By Larry Hodgson
Question: Last week, we had a late frost and several perennials in my garden were damaged. This is the first time that there has been so many. My beautiful big knotweed is now all brown and withered. I’m afraid its flowers may have frozen too. What should I do?
Answer: Most perennials are quite cold hardy and brush off a light frost without any damage whatsoever, but sometimes, especially when frost hits just when they’re in full growth, the cells that make up the new stems, leaves and flower buds, since they are still very young, have not yet developed the full cold resistance mature cells would have offered. As a result, the new growth freezes: sometimes completely, sometimes only at the tips of the stems or around the edges of the leaves, leaving soft, droopy, often mushy growth … that eventually dries up and turns brown.
The risk of late frost is greatest when spring arrives unusually early, because then, to anthropomorphize a bit, the plants “wake up too soon,” reacting to the unseasonably warm weather as if summer had arrived. However, Mother Nature then tends to like to remind us that she is in charge and send us at least a few days of frigid weather just before summer really does set in. (There is much less damage when the spring has been on the cold side, because then perennials are much slower to start producing new growth … and a still-dormant or near-dormant perennial is not going to be much bothered by a late frost.)
What to Do?
The only possible way of fixing frost damage on a perennial is to prune out the dead parts. Sometimes it’s only the leaves and buds at the tip of the plant that freeze, so a light trim will suffice, but in other cases, the damage is more serious and the plant has to be cut back severely, sometimes almost to the ground.
Fortunately, Mother Nature provided perennials with a backup plan: over the millions of years of their evolution, they “learned” to produce extra buds from which they can regrow. Therefore, even if heavy pruning has left the plant with clipped off stems or truncated leaves that are very noticeable at first, the plant will produce new stems and leaves fairly quickly. Most of the time (although each plant has its own particular way of recovering), new growth will hide most of the damage.
In fact, even if you don’t remove the frozen bits, the new growth still often hides the damage. So, if you’re able to tolerate the appearance of a forest of mushy stems or brown leaves for a few weeks, you can simply react to frost damage by doing nothing at all!
Do be aware, though, that secondary growth is rarely as extensive, tall or dense as usual, so the plant will not probably look as good as it does most years. Also, if the plant was in bud at the time of frost, it is very likely that it won’t bloom that year.
On the other hand, a perennial is a perennial—a plant that comes back annually—and will normally grow again the following year and in perfect condition at that, with its customary flowering. So, frost damage to perennials is mostly a very temporary problem.
However, let’s hope Mother Nature doesn’t get into the habit of supplying our plants with an early spring followed by a late frost every year!