When a herb looks wrong, the symptom is rarely the whole story. Yellow leaves on rosemary usually mean too much water, while yellow leaves on basil usually mean too little food. A leggy herb is usually short of light. A herb with no flavour is usually over-fed or over-watered. The skill of troubleshooting is working through the likely causes in order, from the simplest and most common to the rarest, and fixing the thing that is actually wrong.
This guide gives you that order for the problems herbs most often show. Use the pest identifier helper and the plant disease checker to narrow down pests and diseases, and check your local extension service if a problem does not match anything here — they can diagnose from a sample or a photo, and they know which problems are common in your area this season.
Yellow leaves: water, food or disease
Yellow leaves are the most common herb complaint, and the cause depends on the herb. On rosemary, lavender and thyme, yellow leaves usually mean overwatering or poor drainage — the roots are drowning. Stop watering, let the soil dry out, make sure the pot drains freely, and repot into grittier compost if the plant is still alive. On basil and parsley, yellow leaves usually mean the opposite: the plant is hungry, especially in a pot where the compost is exhausted, and a half-strength liquid feed and a compost top-dressing will usually revive it. On mint, yellow leaves can mean either — check the soil first: if it is wet, the plant is drowning; if it is dry, the plant is thirsty.
Disease is less common but possible. If yellow leaves come with spots, a white powdery coating, or a pattern that spreads from one plant to its neighbours, suspect a disease: powdery mildew on mint and sage yellows the leaves, and downy mildew yellows basil. Remove affected leaves, improve airflow, water at the base in the morning, and do not compost diseased material. Our guide to powdery mildew treatment covers the most common case, and our herbs disease prevention guide covers the rest. If you cannot tell whether it is a water problem or a disease, a photo sent to your local extension service will settle it faster than guessing.
Leggy, pale growth: not enough light
A herb that grows tall and thin, with wide gaps between the leaves, pale colour, and few leaves, is short of light. This is the most common problem for indoor herbs, and the most common problem for herbs grown in a shady garden. Basil on a north-facing windowsill, rosemary under a tree, and thyme in a shaded bed all show the same symptoms: stretching towards the light, weak stems, and little flavour. There is no fertiliser that fixes a lack of light — the plant needs to be moved to a sunnier spot, or given a grow light.
Move the plant to the sunniest place you have, turn pots a quarter turn every few days so the plant grows evenly, and pinch the tips to encourage branching. A leggy basil plant can often be rescued by pinching it back hard and giving it more light — it will branch from the lower leaf joints. A leggy rosemary is harder to rescue, because you cannot cut it back to bare wood, so it is better to take cuttings from the healthy tips and start again. Indoors, a grow light on for 12 to 14 hours a day makes a real difference in winter — our grow light calculator can help you work out what you need.
Weak flavour: too much food or water
A herb that grows well but tastes of little is almost always over-fed or over-watered. Herbs produce their essential oils — the compounds that give them flavour — when they grow slowly in lean, sunny, well-drained conditions. A rosemary or thyme plant in rich, moist, heavily fed soil grows large, soft, pale leaves with far less fragrance than the same plant in lean soil. A basil plant fed heavily with nitrogen grows fast and tall, but its leaves are thin and weak-flavoured. The cure is to stop feeding, let the soil dry out a little between waterings, and give the plant more sun.
Other causes of weak flavour are picking at the wrong time and picking the wrong part. Pick herbs in the morning, after the dew has dried, when the oils are at their most concentrated — leaves picked in the heat of the afternoon have already lost oils to the sun. Pick young, tender leaves for the best flavour, and pick before the plant flowers, because flowering diverts the plant's energy into seed and the leaves lose flavour. And use home-grown herbs fresh when you can: dried basil and parsley have little flavour compared with fresh, which is why freezing suits them better than drying.
Bolting, flowering and going to seed
Bolting — a herb sending up a flower stalk and stopping leaf production — is the plant's response to heat or stress, and it is the end of useful leaf production for that plant. Cilantro bolts at the first sign of heat, basil flowers in hot weather, and dill and parsley bolt when they dry out or get too hot. Once a plant has bolted, it cannot be reversed, so prevention is everything: sow cilantro in cool weather and again in late summer, give soft herbs afternoon shade, keep the soil moist, and pinch the flower buds off basil as soon as they appear.
Flowering is not always a problem. Oregano, thyme and chives in flower are beautiful and bring bees into the garden, and their leaves are still usable, though less flavourful. Lavender and chamomile are grown for their flowers. The rule is to let one plant of each kind flower for the pollinators, and to keep the plants you pick for the kitchen pinched and leafy. And sow a little every few weeks, so there is always a young plant coming along behind the one that has bolted.
Wilting, drooping and sudden collapse
A herb that wilts in the heat of the day and recovers by evening is usually just thirsty — water deeply and mulch. Basil wilts dramatically when its pot dries out, and it usually recovers within an hour of watering. A herb that wilts and does not recover is a different problem: it may have root rot from wet soil, stem damage from cutworms, or a wilt disease. Check the soil first — if it is wet and the plant is wilting, the roots have probably rotted, and you should stop watering, improve drainage, and repot into grittier compost if the plant is still alive. Check the stem at soil level for damage, and check the roots: white and firm is healthy, brown and slimy is rot.
Seedlings that collapse at soil level have damping off, a fungal disease of cold, damp compost — see our guide to damping-off seedlings. A mint plant that collapses suddenly may have rust or mildew weakening it, and cutting it back hard will usually produce clean regrowth. A rosemary that dies suddenly in winter has usually been killed by cold and wet together, not by a pest — improve the drainage before you replant. If a whole bed collapses suddenly, check the drainage first and the watering second — most sudden collapses in herbs are water problems, not pests.
Pests and holes: who is eating your herbs
Holes in herb leaves have many authors, and the pattern tells you who. Aphids cluster on soft new growth, curl the leaves, and leave sticky honeydew — basil and dill are the usual victims, and a jet of water is the first control. Spider mites cause fine yellow stippling and webbing on the undersides of leaves, worst in hot, dry air — rosemary indoors is the classic case, and raising humidity and washing the leaves helps. Slugs and snails leave ragged holes and slime trails, worst at night in damp weather — parsley and cilantro are favourites, and hand-picking after dark is the best control. Flea beetles make small round holes in mint and basil leaves.
Do not spray first and diagnose later. Identify the pest, check whether the damage is actually serious — a few holes in a mint leaf do not matter — and start with the gentlest control: hand-picking, a jet of water, barriers, and encouraging predators. Our guide to aphids control and our herbs pest control guide cover the options in detail, and our pest identifier helper can help you work out what you are dealing with. If a pest problem is beyond hand-picking and water jets, check your local extension service for the products approved for edible herbs in your area.
Common mistakes
The biggest troubleshooting mistake is treating the symptom instead of the cause — feeding a herb whose real problem is shade, or watering a herb whose real problem is wet soil. The second is changing three things at once, so you never learn which one fixed it: change one thing, wait a week, and see. The third is ignoring the pattern: a problem on one plant is likely a local issue, while the same problem on every plant is likely water, soil or light. The fourth is letting basil and cilantro flower, which ends leaf production. And the fifth is waiting too long — most herb problems are easy to fix in the first week and hard to fix in the fourth, so check your herbs every few days, keep notes, and ask your local extension service early rather than after the plant is lost.
| Symptom | Most likely cause | First thing to check |
|---|---|---|
| Yellow leaves on rosemary, thyme, lavender | Overwatering; poor drainage | Soil wetness; drainage holes; repot into gritty compost |
| Yellow leaves on basil, parsley | Hunger; exhausted compost | Feed half-strength; top-dress with compost |
| Leggy, pale growth | Not enough light | Move to sunniest spot; grow light in winter |
| Weak flavour | Over-feeding or overwatering | Stop feeding; let soil dry; more sun |
| Bolting | Heat or dry soil | Shade, steady water, earlier sowing |
| Wilting that does not recover | Root rot | Soil wetness; roots brown and slimy means rot |
| Holes in leaves | Slugs, aphids, flea beetles, mites | Look under leaves; identify before spraying |
Frequently asked questions
Why are my herb leaves turning yellow?
On rosemary, lavender and thyme, yellow leaves usually mean overwatering or poor drainage. On basil and parsley, they usually mean the plant is hungry, especially in a pot with exhausted compost. Check the soil first — wet soil means drowning, dry soil means thirst — and look for spots or powdery coating, which point to disease.
Why is my basil tall and leggy with few leaves?
Leggy basil is short of light — it is stretching towards the window or the sun. Move it to the sunniest spot you have, turn the pot every few days, pinch the tips to encourage branching, and consider a grow light in winter.
Why do my herbs taste weak?
Weak flavour usually means over-feeding or overwatering — herbs make their strongest flavour when they grow slowly in lean, sunny, well-drained conditions. Stop feeding, let the soil dry between waterings, give more sun, and pick in the morning before the plant flowers.
Why did my cilantro bolt?
Cilantro bolts in heat and cannot be reversed. Sow it in cool weather, give it afternoon shade, keep the soil moist, and sow a little every few weeks and again in late summer for an autumn crop.
My rosemary wilted even though the soil is wet — why?
Wilting in wet soil usually means root rot — the roots have rotted and can no longer take up water. Stop watering, improve drainage, and repot into grittier compost if the plant is still alive. Cold and wet together kill rosemary more reliably than cold alone.