Keep spider mites in check to protect your plants
“It started as just a few brown spots near the bottom of the tree against the building,” my friend said as she pointed to a nearly dead dwarf Alberta spruce.
Only weeks before, it had been green and full, but now only the tips of the branches showed life. All the internal needles were brown and falling; the tree would never recover.
It’s a story that repeats itself every year in the Inland Northwest when spring dries up into summer and aphids give way to spider mites.
Like aphids, spider mites have piercing mouthparts that they use to feed on the nutrients inside plants. Like aphids, they are found on the underside of plant leaves, but unlike aphids, spider mites produce webbing that eventually covers the underside of the infested leaves or branches.
At only 1 mm in size, spider mites can be hard to see without a magnifying glass.
To determine if your plant has them, shake a leaf from the host plant over a blank sheet of paper. If you see small dots running around the page, you probably have mites.
Spider mites have a wide range of host plants, including many deciduous trees and shrubs, as well as conifers. They can infest annuals, perennials, vegetables and even house plants.
They favor hot, dry conditions and are active in our area from June through frost. Symptoms include stippled yellowing foliage (or browning needles on conifers) that move up from the bottom of the plant and from the inner foliage toward the tips. Damage is progressive and in severe cases leads to defoliation and even death.
Fortunately we have some powerful weapons in the war against these tiny terrors.
Space your plants to prevent overcrowding, which helps spider mites spread. Fertilize with slow-release nitrogen to prevent too much succulent new growth, and if you suspect mites, spray the underside of the leaves with water, to wash them off the plant.
On woody plants, use a dormant oil in late winter to destroy their eggs.
Insecticidal soap is effective against mites, but test it on your plant first, to make sure it doesn’t burn the foliage. Spider mites are not insects, so any pesticide you use must include a miticide, not just an insecticide.
Always follow the directions on any chemical application.
Spider mites have natural enemies, including predator mites, so steer clear of broad-spectrum products that will kill the good with the bad. If you decide to spray, call the Master Gardener Plant Clinic for recommendations.
Once a plant has spider mites, they usually will reappear every year, so if you have a plant that keeps getting hit, you might consider replacing it with a different species.
This week in the garden
It’s a good idea to take a walk around to spot any potential problems. It’s easier to fix a small problem than a large one, and addressing problems now might spare you from hard work in the heat of July.
Pull weeds before they can disperse seeds, and if the weed is in flower, don’t compost it at home or you’ll have seeds all over your garden next spring. Cut spent flowers off annuals and perennials.
After roses finish blooming, cut the stems back to the first leaf with five leaflets.
If you need to fertilize, do it before the Fourth of July, to prevent stressing the plants during the hottest part of the summer.
Now is also a good time to cut suckers and water sprouts (those fast-growing branches that shoot straight up) off your ornamental and fruit trees. You also can prune spring-blooming shrubs, like lilacs, as soon as they finish flowering.
About a month after iris finish blooming, you can divide and replant them. Remember to leave a little bit of the rhizome exposed when you replant them.
Don’t forget to relax and enjoy your landscape. After all your hard work this spring, you’ve earned it.