Keep The Ground Cold With Winter Mulch

We’re now nearly two weeks into winter so, for the optimists among us, the days are getting longer and spring is right around the corner. The question remains though, will this be a long ride to the corner or a short one? For now though, it doesn’t matter because we’ve already had two shots of frigid polar weather and we need to talk about an aspect of winter garden care.It’s about as cold as it’s going to get, though statistically I think February is our coldest and snowiest month. But we want to be ahead of the game and not behind it. One way of doing that is to get your winter mulches down. In this case though, the mulch serves a very different purpose than our rejuvenating mulches, weed mulches and moisture retaining mulches of the summer. Here our goal is to keep the ground cold, even frozen, for as long as possible.

It may be counterintuitive, but here’s the logic: Recently planted perennials and shallowly rooted shrubs may not have established deep anchoring root systems. Should we have a winter with little snow but plenty of cold, the ground will freeze at night then possibly thaw during the day as the sun hits it. This is particularly true of south facing gardens or plantings. This is where we get into the freeze/thaw cycle where the continuing freezing and thawing of the ground results in heaving or moving plants that aren’t well rooted. As a result they can dehydrate and die a slow death. To prevent this from happening, a winter mulch is added when the ground is very cold or even frozen. This mulch keeps the sun from hitting the soil and evens the cycle or eliminates it by stabilizing the upper soil temperature. As a result there is little to no heaving, and new or marginally hardy plants are able to survive those days or even weeks of 15-degree nights.

The practice is not without risk though. The first risk happens when you apply your winter mulch too early or when the ground isn’t cold enough. In this case the mulch results in keeping the ground a bit warmer and not stable as the air temperature cools and it keeps the soil cold and wet, which is not great for some plants. This is why we try to wait until the ground actually freezes or gets as cold as it will before mulching. You can also begin with light layers of mulch and increase the density or fluff as it gets colder. But this calls for extra work, and then what do you do if it snows and you’ve only got a light layer of mulch down? Not to worry, snow is the best mulch available. It keeps the soil just above freezing but cold enough to ensure dormancy while not resulting in rotting problems.

Problem two is a bit more vexing and it comes in the form of a small rodent called a vole. No, not a mole, a vole. They are very different and they eat very different things. When the ground freezes moles go deep into the soil because their source of food, worms and grubs, become impossible to find or they dig deeper into the soil. Voles, on the other hand, are herbivores. That is, they eat plants and plant parts like roots, crowns, tubers and bark. They are shy animals and they know that if they show their tasty little bodies in full view that some hungry fox or hawk will gladly pounce on them for a meal. I’ve seen foxes stalk them under a foot of snow just by listening for them and hawks swooping down from a hundred feet away when one is caught in their sharp view. So, voles are great at hiding.

A winter mulch is the perfect medium for a vole to hide under, as is snow. They are not true tunnelers in that they won’t tunnel through soil but they will burrow under leaf litter and create networks of connections under the snow so they can get to the plant crowns, near surface roots and the bark of certain trees and shrubs. If you have apple trees or quince and you leave leaf litter against the trunk at ground level, sooner or later you’ll find voles have girdled the bark and probably killed the tree. There are three solutions. First, have a cat that’s a good mouser. Second, don’t allow mulches within a foot of the trunk of trees or shrubs. If the voles can’t hide in the mulch, tall grasses or leaf litter they won’t take the chance on being exposed and eaten. The third approach is trapping. They can’t resist little bits of apple and the old-fashioned wooden mouse traps baited with apple do the trick.

Your winter mulches don’t need to be elaborate mounds and you won’t want to or need to mulch everything. Fall planted perennials and marginally hardy perennials are the plants that benefit most from this. While I lightly mulch my entire perennial garden, only newly planted lily bulbs and perennials get the full treatment as well as some plants that I know from experience need the protection. But, what do you use? First, use what you have. Remember we are looking for a light and fluffy mulch that will block the sun and offer a small amount of insulation. I bank maple leaves along with some ash leaves and try to keep them dry until late December, then when its good and cold, as in the 20s, apply a fluffy covering of no more than six inches and extend the cover a bit outside the planting area.

Oak leaves, on the other hand, should be avoided. They tend to mat down and as a flat layer they offer little protection, don’t easily degrade and can result in overly wet soil with poor air circulation. Wood chips are great for the summer mulch but not as a winter mulch. You can buy plastic bales of straw and hay that are seed free (Mulch Master is what I’ve found) and a 1- to 2-inch layer of this material also works great without leaving behind seeds of weeds and other plants.

The all-time best winter mulch is a favorite of many East Enders and it’s called salt hay. It has its own controversies and detractors though. Salt hay is harvested in the salt marshes of New Jersey. The harvesting is, in and of itself, an issue and it’s now tightly controlled by environmental laws. It is weed seedless though, it has a great fluff factor so a little goes a long way and it won’t break down and degrade during the winter. If and when it’s available, you can get it at local garden centers.

Remember, the purpose of these mulches is to keep the soil cool, not warm. All will attract voles so you need to monitor for them (trap) and keep your mulch away from tree and shrub trunks at the soil line. It can be a little tricky figuring out when to remove these mulches and a late frost can do lots of damage to tender emerging shoots, so I like to pull the mulch aside slowly and remove it in layers with a little left behind for that one last cold and frosty night when it can easily be raked or thrown back over tender plants. Keep growing.