Mulching provides insulation
As Canadian garden writer Lorraine Flanigan explains, "mulch acts like a deep blanket of snow to keep the cold in the ground, moderating fluctuating temperatures so that plants remain safely dormant all winter long. This protects roots from freezing during sudden cold snaps and thawing during mild spells, a cycle that can heave plants from the ground, damage roots and place plants under stress." Flanigan is referring to the freeze-thaw cycle, and it's a killer. More often than not, when gardeners report losing plants over the winter, it's because we had little snow cover and they did not protect plants with leaves.
Snowfall can be unreliable
We can't always count on sufficient snowfall to provide necessary insulation. Without snow cover, plants are "exposed to weather extremes," says Flanigan. I think climate change has made mulching even more necessary in the northern garden as we see mild days well into the late fall and bone-chilling temperatures in deep winter.
Most leaves make good mulch
Small is best, though. The small leaves of birch or crabapple "allow rainfall and natural moisture to percolate through to the soil surface, ready for plants to absorb in the spring," says Flanigan. I prefer Manitoba or silver maple leaves as they are thin and deeply lobed. They decompose quickly. In the spring, they can be worked in to improve soil texture.
The large leathery leaves of Norway maples tend to stick together, forming a dense mat. In the spring, they can hold moisture against awakening perennials, causing them to rot if not removed quickly. The remedy is to run over large leaves with a lawn mower or put them through a shredder. If some grass cuttings mix in with the chopped leaves, that's alright. Keep the mulch as dry as possible if you are storing them until it's time to mulch.
Timing is everything
Once again, Lorraine Flanigan says it best. "Because the goal is to hold the cold in the soil over the winter, mulch should be applied as soon as the ground freezes. Earlier than this and rodents, such as mice and voles, will bed down in this cozy winter home and munch on succulent plant roots." Some gardeners recommend mulching once the top inch of soil has frozen. You can mulch over a dusting of snow.
Use enough mulch
A layer of 2-5 inches (6-12 cm) will keep plants protected. I used twice that amount on my perennials beds last year because they were newly transplanted and all my plants survived, with the exception of two heuchera which are marginally hardy in Thunder Bay. So I'm a believer! TO keep leaves from blowing, evergreen boughs will help to hold things in place as will the cut stalks of taller perennials.
Compost leaves in spring
Once the ground warms again in spring, remove the leaf mulch and save it for compost. I know this seems like a lot of work, but it's all part of the gardening cycle.
- Fallen leaves carry 50 to 80 percent of the nutrients a tree extracts from the soil and air, including carbon, potassium, and phosphorus. – Sydney Eddison, Fine Gardening
- If leaves show signs of mold, black spot or rust, you should not use them as mulch and it is likely wise not to compost them either.
- Contrary to conventional thinking, oak leaves can be used as a mulch. "Although oak leaves have an acid pH (4.5 to 4.7) when they are fresh, the breakdown products are neutral to slightly alkaline." from Mulching Woody Ornamentals with Organic Materials, p.4