There are several reasons to spray deciduous plants in the winter. During the year, when trees and shrubs are fully leafed out, it is hard to get at every surface that might host detrimental insects or harbor fungal spores. After the leaves have fallen (and been composted to disarm those pathogens), it is much easier to get into the cracks and crevices where the last holdouts may be hiding.
As always, it is best to use the least toxic materials to deal with these pests. For insects like scale, mites, and mealybugs, horticultural oil is a purely physical treatment that is safe for the trees. It works to smother the insects, even in their dormant state (often as eggs that will hatch in spring), by coating them with a fine film of oil. There are products that combine oil with other pesticides, but the oil is sufficient to do the job. Buy only pure oil or products that may have a soap added to make them spread and stick to the surfaces. Remember that any product you use to control a pest will likely also affect some of the beneficial insects, so use caution. Complete coverage is paramount for good control, but it should not be necessary to spray every year.
Fungal pathogens are a little harder to squelch. If your infestation is spotty, try an oil spray and meticulous cleanup of infected foliage. If peach leaf curl-which can infect nectarines, plums, and other stone fruits-is rampant, then you may have to resort to more toxic concoctions. For very serious infestations, a lime sulfur compound can be used with due caution. Do not make a habit of this practice. It is better to try and promote a natural ecosystem of other fungi and bacteria that can keep a healthy balance between the bad guys and the good guys. If you are making compost tea, spray trees while they are dormant and continue to spray throughout the growing season for the least invasive treatment. Whatever you use, you may want to repeat it a couple of times during the next month or so before buds swell. Hold off if rain is predicted and allow at least 48 hours of dry weather to be effective.
•Enjoy the beginning of a long season of orchid flowers as cymbidiums begin to bloom this month. Fertilize with a dilute balanced formula to keep them going.
•There is still time to plant bare-root fruit trees, roses, and other deciduous woody plants.
•Shop for winter-flowering shrubs such as camellia and azalea. Both benefit from soil very rich in organic material and mulched with more acid materials like pine needles.
•Watch for snails and slugs. As temperatures begin to rise (spring does come early here), they will be hatching and munching.