Purpose

During the weeks of late fall & early winter, many landscapers apply their annual anti-desiccant sprays to broadleaf evergreen plants. Some of the common trade names of these liquid resin, spray-on products include Vapor-Guard, Wilt-Pruf, Stress Guard, and Nu-Film. Although plants go dormant during the winter, evergreens will continue to transpire given certain conditions. These applications help reduce excessive water loss from leaves during the dry winter months when the ground is frozen. An anti-transpirant is a film-forming complex of polyethylene’s and polyterpenes that when applied to foliage will reduce the moisture vapor transmission rate. Although much of the transpiration from leaves occur through small openings (stomates) under the leaves, a significant amount of water loss can also occur directly through the leaf cuticle or epidermis. The anti-transpirants function by increasing cuticle thickness of broadleaf evergreens.

This Rhododendron may have some winter injuries, but the primary cause for the plant decline is from wood borers. (Photo Credit: Steven K. Rettke, Rutgers Coop. Ext.)

Desiccated holly leaves (Winter Burn) with obvious symptoms. (Photo Credit: Steven K. Rettke, Rutgers Coop. Ext.)

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Dormant Oil Guidelines:

Dormant oils at 2-4% rates in the late fall & late winter seasons can aid in the control of overwintering insects and insect eggs. Consider treating for such pests as aphids (eggs), southern red mites/eggs, spruce spider mites/eggs, oak spider mites (eggs), eriophyid mites/eggs, spruce gall adelgids, lace bugs (deciduous plants), cankerworms (eggs), leaftiers, psyllids, plant bugs, etc.

Some landscapers have been successful applying both late fall and early spring dormant oil treatments to the same plants on the same properties. This is a reasonable approach since a single dormant oil treatment will not provide 100% control and it gives the field technician another opportunity to monitor the landscape for problems.

Dormant oil sprays require excellent coverage & often repeat applications. (Photo Credit: Steven K. Rettke, Rutgers Coop. Ext.)

Japanese maple scale infested branch will likely require multiple dormant oil treatments to suppress. (Photo Credit: Steven K. Rettke, Rutgers Coop. Ext.)

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