Home & Garden: Protecting fruit buds from frost
Free Press Gardening Columnist
Peach, nectarine, apricot, cherry, and plum trees have two types of buds — vegetative buds responsible for shoots and leaves, and flower buds. Each flower bud on stone fruit trees contains one blossom. Apple, pear and other pome fruits have vegetative and mixed buds. The mixed buds produce both shoots and leaves, and a cluster of five flowers.
Apricots, at least down here in the valley, are already starting to pop their flower buds. As the flower buds expand, they become more sensitive to frost. And if not provided some protection, the crop of apricots will be lost.
The question I’m often asked is how to ensure your apricot tree produces fruit. My first recommendation is to hang a light bulb in the trees and turn it on. The purpose is not to warm the air around the tree, but to provide radiant energy to the opening blossoms. This is similar to quartz heaters, which warm you without heating the air. This technique only works if the buds are in direct line of the light bulb, so it may take several light bulbs to protect all the buds in tree. Even a five-watt light bulb is sufficient to provide the radiant energy to keep buds from freezing.
Commercial orchardists turn on their wind machines in hopes of raising the temperature around their trees to protect them from spring freezes. This only works when there is an inversion of warm air resting above cold air that hugs the ground. Wind machines bring air from above down to mix with the cold air near the ground. When we have windy conditions, as is happening this spring, wind machines are not very effective due to the lack of an inversion.
In the old days orchardists would use smudge pots to burn oil and other fuels. They even burned old tires to create smog to trap ground heat around the trees. These fires also provided radiant energy, protecting the buds in direct view of the heaters. Some growers still use propane heaters to protect their crops from spring freeze damage. The combination of propane heaters and wind machines are more effective than when either is used separately, but it is extremely costly.
In parts of the country where humidity is high, flower buds are protected by sprinklers placed above the trees. As the water on buds freezes, it gives off heat and raises the temperature of the buds providing frost protection. With our low humidity this technique does not work; as water freezes on the buds it pulls heat from the tissues increasing freeze damage.
Trees that have been pruned to keep them low can always be covered with a sheet or some other fabric to help trap ground heat around the tree. Apricots, sweet cherries, and other frost-sensitive fruit trees can be protected under high tunnels (hoop houses). Hoop houses are used by nurseries for the production of flowers and perennials, but can be adapted for fruit trees as well. The use of such a structure would require pruning to keep the trees low enough so a fabric such as a spun-bonded polyester frost barrier (also called a frost blanket) could be stretched over the trees. The frame work of the structure could be either temporary or permanent.
If built over a sweet cherry tree, it could be used for spring-frost protection and for netting later in the season to prevent birds from feasting on ripening cherries. While this would be an excessively costly adventure for commercial orchardists with acres of trees, a hoop house would work well for a homeowner with just a few trees. If the crop was valuable enough, a commercial grower might be able to recoup the cost of such a system within a few years. Growers would need to run the figures to determine if this would make financial sense.
A few orchardists in western Colorado have installed structures allowing them to pull shade cloth over their trees. These systems help keep ground heat from escaping into the sky and protects the crop from hail and sunburn.
Apricot blossoms need to be pollinated and honeybees don’t fly when they encounter stiff winds. They also rarely work when temperatures are below 57 degrees and don’t fly below 55 degrees. Without fertilization the undeveloped fruitlets drop off the trees even when they have not been killed by spring frosts.
Unless you provide frost protection for your apricot tree, you should not expect fruit. Covering the tree with an old bed sheet and hanging light bulbs in your tree should ensure at least a partial crop. Once we get the frost protection problem solved, you need to encourage the honeybees to be more active.
GJ Free Press columnist Dr. Curtis E. Swift is a retired horticulture agent with the Colorado State University Extension. Reach him at Curtis.Swift@alumni.colostate.edu, 970-778-7866 or check out his blog at http://SwiftsGardeningBlog.blogspot.com. He owns Swift Horticultural Consulting and High Altitude Lavender.
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