Snow Impacts on Woody and Herbaceous Plants
This tip warns about the use of horticultural oils. A must read if planning on using horticultural oils on a woody plant.
The following precautions are recommended whenever using an oil on a woody plant:
- Avoid using oils on plants that tend to be oil-sensitive (Table 2). Avoid drift onto sensitive plants.
- Do not apply when temperatures are excessively high (above 100 degrees F) or low (below freezing). High temperature limitations are primarily related to the drought-stress status of the plant. Plants under stress may be damaged. Those not stressed are much less likely to be damaged by an oil application. Dry conditions without plant stress generally reduce risk of injury by oil, because evaporation is more rapid.
- Do not apply oils during freezing weather. This can cause the emulsion to break down and produce uneven coverage.
- Do not apply oils if plant tissues are wet or rain is likely. These conditions inhibit oil evaporation. High humidity (above 90 percent) also may contribute to injury risk, while low humidity generally reduces it.
- Do not spray when shoots are growing.
- Avoid treating plants during the fall until after winter hardening has occurred. Fall treatments have sometimes caused increased susceptibility to winter injury.
- Do not apply oils in combination with sulfur or sulfur-containing pesticides such as Captan or Karathane. They can react with oils to form phytotoxic compounds. Because elemental sulfur can persist for long periods, label directions on most oils prohibit their use within 30 days of a sulfur application.
Table 2: Plants that tend to be sensitive to oils.
- Black walnut
- Junipers and cedars
- Maples (particularly Japanese and red maple)
- Smoke tree
- Spruce (particularly dwarf Alberta spruce)
- The neem oil insecticides (Trilogy®) have been most widely used on greenhouse-grown ornamentals. They have shown good plant safety, but there are some precautions for use on impatiens, fuschia, hibiscus, some roses, ornamental olive and some carnation varieties.