By Christopher J. Starbuck
Raised-bed gardening is a popular technique for growing plants. Beds are both useful and attractive in the landscape. In addition, they offer some advantages:
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 provides a more optimum soil environment 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.
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 out faster, permitting planting to proceed between rains.
Because plants are growing 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.
Railroad crossties are popular materials for use in raised bed construction. Generally, crossties are less expensive than concrete, stone or masonry materials. Take note that creosote, which is used to treat railroad ties, may cause injury or death to plants that come into direct contact with it. After a few years the effect diminishes. Old, discarded ties do not injure plants. However, injury may occur if ties are still oozing black, sticky creosote or smell intensely. If you are uncertain, place a heavy plastic liner between the cross ties and soil used for growing plants to prevent direct contact of plant roots with the treated lumber. Be careful not to tear the plastic when turning the soil in the bed.
Typically, raised beds are laid out in a rectangular pattern. Level the area first to make a flat base for starting the building project. A convenient width to use for beds is 4 feet. At this width, the center of the bed is easily accessible from either side. A crosstie cut in half is about 4 feet, minimizing the amount of sawing necessary and the amount of waste produced in building the bed. If the bed is accessible only from one side, limit the width to 3 feet. Most gardeners find it uncomfortable to reach farther than 3 feet to tend the bed.
The length of a raised bed is not critical and 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 like wheelbarrows should not be permitted to go through the raised beds. For example, instead of building one long bed, breaking a 50-foot length into two 24-foot long beds with a 2-foot walkway between them will save gardeners many steps.
The depth of your raised beds is to a great extent up to your discretion. 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 holes being drilled through the stacked ties and a length of rebar being driven into the ground to offer stability to the walls. Drill holes all the way through each layer every 4 feet, staying 6 to 8 inches in from the ends of timbers. Tie individual layers together by driving spikes from one layer into the next. Such stakes to hold raised-bed walls in place should be twice the height of the raised bed.
Make pathways between raised beds wide enough for easy access. For foot traffic only, 1-foot wide paths are adequate. However, keep in mind that plants at the border of raised beds will hang over the edge, cutting into the available walk space. To allow room for a wheelbarrow or garden cart, plan on 2 to 3-foot wide walkways. To conserve space, one option is to make most paths narrow, occasionally adding a 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 crosstie-raised beds by capping the walls with a 2 x 6 or 2 x 8 inch board. If you regularly use a roto-tiller 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.
Good quality existing topsoil may be used in raised beds. However, add additional organic matter to soils with a high clay or sand content. 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.
Soil in raised beds warms faster and dries out more quickly than soil at ground level. In spring and fall, these traits are desirable. But through the heat of summer, soil temperatures are higher and drying in raised beds is faster than in surrounding soil.
Use of organic mulches, like 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 may also be used.
At the end of the growing season plant residue can be tilled into the soil, adding organic matter. Additional compost may be added before successive plantings. 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 like 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.