When and why do you spray trees and shrubs with Dormant Oil

Recently Gibbons in Arlington Heights asked about the tiny woodpecker holes in his Flowering Crab.  I told him the Woodpeckers were after the Insect eggs dumped in all the cracks and crevices and he should spray it with Dormant Oil.

Dormant spray or Lime-Sulfur Spray can be purchased at any local nursery, or can be put on by a licensed technician.  It's considered one of the best organic defenses against diseases because the spray smothers over wintering spores and fungus. Because Dormant Spray can damage leaves and needles on trees and shrubs, you have to spray your fruit trees when they are dormant, before the buds have begun to open (bud-break). The spray can also damage surrounding evergreens and perennials, so it's advisable to cover any susceptible plants in the over-spray zone with a tarp. It's also important to choose a clear dry day, with no breeze and preferably above freezing.
This would be an excellent time to prune out any dead or broken branches, before you apply the spray.
Put on your goggles and carefully read the instructions on the bottle of Dormant Spray. Different vendors call for different measurements that can be found on the inside label. Measure, according to the directions, into your pump sprayer. Hopefully, the sprayer has measurements on the side of the holding tank, if not get a set of measuring spoons from the thrift store and keep them with the garden tools. Add the recommended amount of water and place the lid on securely. Shake or swirl lightly to mix and pump a few times to get the mixture flowing.
Focusing at the top and middle of the tree, begin applying the spray. Avoid standing where you'll get dripped on, work the spray towards the outside branches. Keep pumping air into the bottle so the air pressure makes the spray reach farther away. You should be applying enough Dormant Spray to leave the branches dripping. Finally spray the trunk and you're done. Wait until the tree is dry before removing the tarp from the protected plants.

Two classes of woody ornamental s sensitive to oil when applied under seasonal conditions.
Oil Sensitive Plants
Plant	Time of Treatment
Maples (Japanese, Silver, Sugar)	Dormant
Hickories	Dormant
Black Walnut, Japanese Walnut	Dormant
Cryptomeria	Anytime
Smoketree	Summer
Some Azaleas	Summer
Brambles (Rubus)	Summer
Butternut	Summer/Dormant
Blue Spruce/ Koster Spruce	Will lose blue color
Alberta Spruce	Late summer
Plants with a Tendency Toward Sensitivity
Plant	Time of Treatment
Beech	Dormant
Japanese Holly	Summer/Dormant
Redbud	Dormant
Savin Junipers	Summer, Spring
Photinia sp.	Summer
Spruce; Norway, White	Dormant
Douglas Fir	Dormant, Flowering time
Table adapted from: Johnson, W.T. 1985. Horticultural Oils, J. of Environmental Horticukure. 3(4): 188 - 191.

Treating blue spruce or blue rug junipers, results in a temporary loss of blue color, although they will not be damaged permanently. If uncertain of the reaction of a particular cultivar, spot treat with oil before infestations occur. This will determine if horticultural oil can be safely used should pest problems develop later.

Horticultural oils are an effective low toxicity alternative for managing pests when they are used properly with an understanding of their benefits and limitations.

References

Baxendale, R.W. and W.T. Johnson. 1988. An Evaluation of Dormant Oil Phylotoxicity on Six Species of Woody Ornamentals. J. of Arboriculture. 14(4): 102-105.

Baxendale, R.W. and W.T. Johnson. 1988a. Evaluation of Summer Oil Spray on Amenity Plants. J. of Arboriculture. 14(9): 220-225.

Baxendale, R.W. and W.T. Johnson. 1989. Update Note Concerning Horticultural Oil Concentrations for Verdant Use. J.. of Arboriculture. 15(2):51-52.

Gill, S. and W. Healy. 1990. Spray Oils May be Your Best Defense in an IPM Program. GrowerTaiks. 53 (11):68-69.

Grossman. J. 1990. Horticultural Oils: New Summer Uses on Ornamental Plant Pests. The IPM Practioner. 12(8)1-10.

Johnson, W.T. 1985. Horticultural Oils. J. of Environmental Horticulture. 3(4): 188-191.

Johnson, W. T. 1991. Rediscovering Horticultural Oils. American Nurseryman. 173 (1): 77-80.

Larew, H.G. and J.C. Locke. 1990. Repellency and Toxicity of a Horticultural Oil against Whiteflies on Chrysanthemum. HortScience. 35(11) 1406-1407.