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.
Raised beds lend themselves to the development of complex agriculture systems that utilize many of the principles and methods of permaculture. They can be used effectively to control erosion and recycle and conserve water and nutrients by building them along contour lines on slopes. This also makes more space available for intensive crop production. They can be created over large areas with the use of several commonly available tractor-drawn implements and efficiently maintained, planted and harvested using hand tools.
Circular raised beds with a path to the center (a slice of the circle cut out) are called keyhole gardens. Often the center has a chimney of sorts built with sticks and then lined with feedbags or grasses that allows water placed at the center to flow out into the soil and reach the plants’ roots.
Materials and construction
Vegetable garden bed construction materials should be chosen carefully. Some concerns exist regarding the use of pressure-treated timber. Pine that was treated using chromated copper arsenate or CCA, a toxic chemical mix for preserving timber that may leach chemicals into the soil which in turn can be drawn up into the plants, is a concern for vegetable growers, where part or all of the plant is eaten. If using timber to raise the garden bed, ensure that it is an untreated hardwood to prevent the risk of chemicals leaching into the soil. A common approach is to use timber sleepers joined with steel rods to hold them together. Another approach is to use concrete blocks, although less aesthetically pleasing, they are inexpensive to source and easy to use. On the market are also prefab raised garden bed solutions which are made from long lasting polyethylene that is UV stabilized and food grade so it will not leach undesirable chemicals into the soil or deteriorate in the elements. A double skinned wall provides an air pocket of insulation that minimizes the temperature fluctuations and drying out of the soil in the garden bed. Sometimes raised bed gardens are covered with clear plastic to protect the crops from wind and strong rains. Pre-manufactured raised bed gardening boxes also exist. There are variants of wood, metal, stone and plastic. Each material type has advantages and disadvantages.
Raised garden bed with lettuce in Asia.