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.
My greatest demise in my nasty far north zone 3 central continent climate, late spring hard freezes dipping in from Polar bear country, with too many damned trees to quickly move. The worst is newly transplanted trees with cold enough to freeze the soil. It makes a man want to take up birdwatching instead.
True true, birds need less moving around…
Thanks Michael. Very informative especially for my area in the Midwest.
Glad this helped- continental areas are harder, but can be managed.
Called for 28F last night. The lowest reported temp I’ve seen is 30F. I’ve got my greenhouse loaded and garage as well. Here’s hoping after tonight this cold will get out of here and we can start growing.
Lot of back and forth in temps here in the Northwest this year-
Some of us in New England call it the tree dance. Move them out, move them in. Turn around and do it again.
Oh yes, that’s a good dance and you don’t really need a partner…
Bonsai people definately are an attentive bunch, if your doing it right. It’s a lesson you need only learn once. Yes the damage is real!
Very true! Some of us need a mistake reminder once every decade…
Perfect timing on this post Michael – thanks much!!!!
Glad it was helpful!
Hi Michael – Are you aware of a theory that spraying frosted foliage with water before the sun reaches the foliage in the morning reduces the impact of frost? I believe the theory has something to do with energy transfer and the energy required to freeze and thaw water.
I doubt that early morning spraying would protect pots but it might save a few buds – and it might keep interns busy while saving backs!
I would be interested in your comments.
Hi Don, yes that’s an industry standard for fruit tree orchardists. It works. Sometimes we use it here in the bonsai garden as well. But as I understand it the best use of the idea is to spray the foliage with water in an expectation of temps dropping below zero, before it gets below zero. Then the frozen water protects the foliage, melting as the temps rise during the day. There’s a difference between that thick ice and what the sun does with that thin frost layer, though you’d need an earth sciences teacher to explain that one to me… Water does give off heat when it freezes, but beyond that I don’t understand it.
As for backs, we get them in bulk from Costco, so don’t feel too much pity about the apprentices… ; )
Are conifers OK to have spring frosts versus the deciduous in your excellent post?
Hi Joe—Many conifers are somewhat more frost tolerant than deciduous trees. The higher elevation conifers in particular. That is, while a late light frost might damage new growth on a deciduous tree they might not on a juniper or ponderosa. A good frost that’s really late can damage extending black pine candles, however, so don’t get too comfortable with frosts…