Fall Colour & Dormant Oil Applications

November-December 2005

NOVEMBER HAS ARRIVED. Here we are in the midst of shorter days, lower light intensities and cool conditions. These environmental cues tend to make me want to curl up in my flannel pajamas and take a long nap. Similarly in the plant world, these cues stimulate the preparation for dormancy with leaf senescence – where reserves are shunted into the root system for next spring’s first flush. These withered leaves are dropping (abscising) all around us and their remains will provide organic nutrients for years to come. Senescence and abscission are adaptive processes that are used to help the plant face an environment of dwindling heat and light.

It is that point between senescence and abscission that makes us so proud to be Canadians. That’s right, I’m talking about fall colour. Like senescence, fall colour is also stimulated by shorter days and cooler temperatures. Original theories explained the fall colour change as the degradation of green pigment (chlorophyll) and the subsequent “unmasking” of other colour pigments that were there all summer. If that is true, then why is colour expression most spectacular after warm, sunny autumn days? Although some pigments may have been there all season long, newer theories about fall colour indicate that at least some fall colour pigments are manufactured as the chlorophyll degrades, and that sunlight is a necessary part of the process. Sunlight is used to build the yellow, orange and red pigments (carotenoids, anthocyanins) as they replace the chlorophyll. The result is very magical, but why does it happen?

Most theories indicate that fall colour is inconsequential but a recent discussion paper in the Journal of Theoretical Biology (Archetti, M. 2000. J. theor. Biol. 205, 000-000) suggests that fall colour may be an adaptive response, part of the co-evolution between trees and other organisms, specifically insects. Archetti states that other researchers have proposed that autumn pigments may have a protective effect for the trees. The theory goes something like this: vigorous trees produce brighter colour intensities than weaker trees sending a signal to insect pests not to colonize the healthier trees but to choose trees with “duller” hues. Although it is an interesting theory, there is no field data to support it (or at least there wasn’t in 2000). If I were to contribute a theory about fall colour, it would be as follows. Brightly coloured trees are greatly prized by humans. By producing the most intense hues in autumn, you can guarantee that your offspring will be distributed throughout the ecosystem whether by sexual (seeds) or vegetative (cuttings, grafting) means. Somehow I don’t think anyone will be calling me about my theory.…

Dormant Oil in the Fall?
Dormant oil. It’s not just for early spring. The cool weeks of autumn are a fantastic time for applying dormant oil to help knock down insect and mite populations. Sometimes the climactic cycles of autumn are a little more gradual in terms of temperature shifts. This makes it easier to schedule dormant oil applications. The other great advantage to fall application is that the insect populations are much easier to detect. Whereas after the winter months, insects like magnolia scale nymphs will have turned grey and become almost undistinguishable from the twigs. I’ve heard great things about dormant oil helping to manage magnolia scale, euonymus scale, spruce spider mite and spruce gall adelgid. This is a non-toxic product of which we could be making more use. (That said, a welcome addition to our toolkit would be a nice pure, microfine summer oil.)

Dormant oils provide a physical barrier which restricts both the respiration and movement of many over-wintering insects (e.g. adelgids, scales, mites). To avoid foliar damage, dormant oil is best used when min/max temperatures remain somewhere between 5 and 15° C during the first week or so after application. This is especially important on evergreens since their foliage is always present.

Always read the label carefully since many plant species are sensitive to dormant oils (see below). I’ve had professionals tell me that they have used dormant oil on Taxus and Thuja for years and never had a problem. Others have seen quite a bit of burn on those two genera – yikes. I’ve also had a lot of reports of foliar burn where sulfur was used alongside the oil, or was used within 30 days of oil application. Remember, oil and water do not mix. You will need to agitate the mixture constantly in order to ensure even coverage. If over-application occurs and extreme temperatures are experienced within a week or two, foliar damage may result. Applications should be made during mild mornings, when no rain is in the forecast, to facilitate drying.

Plants Sensitive to Dormant Oil Treatments
• Acer palmatum (Japanese maple), Acer rubrum (red maple), Acer saccharum (sugar maple)
• Carya (hickory)
• Cryptomeria, Juglans sp. (walnut)
• Juniperus (blue cultivar selections)
• Picea pungens glauca (blue Colerado spruce)
• Pinus strobus (white pine)
• Quercus rubra (red oak)
• Taxus (yew)
• Thuja (cedar)

Other, Less-Sensitive Plants to Dormant Oil Treatments
• Cercis canadensis (redbud)
• Fagus (beech)
• Ilex crenata (Japanese holly)
• Picea abies (Norway spruce), Picea glauca (white spruce)
• Pseudotsuga menziesii (Douglas fir) ©

— Jen Llewellyn, Ontario Ministry of Agriculture and Food (OMAF) Nursery Crops Specialist

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