In our part of the world cold nights can happen in just about any month of the year. As we move from the warm, sunny days of summer towards winters chill, the National Weather Service will often issue a Frost Warning, Freeze Watch, or Freeze Warning when the temperatures begin to dip.
Although in our area the average first frost date for temperatures that fall below 32.6 degrees is September 30th and the average first freeze date for temperatures to fall below 28.6 is October 15th chances are you’ve already seen either some frost or freezing in your part of our world.
Generally these alerts are only issued during the growing season, but sometimes there is no frost or freeze warning issued so gardeners need to be aware of night time temperatures throughout the growing season, especially if you live in the higher elevations, and there is a difference between these alerts.
Frost is caused by radiational cooling which occurs when the earth loses enough heat so that nighttime temperatures drop below the freezing point at ground level. This commonly happens when air temperatures cool off and the ground gives off the heat it has held. The clearer the night sky, the more heat is released and the colder it will become. If enough heat is given off and the cool air settles in, frost occurs. It is possible for frost to hover above ground level though and sometimes only the tops of your plants will be nipped by this light frost. Frosts are usually short term events that occur overnight or many times during the early morning hours.
The National Weather Service will issue a Frost Advisory when the minimum temperatures are forecast to be between 33 to 36 degrees. In the fall, a Frost Advisory is normally issued when nighttime temperatures begin to drop and up until the first widespread freeze which will normally mark the end of the growing season. In the spring, you’ll see Frost Advisories issued at the beginning of the growing season until the last frost date for your area has passed.
On the other hand, freezes are caused by advective cooling. This is when a mass of cold air from somewhere else moves into the area and settles. The weather folks often talk about an Arctic air mass known as an Alberta Clipper which is a good example of advective cooling. This freezing air generally means the end of the growing season for any tender plants.
Freezes can be long or short but even a short freeze can mean death for the plants in your garden. If you see a Freeze Watch out for your area it means that there is a potential for a significant, widespread freeze within the next 24 hours, but sometimes it doesn’t happen in your yard which is why it’s worth a try to cover your plants. Here are a few tips to help protect your garden through Frost Advisories and Freeze Watches.
The key here is protection. Just as blanket will keep you warm, it will also keep your plants warm. Cover them with row cover, frost guard, a sheet, newspaper, or bucket. Make a tent using poles or some other kind of support and drape a blanket, sheet or even newspaper over it to keep the frost off tender plants. Avoid using any kind of plastic coverings. Plastic attracts the cold and may actually do more harm than good.
Water can be your best friend when you are trying to protect against frost. If you have access to water and a timer, set it to come on between 3 am and 4 am. This is usually the coldest time of night. Let it run until the sun is up. If plants do show some signs of frost damage, wait a few days in order to give them time to recover before pulling them up.
The more severe Freeze Warning means that significant, widespread freezing temperatures are expected with in the next 24 hours and this will usually mean the end of your garden. Unfortunately there isn’t much you can do to save your tender plants when there is a hard freeze, but sometimes it’s worth a shot to try some of the same protection methods if you really want to save what’s in your yard.
Knowing the difference between these advisories, watches, and warnings will be the key to you being able to enjoy your vegetable garden or blooming annuals for just a little bit longer this season.