Five tips for using horticultural oil

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Article by, Nicole Sanchez,
Horticulture, OSU Extension Service, Klamath County

Before fruit trees leaf out in spring, before the first signs of aphids, powdery mildew or obscure scale, home orchardists can undertake an important control step to minimize pesticide use later in the season. Application of dormant oil is highly recommended in the Klamath Basin for most fruit trees. Dormant oil, a preventative pesticide used both in organic and conventional production, is used against piercing-sucking insect pests, mites, and even some plant diseases.

Examples of problems remedied or reduced by dormant oil include scales and mites, aphids, pear psylla, leafrollers, and powdery mildew. Powdery mildew is a challenge for Klamath gardeners in years with a warm, wet spring. Spores move around on wind and water, infecting blossoms and buds. Damage shows up much later, when control is difficult to impossible. Using dormant oil- a low risk pesticide approved in organic agriculture- knocks back pests before they can establish for the growing season. This reduces the need to apply less environmentally friendly products later.

In most home orchards, an application of dormant oil is preferable to other types for another important reason: ease of application. Getting good coverage of all the cracks, crevices, and fissures in the bark where insects, eggs and diseases hide is crucial. Getting the pesticide where it needs to be after the tree is leafed out is considerably more difficult, and requires more product. Dormant oil can be applied before leaves open and new buds swell, getting in the way.

Timing is critical.
Soon, fruit trees in the Klamath Basin will be actively growing. Once the trees “wake up”, the window for dormant oil is passed and a different formulation will be needed. All the cracks, crevices, and buds where pest eggs lurk will become difficult to access. Dormant oil applied when trees are actively growing can cause damage.

Levels of refinement.
Dormant oil, horticultural oil, and superior oil are similar petroleum-based products. Horticultural oil is often used as a generic “catch-all” to indicate any of the oils, while dormant oil is specific to the formulation applied only before active growth starts in the spring (during dormancy, thus the name). Horticultural oil and superior oil are more refined than dormant oil. These further-refined oils can be used later in the growing season, but should be applied only after a careful check of the label to check temperature, plant type, and timing parameters. A few formulations can be used in both dormant and growing stages.

Double dose for scales.
Scale insects are protected by “shells” that make penetration by any pesticide difficult. Oils dessicate and break down these shells, reducing the scale’s protection. The best approach for a scale infestation to apply dormant oil before bud break, then a second application of a horticultural oil – one timed specifically to the life cycle of the scale insect. Right after hatching, scale insects are in a mobile stage called “crawlers” where they move round searching for feeding spots- and have not yet developed the protective shells. Application of oil during the crawler stage is highly effective to reduce scale populations.

Organic and conventional.
Like the copper-based fungicides and neem products, many oil formulations are used in both organic and conventional agriculture. Because dormant oil is applied so early in the season, unintended effects to pollinators or other beneficial insects are minimal. Other horticultural oils should not be applied while plants are in bloom, as some pollinator toxicity to horticultural oils has been observed. In general, any pesticide application should be timed to avoid blooming plants.

Temperature and tissue limits.
Dormant and horticultural oils are not a silver bullet: their dessicating properties are not for every plant type, and potential for damage increases at high and low temperature extremes. Generally, the temperature range for applying dormant oil is between 40-80F. Oil applied outside that temperature range may cause damage. There are some plants that are sensitive to the oils at any temperature: often listed are maples and some types of evergreens. If used on “blue” evergreens, the bloom that causes the blue color may be affected. Every brand or formulation varies- it’s always best to read the whole label before application.

In past decades, preventative pesticide treatments were frequently recommended, even for home gardeners. Practices around pesticide application have evolved: : now most preventative actions are based on models that predict when a particular pest is likely to become an issue, rather than on a calendar. Application of dormant oil for fruit trees is one of the few calendar-based preventative treatments that still makes sense- ecological risks are low, and the treatment helps reduce the need for use of other pesticides later in the growing season.