Organic Plant Protection with Improved Horticultural Oils

Organic Plant Protection with Improved Hort Oils Featured Image
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

Magnifying glass focusing on aphids on leaves
These oils are a great remedy for soft-bodied arthropods, such as aphids.

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

Spraying hort oils on plants
Many older hort oils were best applied in late winter or early spring.

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

Spraying hort oils on flowers
Many new hort oils can be applied at almost any time of the year.

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.

About Russell Stafford


Hortiholic and plant evangelist, Russell Stafford, transplanted his first perennial at age 7 and thereby began a lifelong plant addiction. He is the founder and custodian of Odyssey Bulbs (and Odyssey Perennials), an online nursery specializing in cool and uncommon plants. Russell also works as a horticultural consultant, freelance writer (Horticulture and The American Gardener magazines), and garden editor. He formerly served as Curator and Head of Horticulture at Fernwood Botanic Garden in Niles, Michigan and as the Horticultural Program Coordinator at the Center for Plant Conservation, then located at the Arnold Arboretum in Jamaica Plain, Massachusetts. His academic degrees include a masters in forest science from Harvard University.

Content Disclaimer:

This site may contain content (including images and articles) as well as advice, opinions and statements presented by third parties. Sun Gro does not review these materials for accuracy or reliability and does not endorse the advice, opinions, or statements that may be contained in them. Sun Gro also does not review the materials to determine if they infringe the copyright or other rights of others. These materials are available only for informational purposes and are presented “as is” without warranty of any kind, express or implied, including without limitation warranties of merchantability, fitness for a particular purpose, and non-infringement. Reliance upon any such opinion, advice, statement or other information is at your own risk. In no event shall Sun Gro Horticulture Distribution, Inc. or any of its affiliates be liable to you for any inaccuracy, error, omission, fact, infringement and the like, resulting from your use of these materials, regardless of cause, or for any damages resulting there from.