Dr. Ajay Nair
Assistant Professor, Department of Horticulture
Iowa State University
As weather warms up, so do the weekend warriors, professional growers, gardeners, and landscapers. We are inching towards official start of Spring which marks the beginning of planning, designing, and installation, and all other activities surrounding vegetable production. We all know that soil or growing medium is an important factor to guarantee high vegetable yields and quality. More for homeowners and backyard growers, one question we all wrestle with is whether to grow vegetable in soil or in raised beds. It is an important question that has direct implications on how well your plants will grow. Below we discuss situations where raised-beds significantly enhance plant growth and yield, simple ways to construct them, and growing medium in raised beds.
What are raised beds
As the name suggests, raised beds are freestanding beds constructed above the natural terrain or grade of the soil. The planting area is raised above the existing soil level and usually enclosed within a structure to form a planting bed. The design of raised beds may be formal or informal, or rectangular or irregular shaped. For a vegetable garden one could go either way. A raised bed does not have to be very deep. Eight to 12 inches is usually adequate.
Why would you need one?
There are various reasons why installing a raised bed can help improve the health and productivity of your vegetable garden. Many gardeners are stuck with soil types that have poor nutrient status, are too wet, compacted and have poor drainage, or infested with nematodes and other soil-borne pests. Such soils hinder seed germination, root growth, root establishment, nutrient supply and eventually affect plant growth. There are situations when it is more desirable to grow plants in raised beds because of less bending and crawling involved. Raised beds also come handy in situations when your backyard or garden is prone to runoff and erosion.
Types of raised beds
Raised ground bed
This is the simplest form of raised bed. It can be easily constructed by creating flap-topped mounds, usually six to eight inches high, and require no additional materials other than additional soil. The mounds are created by bringing in additional soil to form the beds, or by excavating three to four inches of soil from pathways between beds.
Supported raised beds
This type of raised bed is constructed by building a framework with wood, stone, brick or plastic. If using wood, it is advisable to use untreated form. Typically supported raised beds are created using untreated rot resistant lumber like cedar. Pieces are usually of 2" x 6" (ht. x width) and 8-12 ft. in length.
What advantages do raised beds have?
Some of the major advantages which raised beds offer include:
- Improved soil drainage, allowing soil to dry and warm faster in the spring, and provide better soil conditions for vegetable crops that need well-drained soils.
- Provide opportunity to raise vegetables in areas with little or no soil, unsuitable soil, or contaminated soil.
- Great resource for people with limited or small space.
- They can raise the height and increase accessibility for people who have difficulty bending and stooping.
- An option to formulate one’s own growing medium with specific formulations and ratios of soil, compost, soil-less medium, etc.
- Reduced soil compaction.
- Earlier planting – The raised bed facilitates better runoff and drainage allowing soil to warm faster in the spring.
- Raised beds could be easily covered with hoops for frost protection and season extension.
What are some challenges
- The time and cost of building materials, construction, and maintenance can sometimes be a barrier to using raised beds.
- If using untreated wood to construct raised beds, the wood frames of raised bed gardens tend to attract and harbor slugs when excess moisture exists.
- Efficient and timely irrigation of raised beds as they dry out faster than native soils.
- Raised beds may limit the types of vegetables you can grow.
- The untreated wood frame will rot and may need to be replaced every 5-6 years.
Building a supported raised bed
Site selection. Site selection is important. 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 6–8 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 shade, such as broccoli, cabbage and lettuce. The soil or the growing medium in the raised bed and the location determine how well a raised bed will drain. Always locate the raised bed in a location with proper and adequate drainage. Setup the raised bed in a level area or one where minor modifications will make it level.
Site preparation and dimension. Till the soil in the spot where the frame will be placed. Next, determine the size and height of the raised bed. It should be no wider than 4 ft. because most people can only comfortably reach 2 ft. to the center. The length of the raised bed varies according to space but is typically 8-10 ft. long. The depth or height of the raised bed varies but for most vegetable crops it is 8-12” or more.
Growing medium. Fill the raised bed with growing medium. Standard potting soil or commercial container mixes can be used as growing medium for vegetables, but are usually too expensive for filling large beds. The growing medium could be soil from a different location or a blend of soil, compost, and soil-less mix. If bringing soil 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 salts. 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.
Irrigation in raised bed. Irrigation is critical for optimum plant growth. Install low-volume irrigation by using soaker hoses or drip tubing to conserve water and keep it off the plant leaves. Sprinkler irrigation is also suitable, but less desirable due to potential disease problems. One should be diligent with irrigation in raised beds as soil quickly dry and cause water stress to plants.
Raised beds can help solve many problems. In areas where the soil contains too much sand or clay, or is too alkaline for plants to grow well, raised beds are a good option. Soil that is poorly aerated because of compaction or poor drainage also will benefit from a raised bed.