Spring Frosts and New Growth–
In any area that freezes, spring is a delicate moment. If cold comes, we can lose years of work in a few hours. In our bonsai garden, we have an outdoor thermostat that triggers an alarm inside the house when it records a temperature below 32F (making apprentices jumpy and sleep-deprived at this time of year).
What is frost? I realized after writing this post that I had an incorrect view of it, thinking it is frozen dew. I’ve an Earth science teacher as a student, and he explains it this way: ‘Frost forms when water vapor from the air crystalizes directly on surfaces that are below freezing. This process is called deposition.’
Frost damage can show up later that day, especially after sun hits the plant, or sometimes later after the plant tries to grow a bit.
Spring frost damage on a tomato
28F / -2.2C is often noted by horticulturalists as the hardy plant temperature to watch for after they’ve shown some spring growth. That early shoot growth doesn’t have the cold hardiness it might have had when dormant.
Length of time is an issue. A late storm bringing freezing temperatures lasting a half day or more can cause extensive damage, while the mild, one hour before sunrise freeze is less troubling. Very light frosts, however, can often damage or kill tropical plants.
Oak showing frost damage that happened when leaves were just opening
Early opening flowers are less damaged by frosts than foliage. But how far along growth is can make a big difference; plants with a bit of bud swell are more resistant to frost damage than those with leaves half open or fully open.
Frost damage is permanent. Preparation and prevention is best, with storing plants under cover, in greenhouses, overhangs and cold frames, or temporarily in a garage. Even after placing outdoors the weather can shift unexpectedly and then they must be returned to their shelters.
Light frost damage can look like disease
Those living in frost-prone areas benefit from:
- taking Ben Franklin’s advice: an ounce of prevention is worth a pound of cure
- daily checking spring overnight temperature forecasts
- moving plants in and out, or using overhead structures that limit plant relocation
- after an inadvertent frosting, protecting plants before the sun hits
Leaving plants in their shelters may be the natural response to fear of frost, though often fully sheltered areas like garages can grow sappy leaves and extensions. The more time spent outdoors on non-frosty days and nights the healthier trees will be.