Yard and Garden: Answers to your raised bed questions

Ajay Nair and Grant Thompson
Iowa State University Extension and Outreach
Raised garden beds can improve plant yields, but proper planning is required.

AMES, Iowa — Raised garden beds are a popular option for many home gardeners. This week’s Yard and Garden answers some frequently asked questions about raised beds, and those answers come from Iowa State University horticulture specialists Grant Thompson and Ajay Nair. 

Question: What are some pros and cons to growing vegetables in raised beds?

Answer: Raised beds provide several advantages over conventional garden areas. Wet, poorly drained sites can be improved by constructing raised beds. A properly prepared raised bed increases drainage, thereby promoting plant growth and increasing crop yields. Additionally, raised beds dry out and warm up earlier in the spring, which allows earlier planting. Raised beds are more convenient for elderly gardeners or physically challenged individuals who have difficulty bending over to the ground. The raised height of the garden allows individuals to continue their gardening activities. Raised beds may also allow gardening to occur where site soils may be ill-suited for edible gardening, such as with possible soil contaminants. 

While generally minor, raised beds do have some disadvantages. Raised beds dry out faster than level garden sites. Accordingly, they have to be watered more frequently in dry weather. Initial construction of the raised bed may take more effort than maintenance of the conventional garden. Depending on the height and volume of the raised beds, this can quickly increase costs and may not be suitable to overwintering crops.

Q: How do I construct a raised bed? What is a good size for a raised bed planter?

A: Raised beds may be permanent or temporary structures. Temporary raised beds are shallow beds (6 inches or less in height) constructed within the existing garden. After the garden is tilled in the spring, the loose soil is raked into raised beds. Permanent beds are usually higher than 6 inches in height, have side supports and are built to last for many years.

The walls of permanent beds can be constructed of decay-resistant wood (such as cedar or redwood), concrete block, rock, or brick. Treated wood can also be used, but depending on the treatment, the inside may need to be lined with woven geotextile fabrics that will allow water to drain through but separate the soil/plant roots from absorbing potentially toxic chemicals.

While there are few rules that must be followed, some guidelines should be kept in mind when planning and constructing raised beds. Site selection is an important aspect. Vegetables require a lot of sunlight; a bed for these plants should be located where it will receive full sun. Select a location that receives at least six to eight hours of direct sun. If that is not possible, select a site that receives morning rather than afternoon sun. If it is challenging to locate a spot with adequate sunlight, try growing cool season vegetables that tolerate some shade, such as broccoli, cabbage and lettuce. Raised beds should be constructed so an individual can reach everything in the bed without stepping into it. If a raised bed is placed against a building, fence, etc., making it accessible from only one side, the maximum width should be 1 1/2 to 2 feet (i.e., approximately arm’s reach). Beds accessible on both sides can be 3 to 4 feet wide. The length of the bed is determined by space limitations, personal preference and convenience. If constructing several raised beds, the pathways between adjacent beds should be wide enough to accommodate garden equipment (e.g., a wheel barrow or cart; about 3 feet wide).

Q: What would be a good soil mix for a raised bed?

A: Raised bed soils should be light and well-drained. An excellent soil mix can be prepared by mixing equal parts topsoil, organic matter (well-rotted manure, compost or peat) and coarse sand. If bringing topsoil from other location, ensure that the soil is not infested with soil-borne plant pathogens or contaminants like lead, pesticides, etc. Likewise, ensure that the compost is well done, mature and does not carry too much salt. Mixing compost with the native soil in the beds will help create structure, add nutrients, improve drainage and enhance the biology. When filling the bed, grade the soil so that it slopes slightly away from the center of the bed to the edge and away from adjacent structures. Use mulch around plants in raised beds to conserve moisture and to control weeds.

Q: Do I need to kill the grass or till the soil before starting a raised bed?

A: Before filling raised beds, remove the grass sod, if possible, and work up the existing soil with a rototiller or spade. Add a few inches of the soil mix, then incorporate it into the existing soil. Continue to add and incorporate additional soil mix until the raised bed is filled. Incorporating the soil mix into the existing soil prevents the formation of distinct layers in the raised beds. Distinct layers of soil impede water movement and discourage root growth. 

If contamination is a concern, place a layer of woven geotextile fabric between the native soil after stripping sod and build the raised bed on top of this. The woven fabric will slow, but still allow water movement into the soil below, but will reduce root growth through the fabric, preventing nutrient (or contaminate) uptake from the native soil. Do not use non-woven fabrics (e.g., pond liner) since these will not allow drainage. If drainage into the native soil is too slow, holes or weeps may be added to the sides of the raised bed to allow for additional drainage.