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Dormant Oil

Summary: Dormant oils, also known as horticultural oils, are mineral-based oils used on woody plants during the dormant season to control unwanted pests. They are considered to be one of the safest methods of control, plus dormant oils mix well with other pest control products to extend their effectiveness.

Have you ever watched your favorite burning bush get defoliated by hordes of aphids and wished you had something to fight them off? Sure, there a loads of pesticide products available on the market, but if you are looking for something that is more environmentally correct I've got just what you are looking for. It's dormant oil.

Certain types of oils used for controlling mites and insects have been around for centuries and many continue to be used in today's modern agricultural practices. Improved oils have overcome some of the known issues of older dormant oils such as phytotoxicity or toxicity to plants and staining. In fact, because the newer oils can be applied over a wide range of applications during the entire growing season, the oils are referred to as horticultural oils rather than dormant oils. That said, you still need to understand how, when and where to use oils properly.

Dormant oil or œhort oil applications do a great job on insects like pine needle scale, aphids, the eggs of tent caterpillars, leafhoppers, spider mites and even some diseases like powdery mildew. There are many other pests controlled by these products, as well.

Horticultural oils control pests in three ways. The most obvious is that the oil gets on the insect and blocks the holes that allow the insect to breath. Some oils interupt the insect's normal metabolism. An application of oil may change the way an insect feeds. It is the feeding process of aphids, for example, that is known to help in the transmission of some plant diseases.

There are some rules you need to follow in quest to dominate with œhort oil. Even with this list of rules, we strongly suggest you review your treatment intentions with your local garden outlet before you apply.

  • Do not apply on wet plants.
  • Do not apply when young shoots are growing.
  • Do not apply in high humidity or high temperatures. (90% humidity or above 95 degrees)
  • Do not apply during freezing temperatures.
  • Avoid drift or over-spraying on oil-sensitive plants like your lawn and flowers.
  • Do not apply until after the plants have become œwinter hardened.
  • Do not mix dormant oil with products containing sulfur.

Identify the plants you intend to treat and fully understand which plants react badly to dormant oils. A few such plants are:

  •  Junipers
  •  Cedars
  •  Japanese Maple
  •  Red Maple
  •  Spruce
  •  Douglas Fir
  •  Redbud
  •  Black Walnut

There are more, so you need to do some research before doing your treatment.

Dormant oil products are available at lawn and garden stores everywhere, but if you want to try making your own, here is an easy recipe:

  • Take 1 gallon of mineral oil, 1 pound oil-based soap and 1/2 gallon water.
  • Mix all the ingredients, boil and mix again.
  • Dilute the resulting mixture with one part of the mixture to twenty parts water.
  • Mix and use it immediately because the oil will separate from the water.

Make sure you are applying the mixture when the temperature is guaranteed to remain above 40 degrees for the 24 hours following a treatment. Soak the branches of your target trees and shrubs thoroughly. The oil will create a film that covers the leaves. Citrus leaves will not tolerate this treatment and require a specialized horticultural oil that you can buy from the garden store.

Remember, dormant oils require that the target pest be thoroughly coated by the spray solution. Faster moving plant insects will not be controlled. In some areas the list of insects that can be controlled with œhort oils is limited, so oil treatments may not work. Do your homework before you jump into this endeavor.

Timing is everything. Apply before leaves or flowers start to bud, but not too early, either. If you apply before insects are actively breathing the oil will have little effect. Oils will not control the stages of insects just before they turn into full adults, nor will they control the adult stage. You need to understand the insects you are trying to control and apply the oils when the insects are soft and slow.It's not easy being green! (credits: Kermit the Frog)

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