Thyme Problems: Root Rot, Gray Mold, Spider Mites and Dieback

Root rot: the one that kills plants outright

By a wide margin the commonest way to lose a thyme plant is root and crown rot in soil that stays wet, caused by soil fungi that only get going in waterlogged conditions. Above ground you see whole stems going gray and dry from the tip while the plant is standing in damp soil, then a sudden collapse over a week or two, most often in late winter and early spring.

The diagnostic test is to pull. A healthy thyme resists; a rotted one lifts away with almost no root, and what root there is is brown, soft and easily rubbed off the central strand. There is no cure at that point. The plant is lost and the work is to stop it happening to the next one.

1. Take cuttings the moment you suspect it Green shoots from an affected plant will usually still root. If the crown is going, that is the only material you will get, so take it early rather than waiting to see.
2. Do not replant in the same spot The soil fungi persist. Move the replacement to a different, better-drained position, or build a raised mound or use a container.
3. Fix the drainage first Work in coarse grit, raise the bed, and plant the crown proud of the soil. Replacing a plant into the same wet hole simply repeats the loss on the same schedule.
4. Get the mulch off the crown Pull bark, straw or leaf litter back from the base and replace it with grit. Organic mulch against a thyme stem holds the moisture that starts the rot.

Note the date and position of any plant you lose this way. Two losses in the same corner of a bed is not bad luck, it is a wet spot, and the answer is to stop planting thyme there.

Gray mold and stem dieback in a congested plant

A dense, unpruned thyme holds damp air in its center, and in a wet spell fuzzy gray mold appears on the dead leaves and lower stems there. It spreads into living stems, which turn brown and die back individually while the rest of the plant looks fine. This is the same problem as the bare woody center, one stage further on.

It is entirely manageable without any spray. Cut out the affected stems into clean green growth, thin the plant to let air through, clear the dead leaf litter from underneath, and stop overhead watering. In most gardens the plant recovers over the following few weeks.

Disinfect or at least wipe your snips between plants when cutting out diseased stems. Moving mold from a sick plant into a fresh pruning wound on a healthy one is an easy mistake.

Spider mites and aphids on stressed plants

Thyme's aromatic oils deter most insects, so pest problems are rare and almost always follow stress or shelter. Two-spotted spider mites are the main one, and they appear on plants kept hot, dry and still, particularly indoors, on a covered patio or under glass. The first sign is a fine speckled paling of the leaves; fine webbing between the shoot tips means the population is already large.

Aphids occasionally cluster on soft spring growth, especially on a plant that has been fed too generously. Rosemary beetle, a metallic striped beetle, chews notches in the leaves of thyme, rosemary and lavender in some regions and is best dealt with by hand.

1. Spider mites: raise the humidity and wash Spray the plant hard with water, undersides included, every few days. Mites need hot, dry, still air. Moving a pot outdoors into the open usually ends an infestation on its own.
2. Aphids: rub them off Squash or hose them off the shoot tips and stop feeding. On a herb you intend to eat, water and fingers are the right tools; if you must use a soap spray, rinse thoroughly and leave it a week before harvesting.
3. Beetles and larvae: pick by hand Check the plant over on a warm afternoon and drop what you find into soapy water. Holding a sheet of paper under the plant and tapping it makes the beetles fall where you can see them.
4. Do not spray oils in strong sun Any oil or soap on thyme foliage in full sun scorches the small leaves. If you spray at all, do it at dusk.

Ants running up and down a thyme plant almost always mean aphids or scale somewhere on it. Follow the ants to find the colony.

Damage that is not a pest at all

Plenty of thyme symptoms have nothing to do with anything living. Blackened shoot tips after a hard frost, bleached patches on a plant moved suddenly into full sun, and general thinning on a plant in shade are all common and none of them need treatment beyond changing the conditions.

Age is the other non-pest cause. A plant in its fourth or fifth season that flowers thinly, roots poorly and looks bare at the base is not sick. It is simply old, and the answer is to replace it from a cutting or a layer rather than to keep nursing it.

Before treating anything, check drainage, sun and the last time the plant was cut. Three problems out of four on thyme are cultural rather than biological.