One of the best tools in the gardener’s arsenal is horticultural oil. Carefully choosing and applying the right oil at the right time can make a big difference in controlling garden pests throughout the year. Some oils can even help control powdery mildew.

Oils are mainly used to suffocate insects, interfere with their metabolism or disrupt how they feed.

Horticultural oils (formerly known as dormant oils) have been refined over the years. Dormant oils can be harsh on trees and plants that are actively growing at the time of application due to phytotoxic compounds, such as unsulfonated residues. Horticultural oils that are safe for use during the growing season are often called verdant or summer oils. They can also be applied during the dormant season.

Success when using horticultural oil is greatly enhanced when the gardener is aware of when it will be most effective. For example, scale insects are especially susceptible to oils during the crawler stage, which occurs immediately after egg hatch. It is important to keep up with pest development so that you can apply the oil when it will do the most good.

A calendar can tell you when you need to start looking closely for pest problems, but it cannot tell you when certain insect life stages are reached, because growing conditions vary from year to year.

Horticultural oils should not be used on hot days (warmer than 90 degrees); on very humid days; any time when temperatures may dip below freezing; or on plants that are water-stressed. There should be at least a three-day period after application when none of these conditions are expected.

Horticultural oils, when applied carefully and only after thoroughly reading the label and following directions, have a low risk of harming beneficial insects.

• UC-certified Master Gardeners are available to answer your gardening questions from 9 a.m. to noon Tuesdays, Wednesdays and Thursdays at 953-6112 or mgsanjoaquin@ucdavis.edu. Questions for Heather Hamilton can be submitted to ucmaster

gardener@gmail.com.