needed.

Prior to constructing the bed, the soil that the bed is constructed over can be aerated with a broad-fork or spade. If the site has good drainage, a landscape fabric can be placed over the area prior to building the frame to reduce weed emergence. Many raised beds are built on concrete slabs or areas with little or no topsoil. The walk path between beds should be approximately 3 to 4 feet, or wide enough to accommodate garden tools, such as wheelbarrows and garden carts.

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Figure 2. The raised bed should be level and square before filling with growing media. (Photo credit: L. Jett)

A raised bed can be easily converted into a four-season cold frame structure by placing hoops made of plastic pipe or metal conduit over the frame (Figure 1). Each hoop is spaced 3 to 5 feet apart and can be placed on rebar to provide support. The hoops can support row cover or a lightweight plastic covering, which will protect young or mature plants from freezing temperature, driving rain or wind. Many vegetables and herbs can be overwintered in a cold frame structure, including spinach, collards, carrots, turnips, mache, sprouting broccoli, shallots, bunching onions and kale. In warmer months, shade cloth or wildlife netting can be placed over the hoops to shade or prevent damage from wildlife.

The soil or media used to fill the raised bed can be topsoil or a mixture of topsoil and soilless mixes, such as peat moss, compost, coconut coir, vermiculite, sand or wood chips. If the existing topsoil in the garden is not suitable for a raised bed, then manufactured growing media can be created. The advantage of compost and soilless mixes is there will be almost no weed seeds in these sterilized potting mixes.

Estimate the cubic feet of soil or media by multiplying the width by length by depth of the raised bed. An average wheelbarrow holds 3 cubic feet, so mixing the materials in a wheelbarrow may be a convenient way to estimate the volume of media needed. Evenly distribute the mix across the bed with a rake so there is uniformity of depth. Lightly water each layer so that there is uniformity of moisture throughout the raised bed. A drip tube can place approximately 2 inches deep with each line or tube spaced 10 inches apart. If the plant rows are perpendicular to the length of the bed, the drip tube can be placed lengthwise down the bed. Raised beds can be watered by overhead watering, but it is much more efficient to water plants from the bottom rather than getting the leaves wet and potentially wasting water.

Seeds or transplants can be planted in linear rows in the raised bed or interplanted throughout the bed as companion plants. To create a square-foot, intensively-spaced garden, the raised bed can be subdivided into individual 12-inch-by-12-inch squares – each square-foot can be used to grow one or several crops. For example, one tomato plant would occupy the entire square but could be interplanted with four lettuce plants (Table 1). There are numerous combinations of garden crops that can be intercropped within the raised bed (Table 1). When choosing plants to grow together, select plants that have different days to maturity. For example, a fast-growing leafy green can benefit from being shaded by a larger plant, such as a tomato or cucumber plant, and will be harvested earlier than the fruiting vegetable. Plants that have deep roots and are considered heavy nutrient feeders are usually excellent companion plants with shallow rooted, light-feeding plants.


Plant

Companion plants

Beans (bush)

Lettuce (leaf), beets, onions (bunching), mustard, Irish potatoes

Tomato

Arugula, basil, beets, carrots, chives, lettuce, onions (bunching), radishes

Broccoli

Beets, lettuce, mache, onions, radishes, spinach

Pepper

Basil, lettuce, spinach, onions (bunching)

Cucumber

Beans (bush), eggplant, lettuce, radishes

Cabbage

Arugula, Asian greens, beets, Swiss chard, sage, rosemary

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Figure 3. Many garden plants can be intensively planted in a raised bed for increased efficiency of space. (Photo credit: L. Jett)


Lewis W. Jett, WVU Extension Service Specialist – Commercial Horticulture