This Week in the Garden: Dormant Oil Spraying

Warmer weather will soon be causing buds to swell on trees and before long, they’ll begin to leaf.  The swollen buds also indicate it’s the proper time to apply the annual dormant oil spray to fruit trees for this year’s healthy crop.

Dormant oil is a highly refined mineral oil that is specifically meant to be used in horticultural applications.  It is usually mixed with a lime sulphur mixture in water.  The oil and the lime sulphur come together as a kit along with mixing instructions.

Safe to use even in organic farming, the spray provides a double punch to both overwintering insects and fungal infestations.  The oil coats the insect egg casings laid along the branches and in the fissures of the bark, causing the overwintering insect to suffocate.  The lime sulphur offers control over any fungal spores remaining from last year that might cause scab or rust.

It’s important to spray dormant oil before the leaves emerge, otherwise the oil can burn the tender emerging leaves.  Temperatures should be above freezing for 24 hours after spraying.  The oil will eventually evaporate over a few weeks.

Always follow the directions on the label.  Some types of trees such as spruce or Japanese maple are susceptible to damage.

Happy Gardening!

Garth Davey

March 2022

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