Here are answers to some of the most common questions we receive about garden plants. You will find concise information on general gardening techniques as well as plant selection and care. For detailed information on specific plant pests and problems refer to our Common Garden Pests and Problems page.

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Horticulture Questions and Answers

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What are horticultural oils?

Oils have been used for centuries to control insect and mite pests. In the past, horticultural oils were generally used during fall and winter as dormant oil sprays on deciduous shade and fruit trees, to kill overwintering aphids, scale, mites, tent caterpillars, webworms and leaf hoppers. Because the oils were heavy and could damage foliage, they were not used during bud break or when trees were in leaf. That has all changed with today's new ultra-refined  superior oils. These new horticultural oils can still be used as dormant oil, but can also be applied in the summer when plants are in full leaf. They can be used on both deciduous and evergreen plants, and some can even be used on vegetables throughout the growing season. They are definitely an effective low-toxicity alternative for managing pests when used properly with a good understanding of their benefits and limitations. Recent research even holds promise for horticultural oil's role as a fungicide against powdery mildew on lilac, rose, horse chestnut and other plant species.

Oils differ in their effects on pest insects. The most important effect is that the oil blocks the air holes (spiracles) through which the insect breathes, causing it to suffocate. In some cases, the oil acts as a poison, interacting with the fatty acids of the insect and interfering with normal metabolism. In still other cases, the oil interferes with how the insect feeds, a feature that is particularly important in the transmission of some plant viruses by insects.

Oils integrate well with biological pest control methods, as they pose few risks to people or to desirable plant and animal species. Oils dissipate quickly through evaporation, leaving little residue. They are easy to apply with existing spray equipment, and can be mixed with many other pesticides to extend their performance.

The main limitation of spray oils is their small but real potential to cause plant injury in sensitive plants. Applying horticultural oils to blue spruce or blue rug junipers will result in a temporary loss of the blue color, although the plants will recover after a season or two. To determine a particular plant's sensitivity to oils, spot treat the plant. This will determine whether oils can safely be used on that plant.

Following are additional precautions to use when using oil on a woody plant.

1. Do not apply oils when temperatures are above 100 degrees or below freezing. Plants under stress may be damaged.

2. Do not apply oils if plants are wet, if rain is likely, or if the humidity is above 90%. These conditions inhibit oil evaporation.

3. Do not spray oil on plants when new shoots are growing.

4. Do not treat plants after winter hardening has occurred, as doing so may lead to increased susceptibility to winter injury.

5. Do not apply oils in combination with sulfur-containing pesticides. They can react with oils to form phytotoxic compounds that will damage foliage.