Frost Damage to Plants
Colorado State University Extension Master Gardener in Larimer County
May 4, 2013
No one who has been in northern Colorado recently has to be reminded of the sudden swath of snow and roller coaster temperatures the past few weeks! Besides catching us personally off-guard without jackets or boots (since we are all so ready for warmer weather), it can wreak havoc on many of the plants in our landscapes which have been lulled into thinking that spring is here for good. April can be a fickle month....warm and welcoming one afternoon, cold and winterish that night. Many plants have already begun to leaf out, only to have tender tissues exhibiting damage caused by low temperatures. Already this year, colorful charmers like daffodils have been cut off at the peak of their bloom time, while perky iris straps, just beginning to emerge, have instead begun to droop.
To understand how fall or spring temperature swings can harm plants, consider the following: woody plants in our temperate zone survive freezing temperatures over winter by first becoming dormant. (Shortening day length and generally cooler fall air sets this process in motion.) As the temperature drops to below 32 degrees, some of the water from plant cells migrate to spaces outside of the cells, and then freeze. Woody plants are not harmed by this process, because the small amount of remaining water still inside plant cells does not freeze, a condition called 'supercooling'. This protection plan, compliments of Mother Nature, also includes hardening woody plants at approximately 40 degrees for a certain number of hours. As a result, plants are protected during dormancy from leafing out too soon in the event of a short-lived warm spell.
On trees, the prime stage for freeze damage occurs just as the leaves are emerging from the buds, once they emerge from dormancy. If the tree has leafed out already, it could actually be spared the most severe damage. Crabapples, lilacs, linden, silver maples and Siberian elms are early leafers, with honeylocust, green ash and hackberry leaves emerging later, so the actual timing of a cold snap will determine which tree species might suffer from the most damage.
Some of the damage will be obvious if leaves and/or stems appear dry, blackened and crisp, since cellular tissues were injured. However, one other sign to look out for is premature leaf drop. However, in most cases, since healthy trees store three years worth of energy, most will recover nicely and re-leaf. A tree with leaf damage will likely put out a second set of new leaves, but from "adventitious buds", which are often structurally unsound. (The latter set of leaves often cluster into a bushy pattern called "witches brooming".) A previously stressed or declining tree could continue on this downward path. If a tree has not leafed out a second time by early June, it probably will not leaf out that year at all.
Damage to emerging flower buds can occur also. There is speculation that some of our flowering plants in the landscape may not have as many flowers because of the recent freezing temperatures, but it depends on your location and how chilly those night temperatures were. Trees and shrubs in more protected locations in the landscape may bloom normally.
Another manifestation of damage you might see is a splitting of the trunk of trees or branches of bushes horizontally, most often on the south or west side of the trunk or branch. This is especially common after a weather pattern characterized by sunny days followed by very clear, cold nights. These 'frost cracks' might even occur on trees considered quite hardy. If not too wide, cracks will often heal themselves.
The majority of frost-damaged trees recover on their own. However, if you feel the need to speed the process, prune out dead branches once summer arrives. Do not increase your previous watering pattern, as overwatering can result in oxygen deprivation to roots, especially in our clay soils. Stick to the healthy practice mantra of "water more deeply, but less often." Hold off on fertilizing, until at least midsummer, and only then if your trees exhibit nutrient deficiencies. Trees planted in lawns usually receive enough nutrients from the grass fertilizer, and probably don’t need extra fertility.
The location of your gardening site can have a huge effect on the amount of damage your plants might receive. Since cold air slides to a lower elevation, creating a frost pocket, having your landscape located at a slightly higher site may prevent a freezing temperature drop. Another factor is the length of time your particular plants have actually been part of your landscape. More established plants are more likely to survive a cold snap, since their root systems are already more acclimated to the unique characteristics of your particular garden area. Plants situated in poorly-drained soils or ones which were fertilized in August or September the previous growing season may suffer more.
As always, damage to plants situated in above-ground containers will be substantial. The fact that roots, normally protected in-ground, can be assaulted by cold on every side makes survival less likely. Some protection can be afforded by situating these containers in areas like garages or sheds, as long as water is provided occasionally. Another option is to sink these containers into the ground in the fall.
What measures can you make in future gardening seasons to prevent further harm from unexpected freezes? One helpful measure is to mulch; in addition to preserving moisture in the summer, it will also add a layer of protection from wild temperature swings in spring and fall. And don’t be surprised what plants may overcome.
1. Freeze injury to ash, taken by Robert Cox
2. Freeze injury to spruce, taken by Robert Cox
3. Iris in snow, taken by Curtis Utley
4. Freeze damage to tulips, by Alexis Alvey
5. Sunscald/frost crack on buckeye, by Alexis Alvey
The author has received training through Colorado State University Extension's Master Gardener program and is a Master Gardener volunteer for Larimer County.
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Updated Tuesday, August 05, 2014