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Saturday, July 21, 2012

Constructing Raised Beds With and Without Side Walls

Q. Hello Professor Morris, I'm trying to figure out how to just post questions in the blog,  so you won't have to reply separately (if the answer is in the newsletter I'll just see it there).  I wonder if there is a best size or height for a raised bed.  I want to put in enough good soil, but don't want to make it higher if I can make it larger, to grow more plants using the same amount of soil. 
Trenching desert soil for creating rasied bed

A. The height is up to you  and what your back can handle. For root crops and asparagus they should be a minimum of 18 inches deep.
Most desert soils can grow excellent vegetables and flowers if they are amended properly at the very beginning. You can use existing desert soils at the location of the bed or you can import manufactured soils from sand and gravel companies or nurseries. If you import a manufactured soil, do not use reject sand. If you are importing a soil to use in place of your soil at the site, select a manufactured soil that resembles the soil at the site as closely as possible. Take a sample of the soil at your site when you are selecting the imported soil so you can compare the two.
Check it to match the texture (how sandy or clayey it is) so the garden bed will drain water into the existing soil. Problems with drainage can occur if the two soils, the soil at the site and your imported soil, are dramatically different from each other.

Adding compost before tilling and forming
the raised bed
Raised garden beds should be oriented north to south if at all possible. Their dimensions should be 3 to 4 feet wide if you can access the bed from either side. If the bed is accessible from only one side then perhaps 3 feet would be the maximum width. Keep garden beds at least 3 feet from foundations and block walls if at all possible.
Once established, you should avoid walking on the raised garden beds other than for possibly planting or harvesting. Use kneeling boards. Never walk on them wet.

I would recommend drip irrigation and using tubing with built in in-line drip emitters. I would use the larger drip tubing such as half-inch and not the smaller laser tubing. Visit with Timmy who runs irrigation department at the Plant World Nursery on Charleston. He will set you up with a drip irrigation system and components that you can run off of a hose bib or garden hose if you want to.

These steps will create multiple raised beds at the site without constructing sidewalls by using and amending the existing soil.
1.         Rip the soil to a depth of 12 to 18 inches with multiple trenches over the entire site. I usually use a trencher.

2.         Remove large rocks.

3.         Flood the trenches with water as best as you can.

4.         Fill trenches with compost.

5.         Collapse the sides of the trenches

Finished, planted and labeled raised
vegetable bed made from raw desert
soil and compost.
6.         Apply 3 to 4 inches of compost to the entire site and rototill as deep as possible.

7.         Remove rocks larger than a golf ball.

8.         Create pathways that are 18 to 24 inches wide between the raised beds by shoveling the amended soil onto the raised beds.

9.         Shape the beds using a landscape rake and garden rakes so that it is a level and the sides of the beds are at a 45° angle. The cross section of the width of the bed should resemble a long trapezoid.

10.       Firm the bed using a roller so that your footprints sink no more than 1/2 inch into the soil when you walk on it.

11.       Install irrigation.

12.       Your bed is ready for planting.

13.       Every time you plant, incorporate a 2 to 3 inch layer of compost subsequently for the next two years. After two years you can reduce the amount of incorporated compost to 1 inch.

These steps will create a raised bed using constructed sidewalls.

1.         Rip or at least disturb the soil to a minimum of 6 inches beneath the raised bed.
2.         Construct the sidewalls with your choice of building materials and to your desired height. For root vegetables and asparagus the depth should be a minimum of 18 inches.

3.         Line the inside of the constructed sidewalls, not the bottom, with heavy gauge pond liner.
4.         If you're using a manufactured soil that has not been amended with compost:
        Mix good quality compost with your desired soil outside of the raised bed in a volume ratio of three parts compost to one part soil (3:1).
        Remove any rocks larger than a golf ball. If you are planning on a lot of root crops then screen the soil so that any remaining rocks are less than 1/2 inch.

5.         If you’re using a manufactured soil already amended with compost:

        Mix good quality compost with your desired soil outside of the raised bed in a volume ratio of one to two parts compost to one part soil (1 or 2:1) depending on the manufactured soil color. If the manufactured soil color is predominantly gray or tan, then use 2:1. If the manufactured soil color is light brown, then use 1:1.

        Remove or screen any rocks in the soil.
6.         Fill the raised bed with the amended soil and settle the soil with water as you are filling it. Fill the raised bed so that the soil is 2 inches below the top of the raised bed.

7.         Make sure the raised bed is firm and not fluffy.
8.         Install your irrigation system.

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