Fifty Degrees In February Not So Fabulous For Plants

Why is it so difficult to have everything? Fifty degrees in February is fabulous, but it makes perennials heave. Now, I’ve got to admit every time I hear a grower say their perennials “heaved,” I try to hide a shouldn’t-laugh-at-a-funeral type of smile. Even though the term is funny, this is serious business.

Remember in science class how we studied the expansion/contraction effects of freezing and thawing as a water-filled milk carton bulged with expansion as it froze, and then contracted and shrunk upon thawing? Soil does the same thing. When too-early extended periods of warm temperatures arrive in late winter with little insulating snow cover, soil freezes and thaws multiple times, causing continual expansion and contraction.

That’s not so bad on bare soil. In fact the freezing and thawing can help loosen heavy clay soil, making it more mellow and workable. But when this happens to perennial flowers, the alternating freezing and thawing tears plant roots as the soil expands and contracts. After roots are torn, the expansion can literally “heave” perennials right out of the ground, usually killing them. We experienced this several winters ago, which resulting in heavy casualties of even tough perennials like peonies, iris, daylilies and hosta.

Frost heaving a perennial

And it’s even worse if perennials and bulbs start to grow. In late February and March, there’s a 100% chance of killing frost.

What to do? If perennials are still under generous snowcover, they’ll usually be fine, if the snow persists through the melting temperatures. But if snow has disappeared, the bare, black soil is especially vulnerable to  warming and freezing. Anything that can be done to maintain the soil in frozen condition, so it doesn’t thaw, will help. The idea is to keep it frozen. If straw happens to be available, that works well. Last fall’s leaves or compost could be piled over perennials. Bags of shredded bark, wood chips or peatmoss might be on garden center shelves by now. Spreading layers of these material will help.

Insulating material added now can be loosened and removed in late March or early April. Come next May if perennials haven’t survived, we very well might look back at these warm February temperatures as the cause.

Here’s hoping our perennials don’t heave.