by Courtney Llewellyn
As far as unpredictable weather months go, October might be champion. Each year it brings a guessing game. Will it stay warm, or even hot? Will it rain nonstop? Will the frost come early? And if the cold does arrive sooner rather than later, do you know how to minimize damage to your fruit?
That’s what Mark Longstroth, small fruit educator with Michigan State University Extension, talked about during this year’s MI Ideas to Grow With virtual conference. First, he covered how freezing actually kills plants: Ice forms inside the cells, which breaks the cell membranes. Plants can avoid freezing by lowering the freezing point of the cell, having ice freeze outside the cell or by super cooling.
For growing regions around the Great Lakes, the lake effect plays a big role. The open lakes prevent extremely cold winter temperatures (seldom dropping below -5º F), and even though there’s heavy snow near the lakes, that can actually provide a layer of insulation.
A majority of fruit trees require a period of dormancy before the next growing season, and Longstroth explained that dormancy and hardiness and chilling requirements, while related, are not all exactly the same. Dormancy is a period when the plant will not grow, even when conditions are favorable. Hardiness is a plant’s ability to withstand cold temperatures without damage. The chilling requirement must be accomplished before growth can begin in spring.
Factors that can reduce winter cold hardiness include early loss of leaves, loss of carbohydrates, early pruning in autumn (which stimulates plants to grow) and late fertilization in autumn (which also stimulates plant growth). However, light fertilization in late autumn can enhance cold hardiness. Longstroth said to wait until the coldest part of winter has passed to prune (usually after February).
Growers should keep an eye on heat transfer in their fruit crops, whether it’s via radiation (how the sun works), conduction (heat moving through an object) or convection (heat rising). The types of freezes depend on how heat is transferred. With radiation, you’ll see clear skies and calm winds; with advection (the transfer of a property of the atmosphere, such as heat or humidity, by the horizontal movement of an air mass), you’ll see a cold air mass, windy conditions or a combination of both. It’s easier to protect against radiation freeze than advective freeze.
Longstroth said to avoid frost pockets, use obstacles, such as dense brush, and take advantage of hills. You’ll also want to improve air drainage – create a low spot at least 60 to 100 feet wide. This will allow cold air to have a place to sink to. On the other hand, you can store heat before a freeze too. Ground cover influences temperature, and a mowed cover crop is warmer than one that’s unmowed. Packed soil is warmer than loose, and wet ground is warmer than dry.
Humid air is also warmer, as it holds more water. A low relative humidity means a low dew point, and low dew points, especially those below freezing, make frosts or freezes more likely to occur. Temperatures will drop rapidly to damaging levels with low dew points.
The key to avoiding damage is keeping heat and humidity in the orchard. Heaters or smudge pots use radiation to provide heat. Overhead sprinklers are good too, as plants can use the heat that water gives off when it changes from a liquid to a solid. Freezing water and condensing water both provide warmth by giving up heat.
Ice is not necessarily the enemy, but it must stay wet if you use it. The temperature of a plant covered in ice will drop below a dry plant if the ice dries. “If the system is working properly, ice will be clear, not cloudy,” Longstroth noted.
He provided these rules for freeze protection with overhead sprinklers:
- Apply water fast enough to keep ice wet at all times.
- Apply enough water to protect the plant.
- High system uniformity is necessary for effective frost protection.
- Overlap is extremely important for frost/freeze protection.
- Higher winds require closer spacing of sprinklers.
Under-tree sprinklers can also be employed. They release heat under the canopy, and they can even use pre-heated water for an extra boost. They work best when you’re under a temperature inversion, which traps warm air below cold air. Under-tree sprinklers are also less likely to damage a crop if the system fails.
Another option for protection is the use of wind machines. Longstroth said they work well if an inversion is low (three to 50 feet). They can protect a large area depending on the strength of the inversion and provide about six to nine degrees of protection. However, they do not work well when natural wind speeds are more than 5 mph.
“You have limited control of cold events,” Longstroth concluded. “And no single freeze prevention technique works all of the time. Most work well in the right conditions and not at all in others.” Ultimately, the proper siting of your crops and planning ahead for cold events will minimize losses. He recommended choosing crops which already perform well in your region, and spreading your risk against cold damage by growing multiple crops.