I got a great question on last weekend’s GardenLine show about the use of dormant oil to control scale on evergreen shrubs.
I had to pause for a moment to factor in the upcoming period of warmer-than-normal weather we are about to experience, then tell the caller to hold off on dormant oil and look for some mineral-oil or all-seasons sprays instead.
Over the years, I’ve often told folks that a dormant-oil spray is a great pre-emptive strike against many insects, especially scale, if used at the height of the dormant season, which is usually January.
If you’ve read any of my books, especially those with monthly checklists, you’ll see I encourage the practice, notably for stone fruit trees. But true dormant-oil sprays are usually made of paraffinic oils, and they can be suffocating for evergreen shrubs right now – they’re just getting ready to come out of dormancy and put on new leaves.
So, instead, I recommend all-seasons oils such as those pictured. All-season oils are almost always mineral oil-based – still good for pre-emptive control of certain insects, and good enough for spraying fruit trees, too.
If any fruit tree is leafless, bud-less and bloom-less, you can and should use true dormant oil to stay true to the normal fruit tree spraying schedules. But if it’s showing signs of buds, blooms or leaves emerging, use mineral oil-based sprays. You could still use dormant oil sprays when you see the very first signs of buds starting to swell, but personally I don’t want to risk using heavier oil on delicate flowers once they have completely emerged. So, it’s all about timing.
Finally, even though a treatment for scale prompted this discussion, consider this a reminder about the many other benefits of horticultural oils at this time of year. Lighter ones like All Seasons from Bonide can be used to smother the overwintering eggs of insects such as mealybugs, aphids, and leafminers, in addition to scale. And on fruit trees, it can really help control plum curculio, mites, leaf rollers, armyworms and whiteflies.
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