Elinor Teague

Horticultural oils are great tools for controlling and reducing pest insect populations and for treating fungal problems.  The oils kill on contact by coating and suffocating the larvae, eggs, adults and nymphs of soft-bodied insects, including aphids, whiteflies, scale, leaf-miners and red spider mites. They also disrupt the insects' summer feeding patterns.

In winter, when sprayed on fully dormant trees and bushes, the oils further reduce pest insects' populations before they become active again in spring. When added to the regular practice of good garden sanitation (cleaning up litter, weeds and pest-infested leaves and debris as well as picking up fallen fruits and removing dried fruit mummies from bushes and trees) home gardeners should see very few, easily manageable infestations of soft-bodied pest insects.

Horticultural oils can also act as preventative fungicides. The oils coat leaf and stem surfaces, smothering the spores or pathogens and preventing them from attaching to the tissue and germinating. Success in controlling fungus problems depends greatly on the type of fungus being treated, weather patterns and timing of applications. Powdery mildew and black spot seem to be more easily controlled with applications of horticultural oils than rust or botrytis.    

There are several weights and types of horticultural oils available to home gardeners and label descriptions often use terms that can be confusing. Horticultural oils can be formulated from petroleum-derived mineral oils or can be plant-based, such as neem oil, jojoba oil and canola oil.  

Superior and supreme oils are interchangeable terms. Both are light weight, highly refined mineral oils and do not contain sulfur, which can harm sensitive plants. Supreme and superior oils can be applied when plants have leaves.  Some newer formulations of dormant oils are now labeled as supreme or superior oils. Dormant oils not labeled as supreme or superior should only be applied during the winter dormancy season.

Dormant oils are slightly more viscous or heavier than summer-weight oils and are sprayed on deciduous plants and trees in winter when the plants are fully dormant and leafless, before bud break in spring. Applying heavier-weight dormant oils in summer when temperatures are high risks burning plant tissues.

Summer oils are a type of superior oil. They can be applied in every season but might not be as effective as dormant oils in winter or during spring rainy seasons.

Year-round oils can be safely used in every season. Check labels for temperature ranges. Neem oil is a year-round oil, as are superior oils.

Horticultural oils should be sprayed to drench every surface – the undersides of leaves, branch crotches and bark cracks.  They are ineffective during freezing weather and can cause damage to plant tissues when temperatures are above 90 degrees. Labels provide information on application rates during every season and also list plants that are sensitive to the specific horticultural oil.

Horticultural oils should not be sprayed on drought-stressed plants. Monitor soil moisture levels and deep irrigate trees and bushes that may be drought-stressed before applying horticultural oils. During our hot summers do not spray plants when they are in full sun even if temperatures are below 90 degrees.

Sources:

Monterey Horticultural oil, GrowOrganic.com

Oils: Important Garden Pesticides, Pests in the Urban Landscape Blog. Author: Mary Louise Flint, March 11, 2014

Horticultural oil spray: How dormant oil works, EpicGardening.com, June 20, 2023

Tha author's neighbor spotted a Wilson's warbler in a female Chinese pistache nearby. The tree produces berries, which attract a wide variety of birds. In California, however, many gardeners opt for male trees, which may produce less litter. (Photo: PIXNIO)

I had never noticed that those three trees were loaded with berries and full of small birds in fall. Maybe that's because almost all the trees in my neighborhood are male varieties; it's rare to see female or flower and fruit-bearing landscape trees in any neighborhood.

The rationale for planting only male trees has been that female fruit, nut and seed-bearing trees trees create litter that can make a mess on sidewalks, patios and cars. They can also attract rats, squirrels and other vermin. Male trees aren't as messy, but they do produce pollen – one of the main reasons allergy problems have increased in recent years. Nurseries often stock only male trees.  

According to a report on the Audubon website, the population of birds in North America is nearly three billion fewer than in 1970. Other sources state that 90% of the decline in the U.S. bird population is in just 12 bird families, including small warblers, sparrows and finches, seed and berry-eating birds that are very common in our urban neighborhoods. The decline is driven by climate change, heat stress, urbanization and the loss of agricultural land. Some birds – including crows, riparian black Phoebes and Anna's hummingbirds – have been better able to adapt to changing conditions, unlike desert birds whose populations have plummeted. 

We try to encourage home gardeners to plant bee- and pollinator-friendly gardens that include a variety of flowering plants that bloom at differing times in order to provide a constant source of pollen and nectar for beneficial insects (and hummingbirds). We might also stress the need to plant a variety of flowering trees and bushes in home gardens, which will provide shelter, nest-building materials, habitat, seeds, fruit, sap and insects for birds throughout the seasons.

Native trees, which will attract and feed native birds, are the obvious first choice, but finding them can be difficult. Many new housing developments have very limited yards or green space and lack parking strips where trees can be planted, so size at maturity is another factor. Nursery labels often don't indicate whether trees and bushes are native, whether they produce seed that is edible for birds, or when fruit and seed production will take place. However, more local nurseries and garden centers are setting aside space for and emphasizing native plants, and their staff is becoming more knowledgeable.

The most bird-friendly Central Valley gardens would include a few smaller evergreen conifers, one deciduous fruit or flowering tree, two or three groupings of perennial seed and berry bushes as well as flowering annuals and perennials that bloom over the length of the growing season and hold seed through the winter.

Sources:

Plant Native Trees That Attract Birds, Birdsandblooms.com, Jan. 21, 2022   

Climate change, urbanization drive major declines in birds, UC Berkeley News, Feb.22, 2023

North America has lost more than 1 in 4 birds in the last 50 years, new study says, Audubon, Sept. 19, 2009

"The Allergy-Fighting Garden," Thomas Leo Ogren, 2015, Ten Speed Press, ISBN: 978-1-60774-491-7