You might have heard the word mulch before, but what is mulching? Why would you bother to mulch? What kinds of mulch are there? Is mulching ever a bad thing?

The basic definition of mulching is “covering soil with a layer of material to provide benefits.” What benefits might be expected, pros and cons of some materials, and mulch recommendations vary, depending on where and when they will be used.

Some benefits of mulching include moderating soil temperature (keeping roots cool in the summer or keeping them from freezing in winter), keeping the soil moist, helping block weed growth, protecting plants from soil-borne diseases, protecting the soil from erosion and/or crusting, and — around trees especially — decreasing the likelihood of damage from mowing or trimming.

There are two basic categories of mulch: organic and inorganic. The use of the word “organic” in this instance simply means that the material was once alive and will break down into nutrients.

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Inorganic mulches can be things like rocks, plastic, or landscape fabric. These materials have the benefit of being a semi-permanent covering, but weeds can — and do — take hold, either from below or from seeds that land and germinate on top. If the material is dark in color it may raise the soil temperature. In the early spring this might be a good thing, but in the heat of summer, not so much. These mulching materials do not add any benefits back to the soil. They may also impede the plants’ ability to get adequate nutrients, water, and air.

Rock paths like this one, while making gardens more navigable, may impede plants’ ability to get adequate nutrients, water, and air. And weeds can, and will, crop up between the stones. (JoAnn Dobis / Special to the Tribune)
Rock paths like this one, while making gardens more navigable, may impede plants’ ability to get adequate nutrients, water, and air. And weeds can, and will, crop up between the stones. (JoAnn Dobis / Special to the Tribune)

Organic mulches such as grass clippings, compost, straw, cocoa hulls, shredded wood, and pine needles, were once living and will break down eventually, giving nutrients back to the soil. Their transient nature, along with a possible issue with nitrogen availability if they are worked into the soil, are drawbacks to organic mulches. On the plus side, weeds that do emerge are easier to pull, and soil structure and water retention are improved with the use of organic mulches.

Organic mulches are often recommended placed around plants, usually at a depth of a few inches, but be sure to keep any mulch away from the base of your plants. A variety of mulches can be used for a variety of uses, for example, shredded wood around trees and straw for a vegetable garden. Rock mulch is probably better used where you are not planting any plants, but you want to cover an area (under the eaves or making a path). Plastic can be helpful in the early spring to warm up the soil faster, and then you can remove it at planting.

When should you NOT mulch? Well, timing is important. Especially in the spring, you want to help (with plastic) or allow (don’t place other mulch) the soil to warm up before planting. Remove mulch in the fall to let the soil freeze, then replace it to keep roots insulated. Some plants do not like having their feet kept moist, so you would not mulch around them. You may have annuals that you want to go to seed, so not mulching there might also make sense. Leaving at least some soil bare helps ground-nesting pollinators.