Garden Detective: Infested impatiens

DEAR JESSICA: For the past three years, small sugar ants have eaten the roots of my impatiens. This year was the worst: They even ate the plants. I tried planting marigolds with them, but they ate the impatiens, anyway. Any suggestions? I do use fresh potting soil every year. Please help! -- Donna Eden, North Babylon

DEAR DONNA: This is a case of things not being what they seem: It's not the ants; ants don't kill plants. I'm guessing your impatiens are infested with aphids.

Also known as plant lice, aphids are tiny, flat insects that typically attach their piercing, sucking mouthparts to the undersides of leaves, where you don't readily notice them, and proceed to suck the life out of your plants, oftentimes injecting a virus for a one-two punch that can be what's killing your impatiens.

Aphids also excrete a sweet, sticky substance called honeydew, which is what attracts the ants. Because they come in droves to partake in the honeydew treat, ants usually are blamed for the damage.

While ants do not eat flowers, they do disturb the soil and can dig up little mounds as they trail over it. And I can understand your need to eliminate them to prevent them from entering your home. An easy way to control them is with store-bought ant traps set on the soil in each container.

It's more important, however, to eliminate the aphids.

Look closely at the undersides of leaves, and you're likely to see them hiding there. Often, they can be washed away easily with a hose. If the problem is extensive, you might try mixing two tablespoons of dishwashing liquid (not clothes detergent, soap containing grease-cutters or automatic dishwashing soap) into a gallon of water and spraying directly onto aphids under leaves and on stems. You should repeat this weekly until they're all gone.

If the problem persists, apply insecticidal soap, which is available at most retail garden centers, according to package directions. When you've gained control over the aphids, most of the ants should disappear, too.

DEAR JESSICA: When is it too late in the season to put down mulch? I had planned to do it earlier in the season but never got around to it and now am wondering if I should wait until spring, or if my established plants should have it for the winter. -- Elke Stoiber, Merrick

DEAR ELKE: Winter mulch is beneficial when applied after the ground is frozen, typically not until November. Applying mulch now could trap heat in the soil and interfere with some plants' ability to go dormant before winter weather hits.

DEAR JESSICA: I have a potted, baby Colorado blue spruce about 10 inches tall. Should I bring it in for the winter? I also have a large hibiscus, which I know has to come inside. The plant is thriving, but the large pot is full of ants. Is there any way to get rid of the ants without hurting the plant, before I bring it in? -- Pat Carrozza, Franklin Square

DEAR PAT: Your spruce should be fine outdoors with the proper protection. Either plant the tree directly in the garden or sink it in the pot up to the rim, to provide insulation for the roots, and add a 3-inch layer of a mulch like straw when the ground freezes. Ridding a plant of ants is easy: Remove it from its pot, gently remove soil from the roots, and rinse the plant and roots in water. Repot it in a fresh, sterile potting mix and sit it by a sunny window. Expect your hibiscus to drop leaves and look sickly over the winter, but it should bounce back outdoors in May.

DEAR JESSICA: I have two crape myrtle trees next to my front door and am at a loss as to how to manage them. It took three years for them to bloom, and now that they have, I am afraid to trim them. How do I prune them and when?-- Jim Baccoli, Farmingville

DEAR JIM: Crape myrtles don't require regular pruning. Just remove dead, diseased or damaged wood as necessary. Because they bloom on new growth, pruning should be timed for late winter when trees are dormant, before new growth begins.