Dormant Oil - Timing and Uses

Dormant oils are an effective treatment for pests on plants, particularly during the dormant season. These oils are applied in late fall or early spring, before bud swelling and bud break. This timing is crucial for controlling overwintering insects and insect eggs on fruit trees and woody ornamentals while minimizing risks to plants and beneficial insects.

Using dormant oils can help manage harmful pests and reduce the need for pesticides during the growing season. However, a single treatment may not ensure complete control, so consistent monitoring is essential to prevent further infestations. Dormant oils are effective only when they make direct contact with pests at the correct life stage.

Why Timing Matters 

Applying dormant oil at the right stage in a pest’s life cycle is critical for success. For some pests, the oil works by blocking the spiracles—the air holes they use to breathe. In other cases, it acts as a poison by interacting with fatty acids and disrupting metabolism.

Correct application targets specific pests and their life stages. Avoid indiscriminate spraying, as blanket applications may not be effective and are environmentally irresponsible. As with any pesticide, remember: “A prescription without diagnosis is malpractice.”

Application Guidelines

Avoid using dormant oil in combination with sulfur or sulfur-containing pesticides, as this can cause phytotoxicity, negatively impacting plant health.

Do not apply dormant oil on sensitive plants such as junipers, cedars, Japanese and red maples, smoke trees, and spruce, especially dwarf Alberta spruce.

Apply when temperatures remain above 40°F for at least 24 hours. If applied just before freezing, the oil’s emulsion can break down, causing it to adhere improperly to bark or leaves instead of targeting pests.

Benefits and Considerations

Monitoring trees is vital. Symptoms from pests like autumn mites may only become evident in spring or summer. Fall applications can prevent such damage by suppressing mite adults, immatures, and eggs. The physical mode of action of oils, causing suffocation, requires thorough coverage to be effective.

Various oils are available for garden use, including spray oil and highly refined supreme oil. While older oils were unsuitable for use after dormancy, modern lightweight oils are safer for foliage. These advancements make it easier for gardeners to apply dormant oils with confidence.

Troubles with your Summer Lawn

The Turf/Tree Team recently had a couple of site visits where our clients are having issues with their lawns. The 1st was fungal issue called Brown Patch, where the grass looked dead and the 2nd issue was an over watering issue where it was impacting his lawn. We also believe the over watering was also impacting the surrounding trees that were under stress. We’ll cover both issue and talk about how to avoid these problems going forward.

Let’s look at Brown Patch impacting a large area of the local neighbors lawn:

    

      No insect problems, just a fungal issue.

Our client swore she was having insect problem, but after we pulled up a large sample of the impacted lawn and took it back to office for analysis. We found both the crown and roots to be intact,  we also found no insect issue, because most insects such as billbugs would be eating away on roots and crown.

What we did find was good soil full of worm eggs and casting and a lot of small roly-polies. The grass though was thin with large patches of light brown grass. Our client had many unfavorable conditions that were going on from overwatering, from causing humidity levels to spike, to having a lawn service come out for multiple fertilizer applications to her lawn. All this added up for a severe case of Brown Patch to develop.  

So we followed up by researching from multiple educational websites that address Brown Patch and how to prevent Brown Patch from happening next year. Those articles included attention to cultural practices, such as avoiding excess nitrogen during the summer heat, which can contribute to reducing disease pressure and help improve fungicide performance on intensively managed turf. Improving air circulation and scheduling irrigation times to avoid long dew periods to also help suppress Brown Patch outbreaks.

There are also numerous effective fungicides that are registered for Brown Patch control. When selecting a fungicide, be aware of other turf disease threats and apply fungicides for these threats at the same time to minimize costs and other potential disease problems.

Our next site visit took us out to West Caldwell where the client was having what appeared to be dieback on multiple trees such as Ash, Pecan and Willow. At the time we didn’t have a answer for his tree problems. But after walking around his lawn area looking at the trees, we notice how soft and spongy the lawn felt. Looking around closer, we found large areas of yellowing grass and when we bent over and tugged at the lawn, we felt the crown area separating from the soil surface. We then ask about his watering and fertilizing schedule. We found out he was watering for nearly hour in each zone every other day and his local lawn care service was out frequently applying multiple applications.

     

          Light Yellowing of grass blades

We talked for awhile about how much water his lawn really needs during the summer months and that his lawn care service may be over doing it. He did turn off the watering station to allow his lawn to dry out some while we were there.

When we got back to office we followed up by putting together a Lawn Maintenance Schedule that included how often to water and what his lawn fertilizing needs were for a whole year. He seemed pretty happy with the follow-up schedule. We also sent an older article we found from USDA where over applying the amount of Nitrogen could be causing problems with his Pecan tree. Even though I wouldn’t rule out the 3 story tall tree house surrounding the pecan tree, it may also be impacting on the life of tree.

So far this summer the Turf/Tree Team has been fairly busy with site visits and providing research based answers to the clients. They have enjoyed the opportunity to provide this service which is part of the Master Gardener Program, Canyon County Extension of the University of Idaho.