What's Eating Your Japanese Maple?

Dormant horticultural oil in late winter

Spray the entire tree with horticultural oil (Bonide All Seasons, ~$15) in late February or early March before bud break. The oil smothers overwintering scale on the bark. Coverage is the whole game. Hit every twig, the undersides of branches, and the trunk crotches. One thorough dormant spray prevents most of the season's pressure.

Summer oil at the crawler stage, every 10 days for 3 rounds

Prune out heavily infested branches

When a branch is lined with egg masses end to end, cut it out below the lowest cluster and bag the prunings. This removes thousands of eggs in one cut and restores the tree's graceful branching habit. Sterilize pruners with isopropyl alcohol between cuts. Best done in late winter while the tree is dormant.

Deep water the root zone first

Drought stress is the trigger. Soak the root zone slowly with a hose at low flow for 30 to 60 minutes, out to the drip line. Mulch 2 to 3 inches deep with shredded bark, keeping mulch off the trunk. A well-watered Japanese maple resists mites on its own. This step alone clears most light infestations.

Strong water blast on the leaf undersides, every 3 days for 2 weeks

Hold a hose nozzle 12 inches from the foliage and spray the underside of leaves at high pressure. Mites can't reattach quickly when knocked off. Repeat every 3 days for 2 weeks. Works on container trees and small landscape trees within hose reach. Pair with the deep water above.

Skip broad-spectrum sprays

Pyrethroids and other broad-spectrum insecticides kill the predatory mites that already keep pest mites in check. Spider mite outbreaks on Japanese maple frequently follow a routine pesticide application. Stick with water and patience. The tree's own ecosystem suppresses mites once moisture and predator populations recover.

Hand-pick at dawn into soapy water

Floating row cover on small specimen trees

For young or small specimen Japanese maples, drape lightweight floating row cover (Agribon AG-19, ~$20) over the canopy through the 4 to 6 week beetle window. Tie loosely at the trunk. The cover blocks beetles without overheating the tree. Remove once peak emergence ends in late July.

Skip pheromone traps near the tree

Japanese beetle traps draw far more beetles than they catch. Hanging one near a Japanese maple guarantees worse damage. If a neighbor uses traps, ask them to place traps at the far property line, away from your tree.

Strong water blast every 2 to 3 days for 2 weeks

Hold a hose nozzle 12 inches from the affected new growth and spray at high pressure. Most aphids dislodge and don't make it back. Repeat every 2 to 3 days for 2 weeks. The fastest, cheapest fix and works without chemicals. Best done in the morning so leaves dry before evening.

Insecticidal soap on the new growth flush

Spray ready-to-use insecticidal soap (Safer Brand, ~$10) on the underside of leaves and tender new shoots at dusk. Coats and suffocates aphids on contact. Reapply after rain or every 5 days for 2 to 3 rounds. Avoid spraying open flowers and skip the hottest part of the day to prevent leaf burn.

Wash off the honeydew once the aphids are cleared

After the population crashes, hose down the leaves and any patio furniture or hardscape below the tree. Sooty mold doesn't damage the tree but dulls the red and burgundy foliage that's the whole point. Clean leaves photosynthesize better and the fall display comes back vibrant.