I have seen in a number of places and not just regarding lilies but they always say to put mulch down after the ground is frozen. Why is that? I don't see why it would matter when it goes down as long as it's there but I hope someone can enlighten me.

The two main reasons I wait until the ground freezes enough to support me @ 185# are: 1. Keep the cold temperature more constant to avoid freeze-thaw cycles. This also keeps the cold in longer in the Spring which helps prevent the new stems from emerging too early (helps prevent frost bitten noses). And # 2. It helps keep rodents like voles out; otherwise, they have easy digging for their favorite winter food.

1 - (same as Lorn) - Since the phase change of water (liquid to solid or solid to liquid) requires an enormous transfer of energy compared to a simple heating or cooling, the benefit of mulching frozen soil is quite substantial in regard to regulating temperature and preventing freeze/thaw cycles.

2 - (same as Lorn) - To delay spring growth and minimize late frost damage, by keeping as much cold in the soil for as long as possible.

3 - To minimize winter wet. Alpine species and most Lilium species are susceptible to cold, wet, unfrozen soils that can cause rot. A soil can be saturated with water, but if the water is frozen, then a plant reacts as if the soil is dry. The water is tied up in crystals and is not available to chemically or biologically interact. I depend on deeply frozen soil here, since my soils are clay based.

Joe, I am in the deep south zone 8a. It will be a long time before the ground freezes here. I have already put a thin coat of mulch down because it helps keep the soil a little cooler (we have very hot summers). I am going to add more in a week or so.

Interesting how mulch helps in such varied conditions. We use it for cold and freeze, and Sally's Mom uses it to keep the ground a little cooler. Neat.

In soils that are prone to frost heaving like the pacific northwest where it rains a lot but hardly ever freezes, (when it does the wet soil heaves) it is more beneficial to mulch before the soil freezes. It is better if it can be done before the soil turns to mud too, makes it easier to get around!