Although generally considered safe, oils can
injure susceptible plant species. Symptoms of plant injury (phytotoxicity) may
be acute or chronic. They can include leaf scorching and browning, defoliation,
reduced flowering and stunted growth. Phytotoxicity may be associated with
plant stress, ambient temperature and humidity, and application rate. It can
vary among plant species and cultivars. To reduce the risk of phytotoxicity, do
not treat stressed plants. Apply when conditions are below 85 °F degrees and 90
percent humidity. Applications during the summer season are best in the morning
or late evening. The longer wet oil sprays remain on foliage, the greater the
chance of phytotoxicity.
During winter it is best to apply oils only when
temperatures are above 40 °F. Apply dormant oils or higher rates of summer oils
only after stems and buds have become winter-hardened and before buds begin to
swell in the spring. Evergreen trees generally should be treated only by summer
rates of all-season oils. Some evergreens, especially those with a glaucous
(waxy) coat, may become discolored following an oil application. This usually
does not harm the tree or shrub.
When treating a new kind of plant, it is best to
apply horticultural oils to part of the plant or to a few small specimens
before treating large quantities of foliage. With oils it is especially
important to read, and follow label instructions and recommendations.
Manufacturers’ labels provide useful information about sensitive plant species
based on extensive testing. Some plants most commonly listed as being oil
sensitive include azalea, carnation, fuchsia, hibiscus, impatiens, photinia, rose,
cryptomeria, juniper, Japanese holly and spruce.
Oils have many
characteristics that make them desirable to growers and homeowners. For
example, they are low in toxicity to humans, wildlife and pets. Since oils are
only active for a short time, they do not Scale insects can be controlled with
horticultural oils. Oils will separate from the carrier. Agitation is necessary
to keep oils in solution. Affect insect predators or parasitoids unless they
are exposed to the direct spray. Oils evaporate quickly and do not generally
contaminate the soil or groundwater sources. Plant and fish oils are broken
down rapidly by microorganisms on plants or soil, and pose minimum risk to
non-target organisms. Oils are also considered one of the few classes of
pesticides to which insects and mites have not developed resistance.