We mulch our gardens to retain moisture, suppress weeds, nourish our soil and prevent extreme soil-temperature fluctuations. In the spring, the first two reasons are most important. In autumn, the last two — especially that bit about temperature changes — are what makes it worth the effort.
All the reasons listed above, and others not mentioned like preventing soil erosion, are in play year round. But the special demands of fall and winter, different than the growing season, guide how we mulch and what we use. Ground doesn’t just freeze solid three months of the year (well, maybe in some places). It’s in a constant cycle of freeze and thaw, something that particularly stresses even dormant plants.
Winter weather conditions that now seem so common — unusually warm winter days suddenly replaced by brutal cold snaps — are particularly hard on plant roots. They’re also hard on the microbial and other life that exists in healthy soil. Mulch helps moderate these extremes.
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Consider how mulch, working as an insulator, protects your plants from rapid swings in soil temperature. No matter how deeply you layer mulch around your plants, it probably won’t keep your soil from freezing. That’s especially true here in Montana, where despite the frigid temperatures we still manage to overwinter our fair share of perennials.
A layer of mulch insulation moderates those temperature swings, preventing soil’s quick freeze as well as its sudden thaw. A good layer of mulch, in certain years in certain climates, may keep ground from freezing at all. And where cold is guaranteed, mulch evens out the unpredictable cycle of freezing and thawing.
What type of mulch you use is dictated by what’s available. Just as in a good down jacket, you’re looking for something that will give you loft, material filled with air-trapping spaces. Straw makes for a good choice as long as it doesn’t carry too much weed seed. Things that compact, like leaves, not only provide less insulation, they’ll resist absorption and encourage run off.
Shredding leaves gives them more fluff, and helps them decompose more quickly back to the soil. Mixing shredded leaves and straw adds to the loft.
Wood by-products are also good for mulching in terms of insulating properties. They won’t easily compact, unless you’re using sawdust (bark chunks are a good choice). But don’t expect bark, shredded or otherwise, to break down quickly into soil. Pine needles, which don’t compact easily and do break down more quickly, are fine, especially under acid-loving plants like azaleas. But they must be piled deeply to provide much insulation.
Here’s a long list of various mulches, charted by their characteristics, from the University of Missouri Extension. Some, like pecan shells and corn cobs, will get you thinking out of the box.
More Tips for Fall Mulching
- Mulch deeply, but not too deeply. Yes, you want maximum insulation. But you also want excess moisture to evaporate. The deeper the mulch, the harder it is for evaporation to occur. Saturated soils encourage rot and disease.
- Adjust your mulch depth to your regional conditions. If you live in the Pacific Northwest with its abundant moisture and infrequent freezes, don’t mulch so deeply. If you live in the high, arid Mountain West, deeper mulch is important. Snow cover provides additional insulation.
- Choose a mulch that won’t blow away. This isn’t an easy task for some homeowners perched on hilltops or in wide and windy locations (see University of Missouri Extension charts above). Rice hulls, shredded leaves, even straw will go airborne when winter winds begin to howl. Anchor your mulch with chicken wire or the like if necessary.
- Don’t let mulch pile up around the trunks of trees or the base of shrubs. Mulch rubbing up against a trunk keeps it from drying out, an invitation to rot and disease.
- Keep beauty bark and other mulch away from the sides of you home and out buildings. You don’t want moisture continually working on your foundation.
- Leaf mulch is one thing, leaf mold is another. If your leaves have started to decompose, or you have a stack of last years leaves rotting in the corner of your yard that you think would make good mulch, think again. Leave mold will compact tightly, providing a skin between the air and the soil. Instead use leaf mold as a soil amendment, digging it into the soil before mulching.
- Mulch your lawn by mowing fallen leaves right back into the yard and leaving them. Mowing, in effect, shreds the leaves, giving them more chance to decompose over winter. Leaves can be a wonderful — and cheap — source of nitrogen that grass will need come spring.
- Consider the aesthetics of mulch. You’ll want a mulch that’s attractive and adds to the visual appeal in landscape features and around garden beds. On the other hand, the mulch covering your vegetable patch is more function than form; it doesn’t have to be so attractive.
- Consider living ground covers, instead of mulch, around your established trees and shrubs. They provide the same insulating qualities as beauty bark.
- It’s never too late to mulch. Even if you hadn’t put your garden to bed before the first frost, even if it’s January and you’ve seen plants popping out of the ground because of a thaw, even if temperatures have already plunged and are staying there, putting down the right kind of mulch can only help. Your plants will be the healthier for it come spring.