Weather Alert! Is it a frost or a freeze? These are actually weather terms but the difference is very important to gardeners and growers. One you can protect against and usually avoid damage from (frost) and the other can be beyond your control to prevent severe damage, die back or even untimely loss.
Why? A freeze occurs within your plants. A frost occurs on your plants.
In weather terms, a freeze is when the air temperature drops and remains below 32ºF and winds are above 10 mph. We may experience these conditions all winter, but in the spring when we the sun is creeping back to the northern hemisphere, our daytime temps get warmer and warmer but the night temps drop back below freezing.
For everyone (except perhaps maple syrup producers) this can have a detrimental effect on our plants depending on how far along they have moved toward flowering due to sustained warm day and night time temperatures that raise soil temperature.
In your garden, when a freeze occurs, the cells within emerged buds or foliage of your plants will actually swell up just like if you put an unopened soda in the freezer. At some point that cell ‘pops’ and releases the fluid inside. It may take hours or days for the damage to fully become visible as the cells warm up and the oozing begins. The result is what looks like burned flower buds, foliage or stems (die back).
Freeze damage usually affects trees and shrubs in early spring because these are plants that are outside year round and you can’t normally go out and just cover them up to protect them. Damage to fruit trees can be especially concerning to commercial growers.
Back to weather terms. Frost, on the other hand, requires three atmospheric conditions to occur: 1) temps near or below 32ºF, 2) little or no wind, 3) a high dew point. This is why frost can not occur when there is air movement (wind) even though the temps might be at freezing.
In the garden, when all three of these conditions occur, ice crystals actually form ON the plant. Most trees, shrubs, perennials are frost hardy. It’s those tender perennials and annuals, like petunias or tomato plants, that can’t take frost.
It’s the ice crystals ON the plant that actually cause the damage to the tissue of the plant. This is why for a frost it is recommended that you cover your tender plants or bring them inside to help minimize the damage. You want to keep the ice crystals off the plants. Frost damage may have the same appearance as freeze damage. But you will see it on your tender plants and generally not on your hardy perennials, trees or shrubs.
At the end of the growing season you will often hear weather people warn of a season ending frost. This may be a harsh enough frost or even a freeze that finally puts the season to an end for your plants.
For the last average frost date for your area, contact your local cooperative extension. It’s also good to keep a garden journal to record your frost dates (last and first) for future reference to help understand the length of your growing season. This is helpful, for instance, when deciding on what tomato variety you want to grow based on the average days to maturity. If you have a shorter growing season, you want to choose plants with fewer days to maturity.
How to prevent damage
There is little that can be done about a freeze especially for those plants that are already in the ground. Again, most hardy, perennial trees, shrubs and other plants recommended for your zone should be fine as long as you have not encouraged them to begin growing early by watering or fertilizing. If they have come out due to a warm up, you may have some damage but most will re-cover. Commercial growers do employe techniques and have equipment to try and prevent freeze damage, but this is typically not recommended for homeowners.
For tender perennials and annuals, it is best to try to not purchase and plant them too early in the season. If you do purchase early, wait on planting until you are beyond your last frost/freeze dates so that you can bring them inside. Worse case scenario is to cover them with a cloth, tarp or buckets.
No, covering plants for a freeze does not work. Remember the damage is occurring in the plant and not ON the plant such as a frost. I know some people try to warm the soil and cover the plants and this may help but generally does not work.
Below are additional links for more information on how to protect plants from a frost.
- What is the difference between a frost and a freeze? from Michigan State University Extension
- Cold protection of landscape plants from University of Florida IFAS
- Protecting landscape and garden plants during cold snaps from NC State Cooperative Extension
- Frost Protection from University of Arizona
- Extend the growing season with high tunnels from PennState Extension
- Basics of frost and freeze protection for horticultural crops from HortTechnology
- Resources to protect plants and gardens from winter damage
- My Plants Map: A Digital Garden Journal