It’s been a mild winter and we have already started racking up some Growing Degree Days in Southern Ontario. The silver maples (Acer saccharinum) are blooming down in Essex county and the flowers are starting to open in the GTA. This puts us between 1 and 25 GDD’s Base 10oC.
Some other plant phenology indicators of this developmental period include: Green Leaf Bud (Gleditsia triacanthos, Syringa vulgaris), Flower Bud (Syringa vulgaris) and beginning Bloom (Acer rubrum, Acer saccharum, Cornus mas, Salix caprea).
Applications of horticultural oil to dormant trees and shrubs have started (daily temperatures between 4C and 12C are ideal). If you can wait for it to get a little milder, overwintering insects will have a higher rate of respiration and will be more sensitive to smothering agents. Timing of horticultural 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 scale nymphs (e.g. magnolia scale, San Jose scale) and mite eggs (European red mite, maple spider mite on silver/red hybrid maples). Keep in mind that most of these insects overwinter on the undersides of twigs, stems (and leaves).
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 very effective against oystershell scale, pine needle scale, juniper scale or Euonymus scale).
Since the oil droplets come out of solution so easily, frequent agitation is VERY important during application. Some plants listed as sensitive to horticultural oil applications include: Japanese maple, red maple, sugar maple, hickory, walnut, blue junipers, Colorado spruce, white pine, red oak, and to a lesser extent: yew and cedar.
Slow or Controlled release fertilizers (especially those that are injected) can start to be applied as soils are starting to dry up a bit. It is too early to be applying fast-release fertilizer in the landscape. Fast release fertilizers can be used when soils warm up and leaves have emerged to facilitate maximize uptake of fertilizer nutrients (the first major active root growth period, usually in May).