Issue: February 17, 2007
Building a raised bed garden
am going to try growing vegetables in a raised bed this year and would like to know what type of soil mixes are good for this? The containers are made of wood, are lined on the sides with a heavy gauge black plastic, and the bottom is perforated tin. Are these materials alright, that is non-toxic, for growing veggies?
Maria Elena W.
The materials you are describing should be appropriate for making the raised bed garden. If the wood for the sides has been treated, your plan to line it with heavy gauge plastic is a good idea. There has been some concern and scientific investigation into the effects of treated wood (creosote and other treatments). The results reported have been inconsistent, but by lining the inside of the wooden structure with plastic, you are avoiding the problem. The plastic lining will also help conserve moisture in the raised bed. However, you will need to take care in subsequent years when you prepare the bed for planting that you don't damage the plastic lining with your garden tools. The wooden structure will also give you a place to sit as you garden and harvest. This will make gardening more enjoyable.
I wonder why you are putting perforated metal at the bottom of your bed. Are you trying to prevent the entry of weeds through the bottom (bindweed or Bermudagrass)? The metal at the bottom may help slow the weeds, but will not prevent them. If possible, don't use the metal at the bottom. I suspect the "tin" is really galvanized steel. It will eventually rust and its effectiveness will be diminished. It will also interfere with roots going deeper into the soil. There may be a reason to keep the metallic bottom, but don't let it interfere with drainage that is necessary to reduce accumulation of salts in the planting soil. Small perforations may not be sufficient for drainage and larger perforations may allow weeds to enter the planting bed. Drainage through the plastic sides at the bottom of the bed may help if the metal is necessary.
Now, regarding the soil mix, almost any New Mexico soil can be improved for vegetable gardening by the addition of organic matter. Well prepared compost, peat moss, or commercially prepared potting soil can help increase the water-holding capacity of the soil. These materials will also release nutrients as they continue to decompose, enriching the soil. They will also decrease the pH of the soil, making it more acid. Most vegetable crops prefer soil that is more acid than most New Mexico soils. You can purchase potting soil and fill your raised bed. This will work well and can be supplemented with native soil to reduce costs somewhat.
Some gardeners first use the raise bed as a compost pile. They put garden, lawn, and kitchen wastes in the bed and allow it to compost in place, either turning it so that soil from below is mixed in, or adding native soil to provide mineral components to the soil helps develop this soil. Once the material is sufficiently composted, it is ready to serve as a garden. You may not have time to do this for this year, but you may consider this for raised beds for future gardens.
Avoid using large quantities of manure as the organic matter added to the soil unless you are certain that the salts have leached from the compost. Often, what is described as "aged manure" has just been sitting under a shed or in a pile for a few years. If the rain and snow melt have not been percolating through the manure, the salts may remain in the manure and can cause problems in the garden.
I'm glad that you are planning to try raised bed gardening. Please let me know of your success.
Marisa Y. Thompson, PhD, is the Extension Horticulture Specialist, in the Department of Extension Plant Sciences at the New Mexico State University Los Lunas Agricultural Science Center, email: email@example.com, office: 505-865-7340, ext. 113.
Please copy your County Extension Agent and indicate your county of residence when you submit your question!