Blueberry bushes are popular, high-yielding garden plants that require precise management for a healthy harvest. The plant’s annual cycle includes distinct periods of vulnerability where external protection is necessary for survival and fruit production. Knowing when to cover the bushes is crucial, involving two separate goals: protecting the plant from cold and safeguarding the developing fruit from wildlife.
Covering for Winter Dormancy and Frost Protection
Protection against cold damage is a two-part process, starting in late fall and continuing through early spring. Blueberry bushes are cold-hardy when fully dormant, typically withstanding temperatures below 20°F without severe injury. Initial winter covering should occur after the bushes have lost their leaves and the soil cools consistently, encouraging complete dormancy.
The greatest risk of cold damage occurs in early spring when the plant transitions out of dormancy, not during the depth of winter. As flower buds swell, their tolerance to freezing temperatures decreases rapidly. An unopened flower bud can survive temperatures around 21°F, but once the bud scales separate, the critical temperature rises to approximately 25°F.
Vulnerability increases as the season progresses, making late spring frost protection a reactive measure. Fully opened flowers are damaged when temperatures drop to 29°F. Newly formed green fruit is at risk of damage at temperatures as high as 32°F. Growers must monitor weather forecasts for late-season freeze warnings and deploy covers quickly to provide a thermal buffer, saving the year’s crop.
Covering to Safeguard Developing Fruit
The second major reason for covering bushes is to protect the maturing crop from animal predation, primarily birds. Timing is tied to the fruit’s reproductive development. The cover must be applied after pollination but before the berries become a visible food source. Applying a cover too early will block beneficial insects, such as bees, preventing necessary pollination and limiting the final fruit set.
The optimal time to cover is once the petals have fallen and the small green berries are fully formed, before they show any color change. As the fruit develops, the accumulation of anthocyanins causes the berries to turn from green to red, then blue, making them highly attractive to birds. Waiting until the berries are blue means wildlife has likely discovered the patch, and some loss is inevitable.
Exclusionary covers should remain in place throughout the ripening process, only being lifted for harvesting. This coverage is also effective against insect pests, such as the cranberry fruitworm or blueberry maggot. Covers must be secured tightly at the base of the bush to prevent ground-foraging animals from accessing the crop.
Selecting the Right Protective Material
The material chosen must align with the specific purpose, contrasting the need for insulation in winter with the need for light and air in summer. For cold protection, the goal is to trap heat radiating from the ground and block harsh winds. Horticultural fleece or frost cloth is effective, potentially raising the temperature underneath by 5 to 10 degrees Fahrenheit, depending on thickness.
Heavier materials like burlap are best utilized as a loose windbreak, especially for container-grown or less hardy varieties, but should not be draped directly onto the foliage. Winter material must remain breathable to allow air exchange and prevent condensation buildup. Preventing condensation is important, as it can lead to mold, mildew, or rot on the dormant canes.
In contrast, fruit protection requires lightweight, fine-mesh netting that allows maximum sunlight penetration and air circulation to prevent fungal disease. Netting with a size of approximately 3/4-inch is sufficient to exclude most birds while permitting air movement. This netting should be supported by a frame or hoops so it does not rest directly on the branches, preventing cane entanglement and allowing fruit to ripen without being crushed.