Ask a Master Gardener: Is horticultural oil a safe pesticide?


Neem Oil

Neem Oil, a horticultural oil used to reduce insect damage, is shown Thursday, April 24, 2014 at the Garden Spot Nursery in Bellingham.


Question: So much of what I read tells me that to reduce insect damage, in my case, in my fruit trees, I need to use horticultural oil. Can you talk about this a bit, and does it really work, safely?

Answer: In the late 1800s, mixtures of kerosene and soap were recommended for treating insect-infested crops. Effective, but it did damage to the tree leaves by the resultant phototoxicity. This can heavily damage the leaves of a tree, ultimately weakening it. Then refined technology produced lighter petroleum-based oils with few impurities, thus reducing the damage to the plants.

Today's oils include vegetable as well as mineral oil products. They are often just as effective as harsher oils but do less damage to plants. And oils also can be used as the carriers, or enhancers, for other pesticides, including Bt (Bavillus thuringiensis.)

Horticultural oils break down into two categories: dormant and summer. Dormant oil sprays are more viscous, evaporate more slowly, and stay on the plants longer. Because they're sprayed in less sunny times or seasons, little damage can be done to the plant by the sun. Summer oils are lighter and capable of doing less damage to leaves.

How does oil work against insects? It creates a physical barrier to respiration by clogging the breathing pores along the sides of adult and larvae abdomens. Oils applied to egg masses inhibit oxygen uptake and decrease hatching success.

Follow the directions on the package carefully. Identify the pest you are going to spray. Don't spray when rain is predicted within a day. Horticultural oils are effective against many pests, including aphids, scale, whiteflies and mites. To get good coverage, you must spray both sides of leaves. There are mixed results against powdery mildew and rose blackspot.

Some do's: Follow directions to the letter. Use the lowest concentration (1 to 2 percent in summer; 3 to 4 percent in winter.) Be aware of beneficial insects and don't spray them. Only apply oils when target pests are present. Keep the oil mixture agitated at all times when spraying.

Some don'ts: Use oils as soil drenches. Mix with sulfur-containing pesticides. Don't apply to wilted or water-drenched plants. Don't let oil drift onto water surfaces; it can harm aquatic organisms. Don't spray conifers with a waxy, bluish cast.

Most veteran gardeners use horticultural oil sprays. When used for the right thing, in the right way, they work. And because they are oil, and not some chemical you can't pronounce, they are relatively safe. Just don't use them in your salad dressing!

Q: What do you think about a drip system for the home gardener? Is it a little too complicated for someone unfamiliar with it? And does it really help that much?

A: Oh ye of little faith! Yes, there's a reason so many stores carry lots of supplies for use in installing home gardener drip systems. It saves a great deal in water use and cost, it is healthier for the plants, and not a minor thing, it saves the home gardener a ton of time. Drip irrigation has many advantages over sprinkler systems. Thirty percent of sprinkled water is lost to evaporation or wind drift, whereas drip irrigation is 90 to 95 percent efficient. My favorite reason for drip over sprinkler is that with drip less soil gets wet, and thus fewer weed seeds germinate.

What's more, it is virtually a cinch to design and install for your garden, be they raised beds or in-ground plantings. Installation requires few tools, and mistakes (little chance you'll make them, anyway) are easily corrected. Even better, the systems won't break the bank, and are reusable from year to year if you take care of them.

I'll introduce you to the first step in how to get started with your own system, but I know you'll find expert help when you purchase a system. You will have to do some comparison pricing to see if it is best for you to buy a whole kit or make up a system of your own. Much will depend upon the area you'll have to water. But don't hesitate to ask for help. It also pays to determine if that help knows anything about drip systems.

The first step is to make a drawing of the area you want to cover with the drip system. Measure and record all distances, as you'll need that information to make your purchases. What parts and pieces you'll need depends entirely on what you're watering, and your particular location and soil.

It may take a few hours to plan and put in a drip irrigation system. But know that you're doing both yourself and your plants a great favor. You'll wonder what you did before you made the leap to the new watering way.


Master Gardener Kathleen Bander is a resident of Bellingham and life-long gardener. For more information on Whatcom County Master Gardeners, go to Ask a Master Gardener will appear in The Bellingham Herald weekly through the summer growing season. If you have a gardening question you'd like answered in the column, please email it to

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