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Great Gardening by Sally Cunningham: Frost, freeze warnings

We live in a world of news-by-soundbite, and often there’s no time for in-depth thinking about what we hear - including about the weather.

Yet for farmers, gardeners, garden centers and the landscape community there’s nothing more important than weather, especially when it comes to frosts and freezes. What does it mean when the meteorologist mentions a frost warning or freeze watch in your neighborhood?

Let’s consider the technical definitions of some seasonal weather terms, and the practical implications for you.

Elusive, tricky frost

During a couple of days this week we gardeners and garden center pros ran around our backyards and nurseries somewhat frantically, because we heard about a frost warning. On Monday night we did have frost in some parts of WNY, including at my house in East Aurora, so our steps and efforts weren’t wasted. But how do we know what’s serious and what these warnings mean?

All frosts aren’t equal. There are light frosts, heavy frosts and killing frosts. Sometimes the timing and suddenness of a frost is more important than the degree of the frost. Plants (as well as insects and many other animal species) adapt to cold weather at different rates and in different ways as changes occur in their cell walls. Some plants handle quick changes better than others — but all plants do best when changes occur slowly.

A frost advisory is reported when minimum temperatures are expected to be 33 to 36 degrees on clear nights during the growing season. In fall, these advisories are issued until there is a widespread freeze — when the growing season is considered done. In spring, frost advisories appear when the growing season has started and the frost could damage new crops or emerging plants.

Now let’s get more specific: In WNY in general the frost-free growing season is supposedly from about May 6 through Oct. 5, according to NCDC data, but that varies according to where you live and the weather pattern of a particular year. Certainly in the southern half of WNY—the so-called Southtowns—gardeners and growers remain fully prepared for frosts all through May, often until the Memorial Day weekend. Also they get ready for frost much sooner than their Buffalo friends.

Both first and last frost dates are important. For the greater area, the average first frost date is about Oct. 19 and the average last frost date is about April 24, but it is so important to remember the meaning of average: The possible dates of the first and last frosts or freezes span several weeks.

For example, the NCDC data for the City of Buffalo shows that in fall the likelihood of a freeze on Oct. 5 is 10 percent; on Oct. 19 it is 50 percent, and on Nov. 1 it is 90 percent. In contrast, in Colden there is a 10 percent likelihood of a freeze by Sept. 15, and it’s 90 percent likely by Oct. 11. That’s about three weeks longer a Buffalo gardener can keep some plants growing. Again, it all depends on your specific location and the micro-climate around your home.

What do you do when you hear about a frost advisory? It depends on what is important to you, and if you think some tender plants are worth keeping for a few days or weeks after the first frost. The frost warning earlier this week was a prime example. I covered my raspberries with a large plastic tarp (or a sheet would have worked) because they taste awful after they have frosted.

Two days later I’m eating them — yum. (If I owned any basil I would have covered it or taken it inside because it is the first to succumb to frost; it turns black. I left the tomatoes exposed because I’ve had enough of them. I had already moved almost all the tropical plants or tender annuals into the house, but I know that some of them — the Norfolk Island Pine — don’t mind the first few frosts so they can wait.

Freeze watches, freeze warnings

Meteorologists use the words watch and warning with specific intentions: Think of a watch as a possibility. A freeze watch means widespread freezing temperatures could occur during the next 24 to 36 hours. A warning refers to a high probability or likelihood. When you hear of a freeze warning you can expect significant widespread freezing temperatures, below 32 degrees.

As for a killing frost, killing freeze, or hard freeze, you’ll hear many definitions, often depending upon whether a farmer, gardener, or casual observer is speaking. Many people call it a hard or killing frost when the grass feels crisp and crunchy underfoot, when seasonal vegetation dies, and ice forms on puddles. Some call a hard freeze any period of four hours or more when the temperature is lower than 28 degrees F.

Do these ominous freeze watches and warnings mean the entire garden is finished? Not entirely. A freeze will end the growing season for most plants, certainly if they are tropical or considered tender (not hardy to Zone 5 or so). Warm-season vegetables such as tomatoes, beans, eggplant, cucumbers, and squash will usually die after the first freeze.

However cold-season or cool-season vegetables will not die, although some foliage may discolor or wither (sometimes called burning). These cold-hardy crops include cole crops (broccoli, cabbage, cauliflower), root crops (turnips, radishes, carrots) and many green vegetables (Swiss chard, mustard greens, collards, parsley, spinach). Brussels sprouts only get better after freezing temperatures; I’ve served them well into November and December.

Most gardeners that I know admit to a certain amount of sadness when the season is winding down. Less sunlight and cold days are hard to face. Still, there is great satisfaction in knowing you have done the best job possible storing and saving the plants you can, extending the season for crops worth keeping, and then finishing your list of gardening chores.

Pay attention to the frost and freeze warnings to come; they are important clues to what to do and when to do it for our plants and for ourselves.

Sally Cunningham is a garden writer, lecturer and consultant.

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