About The Author

Gardening with Allen: Opt for bark over topsoil

I am considering doing some major relandscaping in my backyard. I have 4 inches where topsoil was added, then a hard layer of clay which is difficult to dig into. Should I add more topsoil or is there a better way?

A typical approach for landscapers when landscaping a home with heavy clay soil is to apply a 4-to-6 inch layer of topsoil. This soil is often a mixture which contains about 1/3 sand and 1/3 bark dust. It is easy to work with and plants grow well in it when surface and sub-surface drainage is good. However trees and shrubs roots tend to grow laterally and are not deep rooted like they should be.

We recently did some major landscape remodeling in our front yard. In the process we dug up and moved a number of plants. It was surprisingly easy to dig these plants compared to digging in our normal clay soil in the backyard. Then I remembered the preparation which I had done previously. When I moved into the house 5 years ago I removed the existing lawn. Then I applied bark dust in a 3-to-4 inch layer and tilled it in to a depth of 8 inches with a large tiller. What a difference that has made over the past 5 years as I have moved and added new plants to the landscape.

Organic matter such as bark dust does several things to improve the soil. Its coarse texture creates large pores in the soil which leaves plenty of room for air. This creates a rich supply of oxygen which roots need for good growth.

When a layer of topsoil is placed over clay soil, it creates a distinct layer where they meet which slows the movement of water between layers. When organic matter is tilled into the soil, there is a gradual transition zone and water moves more freely into and through the soil.

Organic particles hold onto water and nutrients longer that other soil particles so they are available to plant roots for a longer time.

As organic particles break down in the soil they gradually release plant nutrients which are available for plant growth.

So my recommendation is to spend half as much for 3 inches of bark dust compared to 6 inches of topsoil and rent a large tiller which will thoroughly incorporate the bark dust into the soil. Dig up small plants that you intend to keep and replant them. Work around large plants. If you do remove some