The latest thinking on ecological landscapes. Useful tips to improve our environment

Kim Eierman

Kim Eierman

Founder of EcoBeneficial!

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To Mulch or Not to Mulch: Protecting Plants in Winter

As winter creeps up on us and the ground starts to freeze in many parts of the country, you may be wondering if you should use mulch to protect your plants.  You may know that I recommend leaving fallen leaves in place to act as nature’s mulch and compost.  Ditto for any fallen plant debris, assuming that those plants have been healthy (always remove diseased plant material to prevent re-infection).  If you are not fortunate enough to have lots of leaves and plant debris, think about mulching your plant beds, and areas around trees, with a high quality, aged, organic mulch.

Let’s make one thing clear – mulch is a functional, temporary material, not a garden ornament!  Let’s do away with mulch volcanoes around tree trunks, and layer upon layer of impenetrable mulch, applied every season whether plant beds need it or not!

Good Reasons to Use Organic Mulch

Here are good reasons to use mulch:

–       To retain moisture in the soil underneath.  Especially useful in periods of drought.

–       To suppress weeds, and ease their removal.

–       To provide organic matter, adding nutrients to the soil, as organic mulch decomposes.

–       To protect expanses of bare soil before planting, if planting must be delayed.

–       To moderate soil temperature and buffer plants from the freeze/thaw cycles in winter, which can uproot plants.

Mulch as a Functional, But Temporary Placeholder

Unfortunately, mulch is often utilized in place of plants.  Think about the many garden beds you have seen where there is far more mulch than there are plants, where no plant touches another, and huge swaths of mulch prevail.  The creators of these gardens have missed the point entirely!

Plants naturally grow together, actually touching, in a matrix of vegetation both above and below ground.  Think of mulch as a placeholder while those plants “get to know each other” and grow closer.  We do not yet fully understand how plants interact with each other, but no doubt that they do.  Your goal is see a network of plants, not a sea of mulch.

Mulching is also a great way to temporarily protect bare soil.  A large area of naked soil is an invitation for weeds, soil erosion, compaction and soil infertility.  Perhaps you have not gotten around to planting an area this fall and you must wait until spring – that is a good reason to mulch to protect bare soil.  Keep in mind that it is always a good idea to leave some patches of bare soil for ground-nesting insects, including the majority of our native bee species.

In cold winter climates the soil temperature below ground can be significantly higher than it is above ground.  When the weather warms up briefly and then cools again, the ground can heave, and sometimes actually push smaller plants up and out of the ground.  This is a phenomenon called frost heaving.  Nature’s mulch (leaves and plant debris) or purchased mulch can help to moderate this.

What Type of Mulch?

What type of mulch should you use?  The best practice is to emulate nature, using materials that already exist in your environment.  This may mean using fallen leaves, pine needles, or bark mulch from trees grow in your ecosystem, etc.  Whatever mulch you use, it should be organic and degradable, providing nutrients to the soil as it breaks down.

What mulch shouldn’t you use?  Anything that is inorganic or not found in nature, including:  plastic sheeting, rubber pellets, artificially dyed mulch, etc.   I am not a fan of crushed pebbles or stone as mulch, except where these materials would be naturally found around plants.  Stone and gravel reflect the sun, heat up the soil underneath, and retain that heat for long periods of time.  This can cook plant roots which stresses plants and can potentially kill them.  Plants that have evolved in desert conditions are another matter.

Another mulch I do not recommend is fresh wood chips, which are thought to tie up nitrogen in the soil, making it less available to plants.  There is some controversy about fresh wood chips, but I suggest that you age wood chips for a period of time before using them in your garden beds.  Aged wood chips break down more quickly, adding nutrients to your soil more quickly.  One way to buffer younger wood chips is to mix them with compost and spread them together.  Remember, you are trying to add nutrients to your soil, not deplete them.

How Much Mulch and When?

How much mulch should you apply?  Well, that depends on the material, but a general rule of thumb is to apply no more than 2 inches of total depth with purchased mulch; you can go deeper with fallen leaves.  You can be a bit more generous around trees, and less generous around smaller plants.  Make sure to keep mulch, particularly bark mulch, a few inches away from the trunks of woody plants.

Only reapply mulch when it is needed.  I have seen far too many landscapes where mulch is applied every season like clockwork and plants are being swallowed alive in a thick, impenetrable crust of mulch.  I often see this around the base of trees  – where the mulch has been piled so high that no tree root flare is visible (that gentle spreading of the trunk at a tree’s base).  These overly mulched trees look like telephone poles plunged into the ground – very detrimental to tree health.  Too much mulch can be far worse than too little.

When should you apply mulch?  That’s easy – at planting time, or, when the soil is bare.  Some people wait until the ground is frozen to apply mulch in the fall or winter.  For spots that need mulch, you can keep mulch in place year round for moisture retention and moderation of soil temperature, until your native plants spread and mulch is not needed.

Vegetate with Native Plants

Remember – mulch is just a placeholder.  The end goal is to vegetate the soil with native plants.  A diversity and abundance of native plants contributes to healthy soil, which in turn, keeps plants healthy and captures more carbon.  Think green mulch!


From Kim Eierman at EcoBeneficial!


Photo: Tree in Winter
Photo credit: Flickr/Patrick McConahay


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