As Sandra Mason of the Illinois State Master Gardener Program, explains, "Sometimes it isn't really the cold temperatures that cause problems, but the fluctuations, especially rapid fluctuations between warmth and cold." This fluctuation causes garden soil to thaw and freeze, to expand and contract. The effect is called soil heaving. As the soil heaves, shallow-rooted plants can lift out of the soil. Roots and plant crowns can actually end up above soil level, exposed to cold temperatures and drying winds. If there is little snow cover, plants can be severely damaged or lost.
Have you noticed how hosta leaves slide into a soft brown skirt that gracefully covers the plant's crown? This is nature's leaf mulch. I top up this covering with more small or deeply-lobed leaves. Manitoba maple leaves – which are abundant in my neighbourhood – are ideal.
When should you mulch?
It's best to mulch after the soil freezes. The goal is to keep the soil frozen longer, allowing plants to slumber on for the whole winter with their roots frozen and resting, even during thawing periods. I always wait for a hard freeze before I mulch, but weather conditions dictate when this fall chore is best done. It's not much fun to be outside in the freezing rain mulching garden beds. If light snow has fallen, you can still mulch. Work leaves around the plants and in wind-swept areas, anchor them with spent plant stalks or evergreen boughs. When the snow comes – or even a crisping frost – everything will be held in place.
About getting leaves ...
I live in a treed neighbourhood. Leaves are everywhere. This year, as my neighbours raked, I asked them to save their leaves for me. 'No need to bag them,' I said. 'I want them for my gardens.' One brought me a huge tarp full. Another leaned a mattress bag full of beautiful dry leaves by my front steps. Marvelous.
Come spring, the leaf mulch will find its way, a little at a time, into my composter. With luck, they will also go into a bin for fine shredding with a weed-whacker, only to return as a finer, weed-blocking mulch for summer. Yes, leaves are definitely my secret weapon.
See Sandra Mason's full article: How plants are affected by cold and winter and how to protect them