To Mulch or not to Mulch?


Sometimes I advise clients to use mulch in their gardens, and sometimes I advise them not to, but most often I advise them to use it wisely.  Why?  Because many times it is used for the wrong reasons and incorrectly, causing more harm than good.

Can Mulch Kill Trees?

This article is courtesy of Steve Elwell, president of SumaGreen:

Is mulch really killing our trees?

YES!!! Mulch can kill trees.

This is a subject of much interest because the trees are suffering. When the soil around a tree gets too high, problems develop. This can occur naturally but is more often caused by people. People who love their trees are hurting and even killing their trees for lack of knowledge. To much mulch around a tree could actually KILL your tree.

Soil is one thing, mulch is another. The difficulty is that both cause problems for trees when applied incorrectly and mulch, good organic mulch that is, becomes soil eventually. The deeper the mulch around the tree trunk the worse the situation for the tree.

Landscapers and builders/developers (sometimes the same but sometimes different companies or different people) cause damage to trees in the normal course of their respective business. Builders prepare the land for building structures and landscapers make the prepared land beautiful. After scraping the earth with heavy equipment the landscapers, and sometimes the builder/developer bring in soil to cover the land for various reasons. The soil level gets raised. Any pre-existing trees that were preserved have to cope with an unhealthy high soil level that actually comes as a mix of detriment and benefit. High soil levels hurt, but new rich soil helps (as long as it does not get to high on the existing tree trunk.

Sometimes homeowners feel it is a good idea to make rings around the trunks of trees by removing the grass and then covering the bare soil with mulch. Sometimes the soil around the trunk will be cultivated, augmented and raised. Then flowers will be planted there. This practice is not good for the trees.

Steve Elwell

How Mulch Can Damage Your Plants

Mulch applied too thickly around the base of a tree will eventually kill the tree.  The roots of all plants, including large trees, require oxygen, moisture, and nutrients.  If mulch is applied too thickly, these required items cannot reach the roots, causing starvation and death of the plant. This theory also applies to ornamental rocks and anything else piled around trees to minimize the growth of weeds and grass.  

How to Correctly Use Mulch

While it is beneficial for keeping moisture in and keeping weed levels down, it must be applied properly.  Applied incorrectly, too thick, or too close to plants, it can cause rot,  mildew/mold, and eventually the demise of your perennials, shrubs, and even large trees.

Never create a pile right up to the base or stalk/stem of your plants.  When applying to your gardens you should leave a space of at least 1 inch (so the soil shows) between the base or stalk/stem/trunk of your plant or tree and the mulch. After heavy rain and in the spring after the snow has moved the mulch, you should reestablish this space as soon as possible, especially for young plants that will rot quickly if the mulch is left too close to the stems.

Mulch applied in a thin layer (one inch thick is plenty) around a tree can be beneficial to keep weeds and grass roots from competing with the tree roots for oxygen, moisture, and nutrients. Just remember, more is definitely NOT better!

These pictures show mulch piled much too thickly around the base of a mature tree (incorrect, left), and removed from the base and spread out (correct, right):


Reducing Weeds

Mulch will keep weed levels down, but it will not eliminate weeds altogether as many people are led to believe.  Weed seeds blow in the wind and will settle in the mulch and germinate there too.  The difference is, when you pull out a weed growing in mulch, it comes out much easier and more completely, with the root intact. This is important because if you do not remove the entire root of a weed it simply grows back, often very quickly.

This is another rant, based on a pet peeve of mine.  I advise gardening clients to use mulch to keep their gardens from drying out and to help reduce weeds.  The problem is, some products out there are full of weed seeds, so when I go back to check out gardens two weeks after planting them I see more weeds than were there before I planted them!  A dead giveaway is that each use of mulch seems to have its own species of weed, today’s was horsetail weed mixed with a coarse grass:

Alternatives Around Trees

Nothing, including grass, should be planted around a new tree for the first five years, allowing the tree roots to get established. After that, shallow-rooted perennials or annuals work best as they do not force the tree roots to compete for required elements.  A few examples of shallow-rooted perennials are geraniums, sweet woodruff, and lamium. (pictured below)  My personal favourite is the perennial geranium (very different than the annual geraniums that I am not so fond of) because it tolerates almost full shade to almost full sun, and is the first plant to green up in the spring.  Perennial geranium flowers can be white, pink, blue, purple, and many shades in between, but are almost inconspicuous in many of the varieties; the foliage is the main attraction to me:

If you do decide to mulch your garden or use mulch under your trees, please use caution and apply it correctly so you do not do more damage than good…

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