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Filling Your Raised Bed

Healthy soil will help you produce healthy crops. Ideal vegetable garden soil should be loose, deep, and crumbly. It both holds water for root uptake and allows excess rainfall to quickly percolate downward. So how do you fill a raised bed? Should you add topsoil, compost, or potting soil? The answer depends on the situation. Read the scenarios below to learn about options and develop an approach that works best for you!

Scenario 1:

Reduce costs by foregoing an enclosure.
Photo credit: Jon Traunfeld

Raised beds without a wood enclosure- formed by “pulling up” loose, fertile topsoil into a 2-6 inch raised bed with gently sloped sides. Spread 1-2 inches of compost on the area before forming the beds.

Scenario 2:

This raised bed is almost 6 inches above grade and was filled with existing soil and compost. Photo credit: Jon Traunfeld

Raised bed with an enclosure located in an existing garden- If the soil is in good shape (topsoil intact, not compacted, drains well) increase the soil depth by adding add 4-6 inches of compost (homemade or purchased) and mix it with the top 4-inches of soil. You could also add a 2-3 inch layer of topsoil from the walking space around the raised bed and replace it with woodchips, bricks, or weed barrier fabric.

Prior to adding the compost, loosen soils with high clay content by pushing
in your garden fork and rocking it back and forth. Move the fork 6-8 inches and repeat across the entire bed. Photo credit: Jon Traunfeld

Scenario 3:

Raised bed placed on lawn– kill the grass and weeds by covering the area with plain cardboard multiple layers of newspaper, or weed barrier fabric (can take 6-8 weeks). Then fill the bed with a mixture of compost and purchased topsoil in a 1:2 or 1:1 ratio. There are vendors who sell topsoil mixed with compost.

You could also fill the bed with compost and a soilless growing mix in a 1:1 ratio. The latter contain ingredients like peat moss, bark fines, vermiculite, perlite, coconut coir, and compost. As in Scenario 2, you can also remove add the topsoil adjacent to the raised bed.

Turf and weeds were smothered and raised beds built on top of the ground without disturbing the soil. It was filled with adjacent soil, compost, and soilless growing media. Plant roots will grow through the raised bed soil into the ground below. Photo credit: Jon Traunfeld

If the raised bed is at least 6 inches deep and it’s time to plant it’s ok to cut the grass and weeds at ground level and cover the area with the selected growing medium. The vegetation will die and decompose in place.

An example of a soilless medium. It’s more economical to purchase a compressed bale of media. Products containing peat or coconut coir are hydrophobic (repel water) and should be thoroughly moistened before adding the raised bed. Photo credit: Jon Traunfeld

Scenario 4:

Raised bed on hardscape or heavily compacted soil- should be at least 8 inches deep for leafy greens, beans, and cucumber, and 12-24 inches deep for pepper, tomato, and squash. Fill the bed with compost and a soilless growing mix in a 1:1 ratio. Topsoil can be added (up to 20% by volume) for beds that are at least 16 inches deep.

This bed sits on a concrete library balcony. It’s about 16 inches deep and is growing a nice crop of tomatoes. Notice the perlite (white specs) in the soilless growing medium. Photo credit: UME Master Gardeners, Frederick Co.

Some tips and caveats:

By Jon Traunfeld, Extension Specialist

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