Oil in your Garden

Norma Evans


Some gardeners may feel their joints will need a bit of oil come spring, but applying dormant oil can be an effective, non toxic and inexpensive control for insects that manage to overwinter in Ontario gardens. Mealy bugs and the eggs of many insects and mites can be controlled. Aphids, mites and scale insects are very susceptible to oil spray because they hide in tree bark and branch cracks and on evergreen needles.

If these pests were alive and well in your garden last year, youíll probably find them again in 2005. Both adult and immature insects breathe through pores on their backs. Once these pores are plugged the insect suffocates.

The description "dormant" oil is confusing. It does not describe the oil, but the time of application (when plants are dormant). Very little oil is required as the ratio is 1 to 5 parts oil and 95 parts water. This works out to be two to six teaspoons of oil to one quart of water. The oil in sprays breaks down rapidly on contact with soil.

Horticultural oil is available in most garden supply outlets. For a goodly number of fruit trees, ornamental shrubs and roses, etc. commercial horticultural oil is recommended. For small areas such as one tree and one bush which is what this gardener sprays, any sort of light oil will do a good job. Some books suggest veggie oil (corn is light), others suggest mineral oil. Baby oil has worked for this author treating a Mountain ash tree for two years and seems to be doing a satisfactory job.

The quicker flowing oils mix better with water, but a couple of drops of detergent will help. Only creatures covered with oil are affected so thoroughly soak the bark. It is also necessary to keep shaking the mixture as oil and water donít stay mixed. Birds landing on oil-treated plants are not affected, nor are bees.

Not all plants are candidates for oil spray. Any blue-green evergreen such as Colorado Blue Spruce loses its bloom, and turns green. Norway and White Spruce, Beech, Hickory, Red and Silver Maple and Black Walnut may be injured by oil sprays.

The optimum temperature for spraying is just before or when dormant buds are about to open and the thermometer is about 45 degrees Fahrenheit or 7 degrees Centigrade. Spraying should be done on a calm dry day when the spray can dry without freezing (drying takes about 48 hours).

Commercial spray kits sometimes come with lime-sulphur to be mixed in with the oil and water. This will control overwintering micro-organisms. If you havenít had problems with fungus or bacteria you donít need this extra control since the lime-sulphur is not preventative but just treats what was previously in existence. For black spot on roses, the lime-sulphur addition may provide some control. Finely ground sulphur is available and can be added to home prepared mixes.

While you are out meandering in your garden, take your pruners with you. The ideal time to prune many plants is when they are dormant. However, some plants bleed when pruned in spring so bypass Birch, Honey Locust, Maple, Poplar, Linden, Walnut, Virginia Creeper and Hickory. Wait until early summer to prune these. Early spring is a good time to prune shrubs that bloom later in the summer like Hydrangea, but spring blooming shrubs should be pruned after blooming since doing so earlier will reduce the number of blooms. Roses are usually pruned when the forsythia is in bloom.

Remove broken, dead or diseased branches. Very old, thin, weak wood is best removed as well. Do not prune while the wood is frozen and try not to take more than 20-30% of the shrub in any one year. Before leafout, it is easier to make decisions as to which branches or limbs must be removed and cuts tend to close comparatively quickly in the spring. If you prune too late in the season, it encourages the plant to develop new growth at the expense of getting ready for winter. The new growth does not have enough time to harden.