Miss Floribunda: Cold snap recovery

Frozen dewdrops on Crocus flowers

Dear Miss Floribunda,

 

The cold snap in March that followed the unusually warm February totally messed up my garden. The huge pink flowers on my tulip magnolia now look like exploded brown footballs. My crocuses croaked. My willow tree snapped off six feet from the base. The forsythia looks like burned scrambled eggs. I went ahead and pruned my roses because the forsythia came out, but all the young leaves were frozen. How bad is the damage? What can be salvaged?
Nipped in the Bud on Nicholson Street

Dear Nipped,

The bad news concerns your willow tree. You’d best just dig it up and replace it. Are you sure it was healthy to start with and that it was in a good spot? A healthy tree would not have been killed by March’s cold weather. Now for the good news. Your tulip magnolia, as well as the various early blooming fruit trees throughout Hyattsville whose blossoms were blasted, is alive and will bloom again next year. If you scrape a branch, you will find that the wood beneath the bark is green. Your bulbs and forsythia are probably already reviving. With the return of warm weather, new buds on the forsythia have formed and bloomed, or else green leaves have pushed out. Even your crocuses may have opened up again. They are certainly not dead. At worst, they do what they always do when they finish blooming. They put their energy back into their underground bulbs and wait for next year to bloom again. Your daffodils, hyacinths and tulips should bloom on time. However, because your tulip magnolia probably had no more buds to open, you will have to wait for next year to enjoy its beauty.
If you optimistically planted pansies or other plants before the cold snap, they might have suffered frost heave. Check to make sure their roots are covered by soil. Tops of pansies, for example, can survive a certain amount of cold, but roots will soon die when exposed to cold, dry air. Tuck them back into the soil and water them well.

Now for your roses. Prune them again. The sight of green wood should reassure you, and pruning them sends a signal to put out new leaves. Although the last frost in this area can be as late as April 20, late frost appears not to affect anything but tender new seedlings and the ground temperature. Because I realized I didn’t understand why, I turned to my friend Mr. Meriweather, who is a hands-on gardener, as well as a meteorologist. He explained that there are two kinds of freezes: advective and radiative. This year, after unusual warmth before March, we suffered an advective freeze — a cold polar outbreak with high winds. Of course, wind increases cold and desiccation. (The nice, moist blanket of snow we had probably helped, rather than hurt, the plants.) Advective freeze is dangerous because it damages plant tissue itself. Should such a freeze after a prolonged warmed spell re-occur in the future, Mr. Meriweather advises us to cover shrubs and beds with tarps, cloth blankets or opaque plastic. He warned against clear plastic, which can heat up during the day and cook plant tissue. Abnormally high heat kills just as efficiently as a freeze does.

Now in our area, serious advective freezes rarely occur in April, but we can still get what Mr. Merriwether told me are radiative freezes. They can strike even after a very warm afternoon if humidity is low and there is no wind. Leaf surfaces exposed to the night sky can give off more infrared radiation than they receive back, and this causes them to cool the air in contact with them. The frost crystals that form on the leaves do not damage them. However, a thin layer of much colder air forms close to the ground and drains like a liquid into low spots and hollows. To protect your plants’ roots, remove mulch after March 20, and keep them well watered in the absence of April showers for a warm day or two. Although this won’t prevent warm, dry air from causing a radiative freeze, the moisture in the bare, mulch-free soil conducts heat to the plant. Mulch would prevent that heat from radiating out. Mr. Meriweather has observed that new tomato plants mulched with leaves have died during late freezes while unmulched ones have survived. Of course, it doesn’t hurt to put a protective cover on plants if you notice the temperature suddenly dropping.

To learn more and to participate in a plant exchange, don’t miss the next meeting of the Hyattsville Horticultural Society at 10 a.m. on Saturday, April 15. It will take place at the home of Joe Buriel and Dave Roeder, 3909 Longfellow Street.