Ferguson: Build a raised bed in time to plant fall vegetables

Published 1:21 pm Friday, August 6, 2021

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This year, with its incredible amount of rainfall, has been a reminder that, for some of us, growing vegetables in flat ground is not practical. While some have sunny areas with well-drained soil, others of us must do what we can with areas that stay excessively wet at times.

If you struggled to grow a garden in the ground this spring, you might consider building a raised bed to be ready for cool-season vegetables, like mustard and collard greens, turnips, cabbage, broccoli, lettuce, shallots, sugar snap and snow peas, carrots, and radishes.

Raised beds are often the best option for growing vegetables at home. Besides providing good drainage (assuming that an appropriate soil mixture is used), they also help keep weeds out of the garden. Another benefit of raised beds is that, as long as the beds are not too wide, tending a raised bed garden is likely to be easier on our backs than working an in-ground vegetable garden. There are even raised beds that are designed so that people in wheelchairs can tend them.

It’s generally suggested that raised beds be no more than three to four feet wide. If you’re gardening with young children, you may want to go with the more narrow width. The important thing is that people be able to reach the centers of the beds, without climbing into them.

A raised bed can be of pretty much any length. Having one long bed will reduce the amount of building materials needed, but having several smaller beds can aid you in practicing crop rotation, which is important for preventing disease problems.

Eight inches is about as shallow as you’ll want to make the bed, unless you’re going to work up the soil under the bed, as well, so that vegetables can extend their roots deeper than the bed itself. Keep in mind that the top of the soil should probably be at least an inch or so below the sides of the bed, so that soil doesn’t wash out of the bed.

Wood makes an aesthetically pleasing bed border, and 2-by-10-inch or 2-by-12-inch pieces of wood can be used to make a raised bed that is adequate for many vegetables. A downside of this is that the sides will not be wide enough to sit on comfortably. Also, of course, wood will eventually rot. Larger pieces of wood, like stacked 6-by-6-inch beams, can also be used to make an attractive bed.

Cinder blocks are a practical option. They won’t rot like wood will, and they can be painted on the outside of the bed, if you want to get creative. For greater stability, you can drive rebar into the soil, within the cinder blocks’ holes.

Cinder blocks are commonly eight inches tall and wide, and 16 inches long. (Just as 2-by-4-inch wood is actually a little smaller than two inches by four inches, some cinder blocks are a little smaller than 8-by-8-by-16 inches.) To make an eight-inch deep bed that is approximately four feet wide by eight feet long on the outside, you need just 16 blocks of this size and about 11 cubic feet of soil. For a 16-inch deep bed, double these numbers. Note that such a bed will have inner dimensions of just about 32 inches wide by 80 inches long, since the blocks are eight inches wide.

When it comes to filling the bed, there is a range of possibilities. Topsoil mixed with a couple of inches of composted organic matter is one option. Our Raised Beds fact sheet suggests equal parts sand, compost, and garden loam. Since top soil or garden soil can vary a good deal with respect to what it contains, it’s a good idea to get a soil test to determine what needs to be added. Some garden centers may be able to provide a soil test report for their products.

Let me know if you have questions.

Dr. Mary Helen Ferguson is an Associate Extension Agent with the LSU AgCenter, with horticulture responsibilities in Washington and Tangipahoa Parishes. Contact Mary Helen at mhferguson@agcenter.lsu.edu, 985-839-7855 (Franklinton) or 985-277-1850 (Hammond).