Garden Variety: Tips for raised garden beds
Gardeners and garden guides often refer to “raised beds” as a method of gardening with little explanation of what these beds really are, how to use them or, more importantly, how to build them. Raised beds are a great way to improve soil for food crops (fruits, vegetables and herbs) in Kansas and define a planting space. Raised beds can also be a fun DIY project.
A raised bed is any planting space that is elevated from the surrounding soil surface. Raised beds can be framed with wood, metal or other materials to maintain form with a level surface, or they can have sides that slope to the ground. Framed raised beds are most common in home and community gardens for food crops. Unframed raised beds are more common in large-scale food crop production and for ornamental plantings. Unframed raised beds are also called berms.
The biggest benefit of raised beds is soil improvement. Existing soil can easily be mixed with compost and organic matter to improve drainage, texture and overall soil health.
Framing on a raised bed holds soil in place and defines the space, which can help keep pets and people from accidentally wandering through. Framing can also provide a space to sit, kneel, or lean while working in the planting space.
Use raised beds for vegetables, herbs, small fruit crops, flowers and some shrubs. Small trees can also be planted in raised beds but are probably better in large berms than in framed raised beds, which can limit root space. The growth of medium and large trees may be limited in raised beds because of their expansive root systems.
To build a raised bed, first select a site. For food crops, choose a site that gets full sun for at least six hours a day, or plants may fail to produce. Avoid planting over the root systems of mature trees if possible. Piling soil on top of tree roots limits air exchange, and the roots will eventually grow up into the planting bed and compete for water and nutrients.
Next, decide how large the bed will be. If the bed will be framed, 4 feet wide by 8 feet long is a popular size. The 4-foot width is about right to work in from the sides without having to step into the bed. Length is less important, but 8-foot boards are common and an easy choice to create the length.
If you have a larger space available, you can create multiple 4-foot by 8-foot beds with aisles between them for standing and working. If you have less space available, try making a 4-foot by 4-foot bed, or whatever other size fits the space.
The height of the bed is dependent on space and personal preference. If the goal is for minor soil improvement, 4 to 6 inches of height is adequate. If the soil below is very poor and most of the roots will be within the raised bed space, go deeper. Deeper planting beds also bring the work space closer to you. Raised beds can even be built at wheelchair height, or high enough to sit on the edge to reduce the need for bending and make work more accessible.
If using boards to create the frame, use cedar if possible. Avoid railroad ties and old lumber treated with chromated copper arsenate, or CCA, which may leach chemicals that can be taken up by the plants growing in the beds. Use of CCA to treat wood was phased out in 2003. Research surrounding newer wood preservatives is controversial, so if you choose to use treated lumber, you may wish to line the sides of it with plastic to prevent contact with plant roots.
Metal, rock and other durable materials may also be used to frame raised beds. Some hardware stores and garden centers sell raised garden kits that contain framing materials if it is easier than selecting boards yourself.
Whatever framing material you use, remember that the weight of the soil inside will push outward once the bed is filled. Connect the boards or other material at the corners and install bracing in the middle of the bed if necessary to help support the weight of the soil.
Use a square and a level to make sure the boards or other materials fit together correctly and are level across the top so the soil will be level when the bed is filled. The bottom of the frame may need to be set into the soil on one side or raised up with soil on one side to get it level.
Once the frame is built, fill it with a mix of garden soil and compost. The soil will settle over time, so fill it as full as possible to start.
The bed is then ready for planting. Plants can be placed in dense plantings in these beds, since all weeding and harvesting will be done from outside of the framework. Plant the tallest crops on the north and east sides of the bed to avoid shading shorter crops. Use a fence or trellis on one side to plant vining crops such as cucumbers, squash and sweet potatoes, or allow these plants to vine out over the sides.
Use straw, prairie hay, or other organic mulch to insulate the soil surface and water plants as needed throughout the summer.
— Jennifer Smith is a former horticulture extension agent for K-State Research and Extension and horticulturist for Lawrence Parks and Recreation.