Now is a great time of year to be thinking about ornamental plant assessment. Deciduous woody ornamentals have little to no leaves present, which makes it easier to inspect the overall structure of the tree or shrub. To help with underlying insect issues that you may not be able to see, a dormant oil application can give you an advantage before the growing season starts.

Thankfully, dormant oil and horticultural oil can be one in the same. The application timing and rates just need to be changed. Using the same oil at a higher label rate can qualify as a dormant oil application when applied to ornamentals during the winter. The same oil at a lower rate can be applied to ornamentals throughout the growing season.

For a dormant application, temperatures need to be at or above 40 degrees for at least 24 hours, with dry conditions and low wind speeds. When using as general horticultural oil, applications can be made up to 90 degrees. Oils can be used to manage overwintering insect eggs, scale (hard and soft), mites, leaf miners, and aphids. In addition, oils can also be useful in disease management for powdery mildew. The oil application is considered a protectant or contact kill, typically through suffocation, but can also disrupt insect metabolism or feeding. Good plant coverage is essential.

Powdery Mildew
Powdery Mildew

I typically recommend dormant oil application on properties that have a history of mite and scale issues or if you are in fruit tree production. I prefer the use of horticultural oil throughout the growing season in combination with system insecticides such as Safari or Merit (Dinotefuran or Imidacloprid). Advanced Turf Solutions offers a great horticultural oil called Tritek, which is OMRI listed, has a smaller droplet size than most conventional oils, and is compatible with many fungicides and insecticides.If your company does dormant oil treatments, late February is typically a good time to make an application in Central Ohio. Avoid using on sensitive plants (Japanese, Red, and Sugar Maple, Redbud, Beech, Juniper, Alberta Spruce, E. White Pine, Yew, Cedar, and Walnut) and do not use after bud swell. A late or heavy dormant oil application can damage developing spring flowers and leaf buds and will remove the blue coloring on spruce needles.