David Deardorff, PhD, former professor and researcher at University of Hawaii and Washington State University. He is coauthor of several books about plants including What’s Wrong with My Plant? (And How Do I Fix It?). KathrynAndDavid.com
Bottom Line: Protect your landscaping without chemicals that hurt people, animals or birds.
The toxic chemicals frequently used to treat common plant problems can be bad for people and animals. They can get into water supplies, be ingested by pets and wildlife or get inhaled when we apply them. But these chemicals are not the only solutions for the pests and diseases that can plague bushes and trees. Here are safe solutions to four problems that many home owners face. In most cases these are affordable, practical do-it-yourself solutions, though in the case of taller trees, you might need to enlist an arborist with equipment that can reach high enough.
A few other insects, such as white flies, excrete a similar sticky varnish. To confirm that aphids are the pest in question, check the undersides of the tree’s or bush’s leaves for large numbers of very small insects that have relatively long antennae.
What to do: Spray an insecticidal soap liberally onto the leaves of affected trees and bushes. These soaps are deadly to aphids but safe for people and animals. Example: A brand called Safer Insect Killing Soap (around $10 for 32 ounces). Apply about every seven days when aphids are present. (Despite what you might hear from a neighbor or online, spraying diluted dishwashing soap is not a good idea—although it can indeed kill aphids, the detergents in these products are bad for plants.)
When you spray for aphids, also check whether there are large numbers of ants on and around the tree or bush. Some ant species herd aphids onto plants so that they can feast on the waste that the aphids produce—it’s full of sugar. Get rid of these ants by wrapping a “sticky tree band” around the trunk of the tree or bush—an adhesive product such as Tanglefoot Insect Barrier spread onto a narrow, paperlike band that’s wrapped around the trunk ($17 for a kit). When ants try to climb the tree, they get stuck and die.
Also sprinkle diatomaceous earth around the trunk of the affected plant. This is a powder derived from the fossils of ancient marine algae. It is not at all harmful to people or animals, but on a microscopic level, it’s sharp enough to slice into bugs’ bodies if they walk across it.
This fungal disease makes affected plants look as if their leaves have been dusted with flour. Rub a leaf gently with your finger—if powdery mildew fungus is the issue, the powder will rub off easily.
What to do: Make a mix of one tablespoon baking soda and one tablespoon liquid hand soap—not dishwashing liquid—per gallon of water. Then spray a generous amount of this mixture onto the leaves of affected trees and bushes as well as surrounding plants. The baking soda raises the pH of plant leaves, which can kill fungus. The soap acts as a spreader and stabilizer—without it, the baking soda/water solution would bead up on leaves and roll off. Apply once a week, and reapply following rain.
Caterpillars eat the leaves of trees and bushes, and some types of caterpillars eat other parts of plants as well, such as the flowers, seeds or fruits. Their voracious appetites can do enormous harm—in large numbers caterpillars can virtually defoliate trees or bushes.
What to do: If you see lots of caterpillars on one of your trees or in bushes—or many of the plant’s leaves are riddled with holes where caterpillars have dined—spray a bacteriological parasite known as Bacillus thuringiensis, often called “Bt,” onto the plant. This bacterium is deadly to almost all common types of caterpillars, but when used as directed, it is harmless to people and animals. Brand names include DiPel (about $25 for a one-pound bag of DiPel Pro DF) and Thuricide (about $9 for an eight-ounce bottle).
Root weevils are beetles, but they start life under the soil as larvae feeding on plant roots, often doing tremendous damage. Adult root weevils climb up the plants to feed on their leaves—you may notice notches along leaf edges. Bushes and trees that can be victimized include azalea, hemlock, lilac, privet, rhododendron, spruce, yew and many more.
Unlike with many beetles, with weevils you’re more likely to notice the notches they eat out of leaf edges than you are to see the insects themselves—even the adults live in the soil and come out mainly at night.
What to do: Apply beneficial nematodes to the soil surrounding affected trees and bushes, ideally in the spring. These are microscopic roundworms that get inside the gut of certain soil-dwelling insects including root weevils and then release a type of bacteria that’s deadly to these pests but harmless to people and animals. Purchase nematodes from an established seller such as Arbico Organics ($36 for NemaSeek, which contains five million nematodes—enough to treat up to 218 square feet). Confirm that the product you select specifically lists weevils among the pests it can control—different beneficial nematodes kill different insects.
There’s one strategy that will greatly reduce the risk to your trees and bushes from an extremely wide range of different pests and diseases—but it’s a solution that’s best implemented before your plants are even in the ground.
Position trees and bushes so that their branches will not touch other trees and bushes of the same species even when they reach their full, mature size. (The nursery where you purchase your trees and bushes, or your landscaper, can tell you how wide a particular tree or shrub will grow.) Or better yet, don’t position plants of the same species anywhere near one another in your yard.
Many pests and diseases affect only a very small percentage of plants. As long as a tree or shrub is not near another of the same type, pest and disease problems tend to remain contained—one of your plants might develop a problem, but it’s unlikely to spread to the rest of your landscaping.