Double digging a garden

Question:

I read a garden book that recommended "double digging". What is that? Do you recommend double digging for New Mexico gardens?

Answer:

Double digging means digging twice as deep as usual (twice as deep as a typical garden shovel's depth). A typical shovel can dig 10 inches of soil if you dig to the full depth of the shovel blade. A typical garden rototiller, on the other hand has tines that go down about 7 to 8 inches (depending on the setting of the drag bar).

A flat bladed garden spade may dig 11 inches and a tile spade (or sharp-shooter often used by plumbers to dig trenches) can dig a narrower swath of soil, but to a depth of 16 inches.

To double dig your garden, follow these steps:

1. Begin by removing a shovel depth of soil from the first end of a row. The soil you remove will be one shovel depth downward, and one shovel depth backwards in the row. The row may be three, four, or more shovel widths across.
2. Take this first soil to the back end of the row where you will finish digging. (You will need it at that location.)
3. Now that you have removed the first layer of soil, you can loosen the second shovel depth of soil at the beginning of the row.
4. Now, move backward and dig the first layer from the next portion of the row. Place it on top of the soil you just loosened.
5. Loosen the soil (double dig) this second part of the row.
6. Continue double digging the row, placing the top layer of soil on the soil you just loosened as you work backward down the row.
7. When you get to the last part of the row and place the top layer of soil on the next to last section, and have double dug the last section, you will need the soil you saved from the very first layer you dug. Place it over the loosened soil at the end of the row. You have now double dug a complete row. After resting and enjoying the pleasant feeling of success, start your next row. Double dig it in the same manner. Illustrations of this procedure can be found in many gardening books (perhaps in the book you mentioned when you wrote this question). Perhaps this explanation will help make sense of the illustrations.

The reason this is good practice is that it loosens the soil deeply, encouraging deeper penetration of rain and irrigation water. This encourages deeper rooting by garden plants and a healthier plant more able to resist the heat of our summers. The fact that gardens are usually near a home or other structure means that the garden soil starts as compacted soil. Construction equipment used to build the structure often compacts the soil by driving over it during construction. This compacted soil tends to shed rain water and irrigation. Plant roots do not easily penetrate compacted soil (even if the soil can be moistened). Double digging overcomes the compaction due to construction. A rototiller or plow that loosens the soil to double the depth of a shovel can be used to accomplish the same thing if there is room for such equipment at the garden site.

Some garden authors recommend gardening without digging. The authors of these books recommend letting the earthworms and winter freezing and thawing loosen the soil. They object to "damaging" the soil's structure by plowing or digging. In our soils, especially soils already damaged by compaction, digging is necessary. Once the soil has been developed, their procedures may be appropriate. You must determine for yourself, in your particular garden soil and local environment, if digging is beneficial after the soil has been developed by a few years of gardening.

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Marisa Y. Thompson, PhD, is the Extension Horticulture Specialist, in the Department of Extension Plant Sciences at the New Mexico State University Los Lunas Agricultural Science Center, email: desertblooms@nmsu.edu, office: 505-865-7340, ext. 113.

For more gardening information, visit the NMSU Extension Horticulture page at Desert Blooms and the NMSU Horticulture Publications page.

Send gardening questions to Southwest Yard and Garden - Attn: Dr. Marisa Thompson at desertblooms@nmsu.edu, or at the Desert Blooms Facebook.

Please copy your County Extension Agent and indicate your county of residence when you submit your question!