Gadenin日本語Get the appWhat actually goes wrong, and why
Lemon balm is not a delicate plant. It is rarely killed by anything, and the list of things that bother it is short. What it does suffer from is congestion: a dense clump of soft stems, damp still air at the base, and leaves that never dry out. Nearly every disease below starts there.
That is convenient, because it means the same action fixes most of them. A hard cut back opens the plant up, removes the infected leaves, and the regrowth comes clean. Before you reach for anything else, reach for the shears.
| What you see | Likely cause | First move |
|---|---|---|
| White dusty film on upper leaf surfaces | Powdery mildew in still humid air | Cut the clump hard, clear the debris away, thin what is around it |
| Orange or brown pustules on leaf undersides | Rust | Cut and bin the affected growth, do not compost it |
| Small dark spots with pale halos | Leaf spot after a wet spell | Remove the worst leaves, improve air movement, keep water off the foliage |
| Fine speckling and webbing, leaves going gray | Spider mites in hot dry conditions | Hose the undersides forcefully, cut back hard, keep the plant watered |
| Clusters on soft shoot tips, sticky leaves | Aphids | Wash off with water or pinch the tips out; they rarely need more |
| Ragged holes, slime trails on young growth | Slugs and snails on spring shoots or seedlings | Clear hiding places at the base and protect new plants only |
| Shoots collapse, base is brown and soft | Crown or root rot in wet soil | Lift, cut away rot, replant a sound piece somewhere drier |
Powdery mildew, the common one
This is the problem you will meet most often, usually in the second half of summer and usually on a clump that was never cut. It shows as a white dusty coating on the upper leaf surface, starting on the shaded lower leaves in the middle of the plant and working upward.
It thrives in humid still air, and it is worse on soft overfed growth and on plants in shade. Note that it does not need rain: a dry summer with heavy dews and no air movement is ideal for it.
The treatment is to cut the whole clump down to about four inches (10 cm), take the cut material away rather than leaving it on the ground, and clear anything crowding the plant. The new growth almost always comes clean. Spraying an uncut clump achieves very little because the infected leaves stay in place.
Rust and leaf spot
Rust shows as raised orange or brown pustules on the undersides of leaves, with yellow blotches showing through on top. It is less common than mildew but more persistent, because the spores overwinter on plant debris around the crown.
Deal with it in two parts. Cut and bin the infected growth during the season, and then be thorough with the autumn cleanup: remove every dead stem and fallen leaf from around the crown rather than letting the litter sit there through winter. A clump that gets rust two years running in the same congested corner is telling you to move it or divide it.
Leaf spot follows wet weather and looks alarming but rarely amounts to much. Take off the worst leaves, stop watering overhead, and it usually stops there.
Sucking pests
Spider mites are the ones to watch for in a hot dry summer, particularly on plants against a wall or in a pot that keeps drying out. The first sign is a fine pale speckling across the leaf, and by the time you see webbing at the shoot tips the population is large. Because they hate moisture, a forceful spray of water on the undersides several days running knocks them back, and a hard cut removes most of them outright.
Aphids gather on the softest shoot tips in spring, especially on overfed plants. They almost never need treating on lemon balm beyond washing them off or pinching out the tip. Remember that the flowers, if you let any open, bring in the predators that deal with them.
Whitefly turns up on plants brought indoors for winter. Outdoors it is not usually worth acting on.
Anything you spray, you may later want to pick and eat. On a plant that regrows this readily, cutting it back is almost always the better answer than treating it.
Wet feet, and the one thing that really kills it
Lemon balm shrugs off cold. What it does not survive is a winter sitting in waterlogged ground. The symptom is a clump that simply fails to reappear in spring, or one that sends up a few shoots that then collapse, with a base that is brown and soft rather than firm and pale.
If you catch it, lift the plant, cut back to firm white root and crown, and replant that piece on a raised area or in a pot of gritty potting mix. If your soil is heavy, plant on a slight mound from the start and do not mulch right up against the crown over winter.
The problem that is not a disease: seedlings
The most common complaint about lemon balm is not a pest at all. A clump that is allowed to flower drops a great deal of fine seed, and the following spring you find dozens of small plants scattered through the surrounding beds, sometimes several feet away. They pull out easily when small and become a real job once they have a taproot.
The remedy is timing rather than weeding. Cut the clump back before the buds open and there is no seed to shed. If you want to leave some flowers for the bees, cut those stems the moment the petals drop, before the seed has colored.
It is worth writing down where and when you first found seedlings each spring, and roughly how many. That record tells you within a season or two whether your cutting is early enough, and it is the fastest way to work out which clump in the garden is doing the seeding.