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Use Summer Oils Safely
A summer oil is a light-weight, highly refined, paraffinic (rather than naphthenic or aromatic) petroleum product that is used as an insecticide and miticide in spring or summer on growing plants. Summer oil is lighter than dormant oil sprays but is still effective on mites and soft-bodied insects. It kills all life stages including eggs, which most miticides will not do. Insects do not build resistance. Summer oil sprays still have a potential to damage desirable plants and effects vary by plant species. Please follow the precautions listed below and on the product label. When in doubt spray only small areas until you are sure of its effects.
The ultra-fine horticultural oils are safer for plants because they evaporate faster and leave less residue. While they are more expensive than some of the dormant oils that also have a summer use rate, the ultra-fine oils are much less phytotoxic. It is also safer to apply oils on bright, clear, sunny days when the humidity is low in order to get faster evaporation. Faster evaporation means less chance of phytotoxicity.
Avoid spraying during prolonged periods of combined high heat and humidity. High humidity slows the evaporation process, which could increase the oil's phytotoxicity. Apply in the cooler parts of the day. Choose days with lower temperatures and lower humidity.
Thorough coverage is essential and can not be overly stressed. A 2% solution for scale in the summer is recommended. Oil and water separate rapidly. Constant agitation is required. If a sprayer has been idle for a few minutes, be sure to spray into the tank for a minute if using a handgun; or turn an airblast on several feet from the crop, to ensure that the oil is thoroughly mixed . Otherwise, the emulsion in the hose and tips will have separated and the first plants sprayed may receive either pure water or pure oil. The oil will burn.
Local grower experience discovered that oil applied during the summer stunted maple growth. I thought the concern on maple was being positive they were dormant before applying dormant oil. It appears that Autumn Purple ash is sensitive. One producer had severe branch and trunk die-back after several applications. A Rutgers University Extension publication lists Japanese maple, 'October Glory' red maple, silver, and sugar maple as being somewhat sensitive to oil applications in the summer. It says to reduce the concentration of the oil. Sources vary a great deal on sensitive plants.
Oil-sensitive plants are injured by oils when applied at normal rates under seasonal conditions. Plants classified as displaying a tendency toward sensitivity have not been damaged, but reports of some injuries warrant a cautionary note. Except for eastern black walnut, butternut and some cultivars of cryptomeria, all other oil-sensitive plants can be sprayed with a reduced rate. - Dr. Warren T. Johnson, Professor of Entomology at Cornell University, pgs. 78- 83, Am. Nurseryman mag. Jan 1, 1991 issue.
Acer (maple) - Dormant
Carya (hickory) - Dormant
Cryptomeria (cryptomeria) - Any time
Juglans nigra (black walnut) - Any time
Juglans cinerea (butternut) - Any time
Picea glauca var. albertiana (Alberta spruce) - Late summer
Rhododendron (limited azaleas) - Summer
Rubus (bramble) - Summer
Tendency Toward Sensitivity:
Cercis canadensis (redbud) - Dormant
Chamaecyparis (false cypress) - Summer
Fagus (beech) - Dormant
Ilex crenata (Japanese Holly) - Summer, dormant
Juniperus sabina (savin juniper) - Spring, summer
Juniperus virginiana (Eastern red cedar) - Spring, summer
Photinia (photinia) - Summer
Picea abies (Norway spruce) - Dormant
Picea glauca (white spruce) - Dormant
Pseudotsuga menziesii (Douglas fir) - Dormant, flowering time