Horticultural Oil Applications Starting (again?) Soon

SSMEggs

Overwintering Spruce Spider Mite Eggs on Abies

Applications of horticultural oil to dormant trees and shrubs will be starting soon (daily temperatures between 4C and 12C are ideal). Timing of dormant oil applications are critical since freezing temperatures, mixing with sulphur and applying at the dormant-rate on actively growing tissue may result in injury.

Apply dormant oil on clear mornings to facilitate rapid drying. The oil provides a barrier that restricts both respiration and movement of overwintering insects. It is quite effective for the management of overwintering spruce gall adelgids, scale nymphs (e.g. magnolia scale) and mite eggs (including spruce spider mite, European red mite, maple spider mite on silver/red hybrids). Keep in mind that most of these insects overwinter on the undersides of leaves and twigs. Note that scale insects that overwinter as eggs protected under adult scale are not controlled by horticultural oils (e.g. dormant hort oil applications not effective against oystershell scale).

Since the oil droplets come out of solution so easily, frequent agitation is VERY important during application. Some plants listed as sensitive to dormant oil applications include: Japanese maple, red maple, sugar maple, hickory, walnut, blue junipers, blue Colorado spruce, white pine, red oak, and to a lesser extent: yew and cedar.

Unless it is soil-injected, slow release, it’s too early to be applying fertilizer in the landscape. Wait until soils dry up a bit and warm up to facilitate root growth and maximize the uptake of fertilizer nutrients (usually when leaves have emerged and expanded). It has been shown that applications of readily available nitrogen to cold, wet soils results in loss of nitrogen to the environment and reduced plant uptake over the season.

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The Crop Protection Guide for Nursery and Landscape Plants, OMAF publication 840, can be found here.

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