Oil-based insecticides have come a long way in the last few decades. Lighter and more versatile than the “dormant oils” of yesteryear, today’s horticultural oils can be used at most times of the year and are effective against a wide variety of insects. They’re also among the most benign pesticides, decomposing within a few days of application and causing minimal harm to beneficial insects and other untargeted organisms. Accordingly, many brands of horticultural oils are OMRI LISTED for organic gardening.
New and Improved Horticultural Oils
Most new-wave horticultural oils derive from petroleum, although an increasing number are vegetable-oil-based. In all cases, they’ve undergone several rounds of processing to remove impurities, such as sulfur that can damage leaves and other soft plant tissues. Their relatively high purity (92 percent or greater) and low viscosity allow them to go places – such as directly on foliage – that are largely off limits for old-school “dormant oil sprays.”
Almost all horticultural oils work not by poisoning pests but by mechanically coating and smothering them. Consequently, they’re an excellent remedy for infestations of slow-moving, soft-bodied arthropods, such as aphids, mites, and whiteflies. They also control a number of plant diseases, including powdery mildew and aphid-transmitted viruses.
Neem oil departs from the norm by disrupting insects’ feeding and development via several biologically active compounds. Virtually non-toxic to humans and other mammals, it’s effective against a relatively wide range of pests, including some that are resistant to other horticultural oils.
Horticultural Oil Conditions
Horticultural oils come with a few provisos. First, they lose their effectiveness in rain, drought, cold (sub-40 degrees F), heat (90-degrees-plus), or high humidity. Additionally, even highly refined horticultural oils can sometimes cause minor damage to flowers and tender new plant growth. Horticultural oils are also said to be mildly toxic to a number of plant species including ferns, Japanese maples (Acer palmatum), and smoke trees (Cotinus spp.), although this may not apply to the highly purified oils currently in use.
When to Use Horticultural Oils
Of course, horticultural oils are effective and ecologically friendly only when they’re properly used on visible pests at vulnerable points in their life cycles. A thorough, targeted coating of oil at the right time will put a serious dent in a susceptible pest infestation. Conversely, indiscriminate spraying will likely do more harm to beneficials and foliage than to pests. It’s always good horticultural practice to know your enemy and to read the label.
Late Winter
The horticultural oil season begins in late winter, as temperatures moderate and overwintering pests begin to shake their slumber. Hemlock woolly adelgids (pest info here), euonymus scale (pest info here), and spruce spider mites (pest info here) are among the insects and mites to look for and treat at this time.
Spring
Mid to late spring is a good time to spray “crawler” stages of armored scales (pest info here). Weekly applications of neem oil will help contain lily leaf beetles (pest info here).
Spring to Fall
Use horticultural oils from mid-spring to fall to control the likes of aphids, lacebugs (pest info here), spider mites (pest info here), powdery mildew (pest info here), and sawfly larvae such as “rose slugs” (pest info here). Mild days in late fall are a good time to spray scales, mites, and other pests that survived early spring treatment.
Oils for Indoor Plants
Indoor plant pests are also fair game, whatever the season. Your spider-mite-infested weeping fig and all your other insect-plagued houseplants will welcome a quick visit to the back porch for a spritz of death-dealing horticultural oil. Or you can give your plants (and their insect pests) a bath by inverting them into a bucket filled with highly diluted horticultural oil. Outside or in, horticultural oils are an environmentally friendly solution to a host of insect problems.