Insects in Mulch

Mulch has many plant benefits when used properly

By: Sandy Feather 2009
Penn State Extension

Q. I have limited time to garden and always have weeds to deal with. Experts recommend mulching, but my area seems to have more than its share of insects, including ants. I really do not like the idea of putting wood chips around my house. Would it be OK to use dried grass clippings instead? Or would that attract insects as well? I've also heard of using layers of wet newspaper to block weeds, but that would be ugly without decorative mulch on top on of it.

A. There are a number of ways to approach mulching while protecting your home from unwanted guests. It is true that mulch -- any kind of mulch -- helps maintain soil moisture, which creates a favorable environment for insects such as termites and ants to build nests. This is true even with inorganic mulches such as river rock or pea gravel. Termites and ants do not use wood mulch as a primary food source -- it lacks sufficient nutritional value to sustain them. They prefer to nest in the moist soil under it.


Mulch around the perimeter of a house

Properly used mulch should not pose much risk at all; most problems occur when too much mulch is piled against the foundation. This is not healthy for the plants growing in those mulch beds, either. The benefits of mulch are so great that you do not want to forgo mulching; you just need to use mulch properly.

Photo of dyed brown mulch made from shredded bark
Dyed mulch has gained favor in
recent years since it holds
its color longer

Two or three inches of mulch is sufficient to maintain soil moisture (for the plants), moderate soil temperature and help keep weeds down. Organic mulches such as wood chips and grass clippings break down and add organic and nutrients to the soil, contributing directly to plant health and vitality. Mulch reduces erosion and runoff by protecting the soil surface. It also reduces soil compaction, which allows rainwater to infiltrate the soil easily, further reducing runoff.

Where Mulch Problems Begin

Mulch becomes a problem when we use too much of it because the underlying soil never gets a chance to dry out, which increases its attractiveness to insects looking for a place to call home. Homeowners who add a few inches of fresh mulch every year so that it looks nice can easily wind up with excessively thick layers of mulch around their plants and near their houses. Rake out existing mulch in the spring and only add fresh mulch where there is less than 2 inches. Never allow mulch to touch building foundations.

Grass Clippings for Mulch?

Grass clippings can be used as a mulch after they have been composted. Fresh grass clippings tend to ferment, releasing heat and ammonia that can damage nearby plants. Fresh grass clippings are high in nitrogen ("green" in composting parlance). It is best to mix them with shredded leaves or straw, materials that are low in nitrogen and higher in carbon ("brown" in composting parlance). Compost happens most efficiently with a brown-to-green ratio of 30:1

You may be able to approximate a good mix of leaves and grass clippings by using a bagging lawnmower for fall leaf cleanup. Allow the leaves and grass clippings to compost over the winter. Turning the pile every few weeks and making sure it is adequately moist (think wrung-out sponge) will speed decomposition. Compost will still happen if you do not actively work at it, but it will take longer.

! Caution !

Avoid using herbicide-treated clippings for mulch -- they may contain herbicide residues that can damage desirable plants.

Photo below: Norway Spruce damaged from being mulched with grass clippings containing the lawn herbicide 2,4-D. The 'twisting' new growth was the revealing clue.
Bending and twisting new growth is a sign of damage from 2,4-D herbicide that is commonly used to kill dandelions and other broadleaf weeds in lawns


Landscape mulches

Mulch problems

Stone mulches


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