Dormant and horticultural oil sprays: what extension services say about rates, temperatures and plant sensitivity

This page reports what the official source says. It is not legal advice: ask the office named here. This page describes what the named extension services say. It does not say which product to use, at what rate or on which plant, and it is not an application guide. Every source sends the reader to the product label for the rate, the plants and the pests. Who may spray for pay is covered on the guide to pesticide licences by country.

What the names mean

Iowa State University Extension (reviewed January 2025) says what was once sold as dormant oil, superior oil or 70-second oil is called horticultural oil, because season-long use makes "dormant" too narrow a word, and that horticultural oils are highly refined petroleum oils mixed with water. NC State Extension (Entomology Insect Notes) says summer oils and horticultural oils are the same thing, and that confusion comes from labels that print winter or dormant directions first and in large type, so a buyer may think a product is a dormant oil when it is a horticultural oil usable in the dormant season too. It names three factors that separate summer from dormant oils: unsulfonated residue, density and viscosity. It gives 50 to 90 percent unsulfonated residue for dormant oils and 92 to 96 percent for summer oils, and says the higher figure is much safer on leaves and stems.

The Pacific Northwest Pest Management Handbooks (article reviewed March 2023) say a minimum unsulfonated residue of 92 is recommended against plant damage, and that pesticidal activity does not rise above 95. They say lighter oils flow and spread more evenly and suit warm weather and heavy foliage, while heavier oils bead up more and are used mainly during dormancy.

Rates and temperatures, source by source

Source	Rate it names	Temperature and weather it names
University of Maryland Extension, Home and Garden Information Center (updated September 22, 2026)	1 percent in season; 2 to 3 percent as a dormant rate.	No application above 85°F. In cool weather, 50 to 60°F or above for several consecutive days with nightly lows above freezing. Not on drought-stressed plants. Not in cloudy, rainy or very humid weather, when evaporation is reduced.
Iowa State University Extension (reviewed January 2025)	Mixed exactly at the label dilution, especially for summer use.	Warmer than 40°F and no hotter than 90°F, on healthy, unstressed plants.
Pacific Northwest Pest Management Handbooks (reviewed March 2023)	Not given as a figure; label instructions followed carefully.	Not above 100°F or below 40°F. Avoided if humidity is expected to stay over 90 percent for 36 hours or longer. Plants irrigated before application in warm weather.
NC State Extension (published May 14, 2018)	1 to 4 parts oil in 99 to 96 parts water, depending on the manufacturer's label.	Warns about application in direct sunlight and at temperature extremes.

Maryland ties the dormant rate to plant type: for deciduous plants only after leaf drop in late fall or before bud swell in early spring, and for evergreens only in winter months before spring growth. It lists the potential for plant damage, serious or cosmetic, as the main drawback of oil.

Timing

Iowa State says dormant oils belong in late March or April before bud break, and calls it a common mistake to spray on the first warm day of February or March, before the insects are active enough to be smothered by the oil. It says to wait until as close to bud break as possible.

Plants the sources name as sensitive

Maryland says oil removes the leaf wax that gives blue and gray foliage its colour and may permanently change the colour of treated leaves, and lists varieties of hosta, juniper, spruce, Arizona cypress, fothergilla and eucalyptus as examples. It says the label lists plants with known intolerance and that a small test area, watched for one or two days, is the route where tolerance is uncertain. Both Maryland and the Pacific Northwest handbooks say oil and sulfur do not mix: Maryland calls oil incompatible with sulfur and with some copper formulations, and the handbooks report that the two together burned grape leaves in one case.

What the sources say oils do and do not control

Maryland describes oil as a contact, broad-spectrum product with very limited residual activity that works by suffocating, disrupting cell membranes and repelling. Iowa State says it is effective only on pests thoroughly coated, which means small, immobile or slow ones, and names pine needle scale, oystershell scale, euonymus scale, aphids, spider mites and small pine sawfly larvae as good candidates, with apple maggot, cabbageworms and white grubs not affected. The Pacific Northwest handbooks say oils work on all stages including eggs, only if the pest is present, and that preventive applications are of no use. NC State says insects are not expected to develop resistance to oils.