Rhododendron Problems: Vine Weevil, Bud Blast and Root Rot

Vine weevil: notched leaves above, grubs below

Vine weevil is the pest that actually kills rhododendrons, and it does all its damage where you cannot see it. The grubs are creamy white, comma-shaped, about a third of an inch (8 mm) long, with a distinct tan head and no legs. They eat the fine feeder roots through autumn and winter and then strip the bark from the stem base, and the first sign above ground is often a plant that wilts, collapses and lifts out of the soil with almost no roots attached.

The adults give you months of warning if you read them. They are matt black, slow, flightless beetles that feed at night and chew unmistakable angular notches out of the edges of the leaves, as though someone had worked round the margin with a small hole punch. Notched leaves do not harm the plant at all. They mean adults are present and laying eggs into the soil, and that is the moment to treat the root zone rather than the foliage.

Containers are far more at risk than open ground, because a pot concentrates the grubs in the small volume of compost the plant depends on. Any rhododendron potted for more than a season is worth checking, and the check is simple: tip it out and look at the outside of the root ball.

1. Look for notches from late spring Check the leaf margins, especially low down and on the shaded side. Angular bites out of the edge are vine weevil adults; ragged holes in the middle of the blade are something else.
2. Hunt the adults after dark Go out with a torch on a mild night and check the stems and the leaf undersides. Adults drop when disturbed, so hold a sheet of paper underneath before you touch the plant.
3. Water in nematodes while the soil is warm Parasitic nematodes are the reliable control for the grubs. Apply to moist compost or soil when it is above about 41F (5C), typically late summer into autumn, and keep it damp for two weeks afterwards.
4. Tip out any container plant and check Slide the root ball out and inspect the outside and the base. Grubs sit against the pot wall. Pick out every one you find, then repot into fresh compost rather than reusing the old.
5. Repeat the following season One treatment clears the generation present. Adults walk in from surrounding planting, so plan on checking for notches each year rather than assuming the problem is solved.

Bud blast: the buds that never open

Bud blast turns flower buds dry, gray-brown and hard through the winter, and they simply never open in spring. What identifies it beyond doubt is what grows out of the dead bud: fine black bristles, a millimeter or two long, standing up all over the surface like stubble. Nothing else on a rhododendron looks like that, and a frosted bud, which is brown and soft rather than dry and bristly, does not produce them.

The fungus is closely linked to the rhododendron leafhopper, which lays eggs into the developing buds in late summer and opens the wounds the fungus enters. Controlling the leafhopper reduces blast, but for a garden plant the practical control is mechanical: remove and destroy every blasted bud you can reach.

Do it through the winter, when the dead buds stand out clearly against the green foliage and there is nothing else to distract you. Snap or cut each one off, taking care as always not to damage the shoots behind it, and put them in the bin rather than the compost pile. The bristles are the spore-producing structures, and leaving blasted buds on the plant seeds next year's infection from a foot away.

Root rot and the difference between wilting and dying

Root rot comes from water standing around roots that need air, and rhododendrons are unusually easy to kill this way because their fine roots sit in the top few inches of soil. Heavy clay, a planting hole dug like a sump and backfilled with compost, a pot standing permanently in a full saucer, or a crown set an inch too deep will each do it.

The signature is a plant that wilts on a day when the soil is clearly damp and does not recover overnight. The leaves go dull olive, hang straight down and stay down, and watering makes it worse rather than better. Scrape the soil away at the base and the roots are dark, soft and sour-smelling instead of pale, springy and fibrous, and the bark at the stem base peels away brown underneath.

Compare that with drought wilt, where the leaves roll into tubes, the soil is dry to a finger pushed two inches (5 cm) down, and a deep soak brings the plant back within a day. The two look alike from the path and want opposite treatments, so check the soil before reaching for the hose.

Rolled leaves in hard frost are a normal water-conserving response and mean nothing is wrong. Judge by the temperature and the soil, not by the leaves alone.

What to treat, what to tolerate, and when to give up

Most rhododendron problems are cosmetic and the plant outgrows them. Leaf spots, sooty mold, a few notched margins, stippling on old leaves: none of these kill, and a shrub with a cool moist root run in dappled shade shrugs them off. Reserve real effort for the two that are fatal and for anything hitting the newest growth.

When you do treat, treat the cause alongside the symptom. A plant baking in afternoon sun on dry soil will be stippled by lace bug every year no matter what you spray, and moving it in autumn, when rhododendrons transplant easily because of their compact root mat, fixes it permanently in a way that repeated spraying never does.

Give up when the roots have gone. A shrub that lifts out of the ground with almost nothing attached, or whose roots are dark, soft and sour, will not recover, and nothing is gained by leaving it. Find out what killed it before replanting, because waterlogging, a buried crown and a resident vine weevil population will all do exactly the same thing to the replacement.

Record the date you first see notched leaves each year, and which plants. Vine weevil returns at a similar point every season, and two years of notes tell you when to apply nematodes rather than discovering the grubs when a plant collapses.