Using dormant oils on Christmas trees

Editor’s note: This article is from the archives of the MSU Crop Advisory Team Alerts. Check the label of any pesticide referenced to ensure your use is included

Dormant oil can be useful in controlling the over-wintering forms of some Christmas tree pests – such as adelgids, aphids, scales and spider mite eggs. Dormant oils work by killing the insects by smothering and suffocating them. These sprays help to reduce the number of insects that survive the winter. In addition, they have little, if any, impact on natural enemies.

To avoid injuring your plants and obtain good control, you must apply your oil at the proper time and rate. Here are some tips to successfully use dormant oils.

  • Proper timing. Oil must be applied when plants are dormant before buds begin to swell.
  • Watch the temperature. Temperatures must be above 40°F and below 85°F. Avoid application when temperatures are expected to drop below 32°F over the next several days, and even when daytime temperatures have warmed to above freezing after a cold night, make sure that melted frost has evaporated before making an application.
  • Be careful with sensitive tree species. Spruce and Douglas-fir may be sensitive to the oils, even when applied at the proper time. Be aware that oils can discolor many conifers that have a bluish, waxy coating on their needles. New, unsprayed growth will again have the blue coloration.
  • Good coverage. Thorough coverage of the plant is necessary for good control.
  • Keep your spray tank agitated. Oil and water tend to separate, even with the emulsifiers added to horticultural oil products. Inadequate agitation may result in deposition of concentrated oil on needles, which is likely to cause burning.
  • Read the label. Always read and follow label directions for safe use of any pesticide.

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