To control overwintering insects and diseases on fruit trees and other deciduous (non-evergreen) trees and shrubs, spray them with a mixture of dormant oil and lime sulfur during their winter’s sleep. This organic pesticide works to kill any exposed insects AND diseases. Common targeted insects include scale insect, spider mites, caterpillars and their exposed eggs. It also does a great job controlling common diseases on fruit trees such as peach leaf curl, apple scab and powdery mildew. Use on roses to reduce black spot.
This dormant spray solution is less harmful to pollinating insects and other beneficial bugs as they are not around this time of year. And unlike other pesticides, the insects and fungi do not develop a resistance; a common issue especially when using fungicides.
Apply while plants are still dormant and have not yet sprouted foliage or buds. Spray when there is no rain, snow or frost predicted for 24 hours. Temperature should remain at 0°C (32°F) or above for at least 24 hours. Avoid spraying on evergreens (cedars, rhododendrons etc.) as it may injure them. Apply in the morning so the plant will be dry by evening. Don’t apply if frost is predicated overnight.
Although this sounds like the window of applying this organic pesticide and fungicide is rather narrow, it's usually doable. And it works. I have noticed a huge difference in my apple tree especially when I neglect to get the dormant oil solution in time - like last year. My poor apple had lots of issues, much more so than previous years when I managed to get the job done.
Usually the end of January to the beginning of February is the window of opportunity, however, since I live in the temperate climate of southern British Columbia, spring sometimes come early, catching me off guard (in a nice way). There's been a couple of years when I the plants sprouted at the very beginning of February. Oh no! For most of Canada the window of opportunity is usually late February and into March.
Look for the dormant oil, lime sulfur duo packaged together at home hardware stores and garden centres. Get an extra box for next year as sometimes they are not in stores in time for us here in temperate and balmy B.C.
Also don't store dormant oil/lime sulfur outside during the winter. I kept mine in my shed and it froze, becoming totally useless. With any product, read the label thoroughly for how to use it, rates of application and cautions etc. Don't spray on a windy day for obvious reasons and avoid getting in the way of the spray while applying. Don’t mix more than you need as you cannot store it for later use. It will stain stone and concrete so cover them with a tarp or plastic before applying.
Avoid using dormant oil, lime sulfur on beech, hickory, Amur, sugar and Japanese maples, walnuts as well as redbuds and evergreens, especially Colorado blue spruce and holly. If you are worried about spray drift getting on these sensitive plants, temporarily cover them before spraying with a tarp or plastic.
Although dormant oil/lime sulfur is not 'toxic' to humans, cover yourself up as it's rather stinky. Wear rubber boots, cover your arms and legs, a hat, wear glasses (don't want it in your eyes) and rubber gloves. Wash your hands and face after applying even if you use a face mask.
Cover the plant entirely so it is dripping off and spray the ground too as it also harbours overwintering pests and diseases. To apply this product to tall plants, I use a trombone sprayer, but a backpack sprayer would be suitable. It makes a fine mist that goes quite high into the tree canopy. You can also use a hose end sprayer for smaller plants, but use the mist setting.
This one time application per year works wonders so consider it if you have buggy and diseased fruit trees, roses and other deciduous shrubs and trees.
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