By Cyndy Crist
What Is Mulching
I prefer to use natural mulches, such as straw, marsh hay, shredded bark, grass clippings, or leaves, for several reasons. One is that they will break down over time and help enrich the soil. Another is that some (notably grass clippings and leaves) are available to me at no cost. One word of caution, though – if you want to use leaves, be aware of their characteristics and shred them first if they’re not a kind that curls up and crumbles as they dry. In my garden, I use the fallen leaves of our neighbor’s silver maple because they form a layer that remains relatively light and loose even after being covered by heavy snow. By contrast, I avoid using the leaves from our boulevard maple (I think it’s a Columnar Norway Maple) because they are large, thick, and remain flat after falling. As a result, they form a dense cover that keeps the ground wet and frozen late into the spring.
Stones, plastic sheeting, newspaper, and landscape fabric can also be used as mulch, but each has potential drawbacks. For example, plastic doesn’t allow anything under it to breathe and can provide a safe haven for slugs. Stones may hold more heat than you’ll want in the height of summer and can be difficult to remove should you decide to change your beds. Newspapers will break down over time but will need a layer of something with more weight on top of them to keep them from blowing away, as will landscape fabric.
Purposes for Mulching
Winter protection isn’t the only reason to mulch. Throughout the year, mulch helps maintain moisture in the soil, reduces weed growth, moderates soil temperatures, and decreases soil compaction. It may also reduce the spread of soil-borne diseases and, as noted earlier, natural mulches will break down over time and improve the soil. In addition, mulch can enhance the appearance of a garden by keeping it looking tidy and providing a contrasting color and texture to the garden beds.
But as the growing season draws to an end, wise gardeners prepare to add a cover of mulch to help plants survive the cold months ahead. As our climate is changing and many of us are experiencing earlier springs and longer growing seasons, some of us are giving into “zone envy” and planting perennials not generally considered hardy where we live and garden. Mulching these tender perennials can increase the likelihood that they will make it through the winter. Mulch can also provide a critical layer of protection for things like newly planted garlic or recently planted perennials that, while hardy, may need a little help getting through their first winter.
When to Mulch
I think one of the things most misunderstood about winter mulching is when to put it down. Many people think it is needed to protect the ground from freezing. However, at least in a northern climate, no amount of mulch will prevent freezing. What it will do is help the soil cool gradually in the winter and warm gradually in the spring and thus prevent the heaving up of plants that can result from temperature fluctuations that create alternating periods of freezing and thawing. Mulch can also help reduce how deeply frost extends into the soil, thus protecting the deepest roots of trees, shrubs, and perennials. In practical terms, this means that it is best to mulch garden beds for winter after the soil has begun to freeze (in most of Minnesota, that’s likely sometime in November).
Another reason that I suspect gardeners are tempted to put down mulch earlier than necessary is out of a fear that snow will cover beds before they have been mulched. According to the experts, however, it is perfectly fine to layer mulch on top of snow if necessary. Yes, it may look a little odd, but it will still provide the protection your garden may eventually need if early snows melt or become compacted. If we could rely on getting and keeping a thick, steady layer of snow through the winter, we wouldn’t even need to worry about mulching our gardens. In fact, I recall the owner of a Michigan nursery that I visited many years ago referring to snow as “poor man’s mulch.” But since nature isn’t that reliable, we are wise to give our gardens a helping hand.
Laying Down Winter Mulch
While mulch used during the growing season is placed at the base of plants, trees, and shrubs, winter mulch should cover beds to a depth of 2-5 inches. For this reason, straw, hay, and leaves are considered the best choices for winter mulching. Per earlier comments, not all leaves are created equal here, so choose carefully if you decide to mulch with leaves. Straw and hay are good choices because they “stick together” better than leaves and thus resist blowing around. I also find them a little easier to remove in the spring. Some gardeners find marsh hay to be preferable to straw because it has far fewer, if any, seeds that can sprout in the spring, but either provides an effective winter cover.
Because your mulch will be covering the garden, there are several things you’ll want to do before applying it. First, remove any diseased leaves or decayed fruits or vegetables that you may find; this will eliminate or reduce the chances of fungal or other problems next year. Next, I like to cut some plants back nearly to the ground and leave others standing to help hold the mulch in place and add a little winter interest in the garden. Those I cut back are usually ones that become slimy and messy by spring (like the leaves of day lilies) or that show signs of late-season disease (such as peonies and phlox with powdery mildew), while I leave standing plants with interesting seed heads and sturdy stems that can resist snow and wind (examples in my garden are Autumn Joy sedum and Baptisia). Finally, once your beds are ready to be covered and if the ground isn’t yet completely frozen, it’s a good idea to give your garden a deep watering to help carry it through the winter.
A Final Word of Advice
In the spring, don’t forget to remove winter mulch once soil temperatures have begun to warm and plants are showing evidence of active growth (usually April in Minnesota). This will help prevent the growth of molds by allowing air to circulate freely around emerging plants. In addition, I sometimes find a spot or two where especially thick mulch and deeper shade have kept the ground frozen, so uncovering that area will allow the soil to thaw, soften, and support plant growth.
Clearly, none of this is rocket science. Mulching is an easy step to take to protect your garden until it’s ready to come back to life next spring. If they could talk, I’m sure your plants would thank you.