RAISED BED GARDENING
By Nancy Kalman
Miami County Master Gardener
What is a raised bed? It is nothing more than an area where the soil is piled 6 to 8 inches above the surrounding soil. You donít have to put frames around the area but that soil will eventually flatten without a frame.
There are several advantages for making raised beds. The soil will warm up earlier in the spring, high quality soil can be added to the beds, there is improved drainage, the bed is easier to maintain, it helps reduce erosion (especially on hillsides), and you receive increased yields. Raised beds can be made high enough to allow a person to sit on the edge of the bed or reach the bed from a wheel chair. Raised beds are also handy in small yards or on patios.
The disadvantage is that the soil will dry out faster in the summer.
Kansas State University publications suggest that a raised bed be no more than 4 feet across, so that it is easy to reach the middle while standing outside the bed. If the bed is against a wall, it should be limited to 3 feet across. The length can be as long as you want. However, walking around a 50-foot long bed would be time consuming. Most raised beds are between 4x4 and 4x12.
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A frame can be made from railroad ties, landscape timbers, planks, rocks, bricks, plastic composite materials, or any material that is resistant to rot. Railroad ties should be really old and not have any dark creosote oozing from them. If you are concerned about the toxicity of the ties, you can line the sides with plastic. If you are growing edible crops you might want to use natural cedar or redwood . If you use pressure treated wood, be sure it is not CCA Ė which can leak arsenic. CCA treated lumber was phased out in December of 2003. Several university publications say that research has found ACQ (Alkaline Cooper Quarternary) treated lumber to be safe.
It is recommended that you use a stake, like a 4x4 or 2x4, that is twice the height of the bed to secure the bed to the ground. The bottom half of the stake is buried on the corners of the bed and the side boards are screwed into the top portion of the stake. More stakes can be used on the sides of longer beds. If the stakes are used on the inside of the bed, they wonít be very noticeable. The side can be 2x6 or 2x8 lumber. Two 2x6ís can be placed one on top of the other. If you use landscape timbers or railroad ties, you can drill through all timbers and insert rebar through them and into the ground. The rebar should be placed every 4 feet along the bed.
The beds should be placed in a sunny location for growing vegetables or sun-loving flowers and have some wind protection. You should remove any grass that is growing where you want to place the raised bed. You then want to dig the soil and mix in some compost or other organic matter in the top 2 or 3 inches. You donít want to leave a sharp distinction between the soil in the bed and that under the bed. Then you are ready to add good topsoil which is amended with organic matter, like compost. To be on the safe side, you should have a soil test performed on the new soil. The K-State Extension Office can help you with the soil test. This soft, amended soil will remain very workable because you shouldnít be stepping into the bed.
One big advantage to growing small vegetable crops in a raised bed is that you can plant in blocks or interplanting. Colorado State compared the production of small vegetables grown in raised beds to those grown in rows in a field. They reported a 15 times increase in production in the raised beds. Your plants can be closer together, which helps shade the ground and reduce weeds. You can add a trellis inside the bed to conserve space. Peas, cucumbers and tomatoes can all be trellised. Be sure to plan the location of the trellis so that it doesnít shade other plants.
Drip tape or soaker hoses can be placed down the length of the bed. Sprinklers can be used, but you will have less chance of fungal diseases by watering from below. K-State recommends that you add 1 to 2 inches of water, 1 to 2 times per week in a typical Kansas summer. You will need more water, rain or irrigation, if the temperatures are high, there is a lot of wind, or you have less organic matter in the soil. A mulch will also reduce the loss of water in the heat of summer, but donít place it on the beds too early because a mulch will prevent the soil from warming. The addition of a little more compost each year and side dressing with an all-purpose fertilizer will keep your raised beds productive.