Water Retention in a Wood Chip Garden

Cilantro growing in approximately six inches of wood chip mulch.

Few characters I dealt with in my years as as a researcher were as colorful as Chuck Redman.  Chuck was a retired solar energy technician who had dedicated

many lab hours during his career to exploring the movement of adsorbed water.   After retirement, Chuck had become fascinated with the hypothesis that significant amounts of water in trees could move by adsorption.  He could not understand why the dominant theory in plant physiology was that water moved within plants via capillary flow. Chuck explained that once water adsorbed to a surface, it spread out across that surface uniformly, much like a gas will uniformly fill a container.

Chuck spent a good part of his retirement time writing letters or calling on plant physiologists, trying to convince them of his hypothesis.  His experimental data collected in borrowed laboratory space was poorly received by established plant physiologists.  The evidence that capillary flow explained water movements in plants was already established, and no big granting agency was offering money to challenge conventional wisdom on the matter.  To my knowledge, Chuck never found the collaborators he needed to help him publish his theory.

¬† ¬† ¬†Recently, I ran into a tree trimmer who was trying to get rid of a few wood chips. ¬† I knew these would provide a great mulch for my garden, so I did my best to take them off his hands. ¬† A few trailer loads and I was able to bury my entire garden in about 6 inches of wood chips. ¬†I use an underground drip system for irrigation. ¬†Before adding the wood chip mulch, the surface of the soil remained dry even after irrigation. ¬†But within days of adding wood chips, I noticed that the bottom four inches of the wood chips were always moist. ¬†I’ve now had wood chips covering my main vegetable garden and my raised beds for about six weeks. ¬† My spinach has never been more prolific, and my soil has never stayed so moist.

¬† ¬†Wood is indeed a very porous material, and one can certainly argue that the capillaries in my wood chips are filling with water. ¬†But in a matrix as disconnected as chipped wood, I find it hard to accept that so much water is finding its way into such disconnected capillaries. ¬†It has been some time since I’ve seen Chuck Redman, but I have no doubt that he could offer an explanation for the high moisture content of my wood chip garden.

All debate aside, wood chip mulches offer an excellent way to retain moisture in any garden.   There are other benefits to wood chip mulches as well. Here are a few:

Some Benefits of Gardening with Wood Chip Mulches

Weed control.  Wood chips are naturally free of weed seeds. A thick covering of wood chip mulch can also make it difficult for weeds to emerge.

Organic nutrient source.  Wood chips break down slowly, providing the ultimate slow release fertilizer for your plants.

Home for fungi.  Wood provides a source of, and a substrate for beneficial fungi that help your plants grow better.

Water harvesting and moisture management.   A thick layer of wood chip mulch can increase the capture of moisture from morning dew by providing more surface area for water to condense on.  It can also capture water from rainstorms that, in the absence of a thick layer of mulch, might run off your site rather than soaking into the soil.  By protecting soil from harsh sunlight, wind, and other elements and by providing a surface for water to cling to, wood chips help soil stay moist without becoming too wet.

Economical.  A thick layer of wood chips can take decades to decompose.  Often, wood chips can be obtained free or at low cost from tree trimmers.

Before you get started

In home gardens, there is little to lose by utilizing wood chips as mulch.  Just be aware that fresh wood chips, like straw or dry leave, may absorb a lot of nitrogen when they are first laid on the soil.  In time, this will be to your benefit, as the wood chips will support billions of microbes that convert atmospheric nitrogen to food for your plants.   But initially, you may cause young plants a lot of undue stress by burying them in wood chips.   We recommend laying your wood chips down 6 to 8 weeks before planting.  This will allow your wood chips time to be colonized by beneficial microbes so that nutrient cycles can balance out before exposing them to new, young, or delicate plants.

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