I want to improve the health and appearance of my garden and recently received a proposal for four separate applications for my trees and shrubs to fertilize and control insects and diseases. Should I sign up for this?
— Paul Stewart, Riverwoods
I would most definitely not subscribe to any blanket spray programs to apply pesticides to trees and shrubs. There can be benefits to a fertility or soil health program and there are some diseases like apple scab that do require preventive fungicide sprays to control should you decide to do so. But any application of insecticides or fungicides should be targeted to control a specific pest or disease that is present in the landscape. The potential harm to the environment is high, as any insecticides applied can kill pollinators and other beneficial insects in addition to the pest. I have only applied an insecticide once over a period of 35 years in my home garden and it was to control viburnum leaf beetle on arrowwood viburnum. This insect will kill arrowwood viburnums when its population gets large enough and the shrubs are defoliated over a period of years. It’s been three years since I treated them, and the damage from this insect now is minor. I do not plan to treat them in the future. Once the arrowwood viburnums are killed by this insect, I will replace them with another shrub. I have a garden full of birds and pollinators with great light shows in the evenings from fireflies in the backyard. I also have two dachshunds that spend a good amount of time in the backyard. If I don’t focus on weeds and some grooming that needs to be done, my garden looks good without a spray program.
The approach I recommend for most home gardens is integrated pest management. Select the right plant for your design goals and the conditions present in your garden, plant them properly and practice good maintenance. This will result in healthy plants under less environmental stress, and will present you with fewer insect and disease problems to address. The plants will also look better. Monitor (or hire someone to monitor) the garden to identify pests and diseases before applying a control. Then choose the proper timing for the best results and the least toxic product for control. Another tenet of integrated pest management is that one accepts a certain amount of damage to the plants before applying any product for control.
One example of managing a pest with an integrated pest management approach is controlling magnolia scale. This pest was prevalent in gardens a couple of years ago and in high enough populations to kill magnolia trees without controlling it. Black sooty mold forming on leaves from the honeydew dripping from the scale tells you it’s present if you have not been monitoring the branches. Depending on the severity of the infestation, you can wait until early spring to apply a dormant oil with very low environmental impact to kill the scale. There is also a late-summer timeframe when an insecticide can be applied to control the insect in its crawler stage. Monitor magnolia trees going forward, but there is no need to rely on an annual spray program for magnolia scale as you should treat this pest when necessary and not preemptively.
For more plant advice, contact the Plant Information Service at the Chicago Botanic Garden at [email protected]. Tim Johnson is senior director of horticulture at the Chicago Botanic Garden.