The Importance of Mulching your Trees

By Andrew Land

If you’re looking to learn far more about the importance of mulching trees than you knew before, then you’ve come to the right place! Other than watering, there’s arguably nothing better you could do to help your new tree get established than add a layer of mulch. Many think that mulch just makes newly planted trees look pretty, but the truth is that it serves several key functions all at once.

Before we even begin, what qualifies as mulch? The quick answer is that more or less any organic matter serves as mulch, from wood chips, to compost, to straw. Carbon is nature’s “sponge,” and in very general terms, organic matter that is brown (dry leaves, twigs, wood chips, straw) is primarily carboniferous and can hold available moisture to feed growing tree roots. In very hot climates, even rocks can be used to shade soil and make condensation available for tree roots, but rocks don’t do nearly as much in terms of imparting nutrition as organic mulches do.

For people, nutrients are either fat or water-soluble, but trees get all that they need from water. Mulches both hold that moisture for trees’ use and are arguably the best way to fertilize a young tree by providing slow-release nutrition that leaches down to the roots below with rain.

In addition, mulches moderate soil temperature to help contribute to vigorous root growth. The vast majority of stabilizing tree roots reside in the top 3 – 4′ of soil where they have access to oxygen, whereas only a small percent dig deep down as many might assume. For that reason, our goal as tree planters is to encourage root growth horizontally. Robert Kourik wrote in his book Roots Demystified that tree roots are famously “lazy” and that their tendency is to grow where conditions are good. That is related to why we aim to dig holes during Friends of Trees planting events that are 2- to 3-times the width of the root mass. Young roots will happily grow into loose soil, but have a harder time punching through the hard-packed clay we often find in urban environments. They’ll grow even more enthusiastically into loose soil that is moist and held to a moderate temperature.

That said, we’ve learned that mulch retains moisture, but another way that it contributes to creating good conditions for root growth is by helping maintain temperatures conducive to root growth. Unlike the canopy of a tree, roots can grow whenever the conditions are right. The primary ingredients for those conditions are adequate moisture and moderate soil temperatures. When a tree is well-mulched, the soil around its roots is slightly warmer during the winter and slightly cooler during the summer. This happens naturally in a forest as twigs and leaves constantly shower the forest floor, so we’re simply recreating what young trees find in a more natural setting to ensure they thrive in years to come.

If that’s not enough, mulch helps to suppress weed growth and is a great way to recycle organic matter. In doing so, mulches help add organic matter to the soil, which serves as a nutrient and water reservoir underground. Because mulch prevents weeds and grass from returning, it also prevents the need for lawnmowers and weed-whackers to run right up to the base of trees. They frequently cause bark damage and can even be fatal if damage is done all the way around the tree. Mulches also foster growth and development of soil microbial activity, including that of mycorrhizae. Mycorrhizae means “root fungus,” but note that they are an example of a very symbiotic fungus which helps increase trees’ uptake of water and certain nutrients.

Finally, mulches protect the soil structure around tree roots from compacting when pounded by rain. Although water is crucial to trees’ survival, too much water too fast can flatten soil structure, impeding root growth. Mulches slow and spread rain water above soil, encouraging it to percolate downward at a more manageable rate. That way, the roots still get the water they need while maintaining good soil structure, which they also need.  How’s that for having your cake and eating it too?

We’d be remiss not to mention the proper way to apply mulch, having touted its many benefits. We advocate for “doughnuts and not volcanoes.” Because mulch holds water, we don’t want our mulch to touch the base of our tree’s trunk, or we risk rotting the root crown and killing our tree. Start your mulch 3″ from the base of your tree, extending it a few feet outward. Mulching to a depth of 2-3″ is sufficient, and you can extend your mulch the following year by adding a new, wider ring of mulch that overlaps the first and forms a concentric circle.

If you’ve been sleeping on the idea of applying mulch to your newly planted tree, wait no longer! We maintain a pile of free compost or wood chips here at the Friends of Trees office that is available to the public, so feel free to bring your yellow glass recycling bin by, load up, and mulch your trees before the weather gets really hot this summer!!

–Land is a Neighborhood Trees Specialist at Friends of Trees.