Gadenin日本語Get the appRoot and crown rot: the main killer
Far more tarragon is lost to rot than to any insect. The pattern is unmistakable once you know it. Stems wilt from the base upward, the lower leaves yellow then go gray-brown, and the whole plant collapses over a few days despite the soil being damp. Pull it and the crown is soft, brown inside and smells sour, while the outer root tips may still look white.
The classic mistake is reading that wilt as thirst and watering more, which finishes the plant within a week. The test is your finger, not your eyes. Push it two inches (5 cm) into the soil. If it comes out damp and the plant is still wilting, the problem is roots that can no longer take up water, and more watering makes it worse.
Rot is not treatable once the crown is soft. What you can do is act early on a plant that is only sulking: stop watering entirely, scrape mulch away from the crown so air reaches it, and if it is in a pot, tip it out and repot into a dry gritty mix. Take cuttings from any healthy top growth immediately, because they may be all you keep.
| Symptom | Root rot | Drought stress |
|---|---|---|
| Soil two inches down | Damp or wet | Dry and dusty |
| Where wilting starts | Lower stems first | Whole plant at once, tips worst |
| Recovery after watering | None, or worse | Firm again within a few hours |
| Leaf color | Yellow then gray-brown, stays on stem | Dull gray-green, crisps and drops |
| Crown when dug | Soft, brown, sour smell | Firm and white inside |
Rust, mildew and other leaf problems
Rust shows as small orange-brown pustules on the undersides of leaves, with pale yellow spotting visible from above. It spreads from the bottom of the plant upward and is worst in a crowded clump in still, humid air. Pick off and bin every affected leaf as soon as you see it, thin the stems to open the plant up, and never compost the material.
Powdery mildew turns up in the same conditions as a white dusty film on the upper leaf surfaces, usually late in the season on growth that is already past its best. It is rarely worth treating at that point. Cut the affected growth away, clear all fallen debris from around the crown, and treat it as a signal that the plant is too crowded or too shaded.
Both problems overwinter in dead material lying on the soil. The single most effective control is autumn hygiene: once frost has blackened the top growth, cut it all off at the base and remove it completely rather than leaving it as a mulch over the crown.
Insects worth knowing about
Tarragon's strong oils keep most chewing pests away, and it is often planted near other crops for exactly that reason. The few that do turn up are manageable by hand.
Aphids cluster on soft new tips in spring, particularly on plants that have been fed too generously. A jet of water knocks them off and the soft growth that attracted them is a sign to stop feeding. Slugs and snails take the emerging shoots in spring while they are still low and tender, which can look like the plant failing to come up at all; check at night with a torch before assuming the crown is dead. Leaf miners occasionally leave pale winding trails in a few leaves, which is cosmetic and needs nothing more than picking those leaves off.
Root aphids are worth ruling out on a potted plant that declines with no sign above ground. Tip the pot out and look for white waxy deposits on the outside of the rootball. If they are there, wash the roots clean, discard the old mix and repot into fresh.
When the problem is age, not disease
A clump four or five years old often looks diseased when it is simply worn out. The center dies away, the surviving shoots are thin and short, growth starts later each spring, and the flavor goes flat. There is no spray for this and no feeding that fixes it.
The answer is division, done in spring as growth begins. Lift the whole clump, throw the dead center away, and replant only the vigorous outer sections with fresh grit worked into a different spot in the garden. Moving it matters: replanting into the same hole often puts the new divisions straight back into the drainage problem that weakened the old plant.
Keeping a record that pays off
Write down the date you first see rust or mildew each year, and where on the plant it started. Both diseases arrive at a fairly consistent point in your own garden's season, and after two years of notes you know when to thin the clump preventively rather than react a fortnight too late.
Also note every plant you lose, with the position and what the crown looked like when you dug it. A list showing three losses in the same bed is the clearest possible argument for raising that bed or moving tarragon somewhere else entirely.