Adapted from the article “Using a high oil rate before bloom: Why do it?” by Franz Niederholzer, UC Farm Advisor, Colusa/Sutter/Yuba Counties in the October 2010 Sacramento Valley Prune News

A high rate of horticultural oil in a dormant spray can give significant benefits to prune growers. However, a high rate of oil can cost $20/acre or more and risks oil burn under the wrong conditions (dry orchard soil or trees), so growers should weigh the benefits against the costs.

“Horticultural oil” refers to superior or supreme oil (440 or 470), regular dormant oil, or dormant flowable emulsion (“mayonnaise”). All these materials are effective at moving bloom and controlling scale. Summer oil (415) is not recommended for dormant application. With superior or supreme 440, a “high” rate is 3-4 gallons/acre. Consider orchard conditions (tree age, orchard moisture, etc), the product label and your PCA’s recommendation when selecting oil rates in dormant or delayed dormant prune orchards.

High rates of horticultural oil can advance prune bloom date 3-5 days when applied in late December through mid to late January. Earlier bloom can help growers avoid heat damage at bloom and can help with equipment and labor management at bloom time if a grower has several orchards and only sprays part of the trees with a high rate of oil.

A high rate of oil before bloom, with no other pesticide, also gives good control of low to moderate San Jose scale populations. The best timing for scale control with oil is in the delayed dormant period. Spraying oil for scale in the full dormant timing reduces scale populations, but not as much as the delayed dormant timing.

When should oil be applied to advance bloom? A good ballpark spray window is the old standby — late December through mid January. If you want to try to fine tune your dormant oil application, consider applying oil once a certain amount of chilling has accumulated — 30-50 chill portions using the new Dynamic Model calculations. [This timing usually works out to being from mid-December into late January.] For information on the Dynamic Model and how to use it to time a dormant oil application to advance bloom, see the Prune Chilling Prediction Model.

So, heavy oil can be used to advance bloom in the dormant period but gives best scale control when sprayed in the delayed dormant. If a grower’s primary concern is advancing bloom, then spray in the dormant period. This timing will also provide good scale control. If scale control is the biggest concern, apply oil during the delayed dormant period. This timing has less risk of oil burn, but will have little effect on bloom timing.