Do as I say, not as I do.
That phrase is such a cop out, but the truth of the matter is that I often give better advice to others than I give to myself.
So what is all this hand wringing about? I sprayed cinnamon oil (a type of horticultural oil) on my nectarine tree. Seems like a smart decision for an organic gardener, right? Yes and no. Horticultural oil is great for solving problems like spider mites, which I was trying to banish from my tree. The mistake I made was to spray it on a day that was hot, which burned the leaves. Most horticultural oils should not be applied when temperatures sneak past 90F. Damn. I knew that.
This is not to say that I think you should throw out your stash of horticultural oil. On the contrary, horticultural oils are great for killing soft-bodied pests by smothering them (which is why horticultural oils are people safe). Horticultural oils can also be used to suppress diseases. And horticultural oils don’t negatively impact most beneficial insects. Bees, ladybugs, green lacewings, etc are either unaffected or likely to fly away while you’re spraying and be unbothered by it when the come back.
How to Properly Use Horticultural Oils
Most horticultural oils should not be sprayed on plants plants when temperatures dip below 40F or rise above 90F. It’s also helpful to apply them to well-watered plants on days that when you are expecting low humidity (below 65%).
You need to fully cover the affected plant. Make sure you spray the top and bottom sides of leaves as well as the stems. The oil must come in direct contact with the insect or egg to kill it. To kill soft bodied insects that have overwintered on your deciduous trees (such as most fruit trees), spray in fall after all the leaves have dropped or after insect dormancy ends in late winter or early spring.
Also, know that japanese maples and junipers do not like being sprayed with horticultural oils, and you should not spray either type of plant.
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{ 13 comments… read them below or add one }
Great post! I found out about the heat/humidity issue when spraying aphids last year with a home-made mixture. The pests died almost instantly but then my pepper/cilantro plants dropped all their leaves a couple of days later. Any info on safe sprays for edibles? Do you wash the plants down with plain water after? Ta! (and love the blog!)
Caty–I have to say, I’m not a big fan of home made pest sprays. A lot of the recipes I’ve seen on the internet are at best benign. I always consult http://ipm.ucdavis.edu/PMG/menu.homegarden.html to learn about scientifically-studied methods of controlling pests (their approach is to use chemical pesticides only as a last resort, but usually never in the home garden).
I would love to see a post/article with links to more info on horticultural oils for many purposes and where to buy them etc. I’m transitioning from the chemical products and need all the help and information I can get.
Cherrie–I would suggest stopping by your local independent garden center the next time you have a pest problem and ask them what organic pest control method they have for that pest. It’s much easier to build up your organic fertilizers and pesticides as you need them.
I use Gardner & Bloome pest control sprays, in part because they’re sold by my employer. But they work really well (when applied correctly!
).
Hey Fern-great and necessary post! I will never forget when I was an intern at the Smithsonian and one of their gardeners sprayed the ENTIRE newly-planted rose garden with horticultural oil in the middle of a hot, muggy DC summer.
Needless to say, it was quite an emergency, and there was much explaining to do by all parties!
I’m sure glad I just burned the leaves on my own nextarine and not on the plants of a public garden!!!
Thank you for the information! I never use traditional pesticides. Is there something to kill blackspot that is organic? Thanks!
This is what the University of California recommends: “A combination of bicarbonate of soda plus horticultural oil can be used to manage black spot (as well as powdery mildew). Use about 4 teaspoons of baking soda per gallon of water with a 1% solution of narrow-range oil.”
More info here on how to prevent blackspot in the first place: http://www.ipm.ucdavis.edu/PMG/GARDEN/PLANTS/DISEASES/blackspot.html
I hate when this happens….
I did the same thing with my vegetable garden! Unfortunately, 90-105 is the usual summer temperature where I live, so I don’t think I can even use the oils anymore. I thought spraying them at night would be OK, but I killed my peas.
Any suggestions on other organic pest control methods?
Lisa–It really depends on the pest in question. A lot of times, pests can be controlled or prevented without spraying anything at all–traditional/chemical or organic. For example, encouraging beneficial insects to live in your garden can keep many pest problems down to levels that you can tolerate/ignore.
Fern thank you for sharing these useful tips. I plan to purchase a couple of fruit and citrus trees in the future and I appreciate every tip that I can find. Good post!
Ah that’s very interesting, have never used cinnamon oil. Thanks for the tip!