Gadenin日本語Get the appRust: orange streaks along the blades
Rust shows as small orange to rust-brown streaks and pustules running lengthwise along the leaf blades, usually starting on the older outer leaves. It appears in warm, humid, still conditions, and it is far worse on a congested clump where air cannot move through the middle. Rubbed with a fingertip, the pustules leave orange powder on your skin, which distinguishes rust from simple sun scorch.
It rarely kills an established plant, but it disfigures the leaf you would otherwise use for infusions and it slows growth. The response is mechanical rather than chemical.
- Cut the affected leaves out Shear off the worst leaf blades and remove them from the garden rather than composting them. On a badly affected clump, cut the whole top back to twelve inches (30 cm); it regrows clean in warm weather.
- Open the clump up Harvest stalks from the outside, peel away dead sheaths at the base, and if the clump is more than a foot (30 cm) across, divide it. Air movement through the base is the actual cure.
- Water at the base only Overhead watering late in the day keeps the blades wet overnight, which is exactly the condition rust needs. Water the soil, and water in the morning.
Spider mites: the winter killer
Spider mites are the single biggest threat to lemongrass, and they almost always strike indoors during winter, where warm dry air near a radiator or heat vent is ideal for them. The first sign is fine pale stippling on the blades, giving the leaf a dusty, faded look. By the time you can see webbing where the blades meet the stalk, the population is already large.
Check the plant weekly through winter by holding a blade up to the light. Stippling is visible long before webbing, and treatment at the stippling stage is straightforward while treatment at the webbing stage often means sacrificing the top growth.
- Raise the humidity Move the plant off the windowsill above a radiator, stand it on a tray of damp pebbles and group it with other plants. Mites breed far slower in humid air, and this alone prevents most infestations.
- Rinse the foliage Take the plant to a sink or shower and spray the blades thoroughly, including where they wrap the stalk. Repeat every four or five days for three rounds to catch newly hatched mites.
- Shear it back if it is bad Because lemongrass regrows from the base, cutting all the foliage off at six inches (15 cm) removes the entire mite population at once. It looks drastic, but a healthy clump replaces that growth in weeks once light and warmth return.
Yellow leaves and rot at the base
Yellowing has three separate causes and they need opposite responses, so identify which one you have before you act. Yellowing on only the oldest outer blades, on a plant growing well, is normal ageing and needs nothing but tidying. Uniform pale yellowing across a whole potted plant in the growing season is hunger. Yellowing that starts at the base, with stalks that feel soft and pull away wet and brown, is rot.
Rot is the one that kills, and it is nearly always cold compost kept wet. Lemongrass at 50F (10C) uses almost no water; watered on a summer schedule at that temperature, the crown sits saturated for weeks and the base collapses.
| Symptom | Likely cause | What to do |
|---|---|---|
| Oldest outer blades yellow, plant otherwise vigorous | Normal ageing | Peel off dead sheaths, nothing else |
| Whole plant pale, growth slow in warm weather | Nitrogen shortage or exhausted compost | Liquid feed every two weeks, or pot on |
| Yellow and soft from the base up, sour smell | Rot from cold wet compost | Stop watering, move somewhere warmer, repot into gritty mix |
| Blades brown and crisp at the tips only | Drought, dry air, or salt build-up | Water more deeply, raise humidity, flush the pot through |
| Fine pale speckling over the blade | Spider mites | Rinse foliage, raise humidity, shear back if severe |
Cold damage and what is actually dead
Lemongrass takes visible damage below about 40F (4C) and the top growth is killed outright by frost. After a cold snap the blades go straw-colored, limp and translucent, and it looks terminal. It very often is not: the growing points are at the base, and a clump whose top has been frosted will regrow from the crown provided the crown itself never froze solid.
Do not throw a frosted plant away and do not cut it back to nothing straight away either. Cut the dead top off at four to six inches (10 to 15 cm), keep the compost barely moist and somewhere above 50F (10C), and wait until temperatures rise. Squeeze the base: firm means alive, mushy means gone.
Judge a frosted clump only after four to six weeks of warmth. Plants written off in winter routinely push new shoots in late spring, and a base that is still firm is worth waiting for.
The minor pests, and keeping notes
Outdoors, lemongrass is largely left alone. Slugs and snails will rasp young shoots on a newly planted division but ignore mature stalks. Aphids occasionally colonise soft new growth in spring and wash off with a jet of water. Rodents sometimes gnaw the sweet stalk bases of a plant overwintering in an unheated shed or garage, which is a reason to check a stored plant monthly rather than shutting the door until spring.
Write down the date you first see rust each year, and the date you find mites on the overwintering plant. Both are strikingly consistent from year to year for a given site, so after two seasons you know when to start checking rather than reacting once the damage is done. Note which clump was affected as well: a repeat offender is usually telling you something about where it stands.