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Nearly every calathea complaint falls into three groups: a pest, a water or air problem, or cold. They ask for completely different responses, and treating the wrong one wastes a season. Start here and then read the relevant section.
| What you see | Most likely cause | Where to look next |
|---|---|---|
| Fine pale speckling, dull leaves, webbing | Spider mites | The spider mite section below |
| Even brown crisp line around the whole leaf edge | Water quality or dry air | Edges and tips |
| Brown patches with a yellow halo, mostly lower leaves | Fungal leaf spot from wet leaves | Leaf spot |
| Whole leaves yellowing from the bottom up | Overwatering, or simply old age | Yellow leaves |
| Blackened leaf margins after a cold night | Cold damage | Cold damage |
| Small flies around the compost | Fungus gnats | Fungus gnats |
Spider mites are the serious one
Spider mites are the pest that kills calatheas, and they thrive in exactly the conditions a heated winter room provides: warm, still and dry. They are under a hundredth of an inch long and live on the undersides of the leaves, so by the time you see webbing the population is already large.
Check for them every time you water. Turn two or three leaves over and look at the underside against the light, and run a piece of white paper under a leaf while you tap it. Anything that lands and walks is a mite.
- Isolate the plant immediately Move it away from every other plant the same day. Mites spread on air currents and on your hands, and a single infested calathea will seed a whole windowsill within a fortnight.
- Wash the leaves before you spray anything Take the plant to a shower or sink and rinse both surfaces of every leaf with tepid water, undersides especially, for several minutes. This physically removes the bulk of the population and makes everything afterwards work better.
- Treat with insecticidal soap or horticultural oil Spray to run off, covering the undersides, the leaf stalks and the crown. Contact sprays kill only what they touch, so complete coverage is the whole job.
- Repeat every five to seven days, at least three times Eggs are unaffected by most treatments and hatch in about five days. One treatment always fails. Three or four at weekly intervals breaks the cycle.
- Raise the humidity afterwards Mites breed far more slowly in humid air. Getting the humidity up to 50 to 60 percent is a permanent preventative, not just a treatment.
Do not spray any oil on a plant standing in sun or in a hot room, and do not use a leaf shine product on a calathea at all. The soft leaves mark easily and shine products block the stomata.
Brown edges and dry tips
A brown edge is the most reported calathea problem in existence and it has four causes worth separating. The pattern of the damage tells you which one you have, and the leaf itself never recovers, so you are diagnosing to protect the next leaf.
| Pattern of damage | Cause | What to change |
|---|---|---|
| Thin even brown line all round the margin | Minerals and fluoride in tap water | Switch to rainwater or distilled, flush the pot |
| Dry brown tips, spreading back | Air too dry, or a draft | Humidifier, group plants, move it from moving air |
| Large brown areas with a soft yellow edge | Compost dried out completely | Shorten the interval, never let it go dry |
| Whole edge blackened, quite suddenly | Cold, usually a night at a window | Move it into the room, above 61F (16C) |
Trim damaged edges with clean scissors following the natural leaf outline, leaving a hairline of brown rather than cutting into green tissue. Remove a leaf entirely only when more than half of it has gone.
Yellow leaves, leaf spot and gnats
These three all come back to the same root cause more often than not, which is compost that stays too wet for too long.
- Yellow leaves from the bottom One or two lower leaves yellowing slowly over months is normal turnover on any calathea. Several at once, with damp compost, is overwatering, and the fix is drainage and interval, not fertilizer.
- Fungal leaf spot Brown or black spots with a yellow ring, usually where leaves stay wet. Remove affected leaves, stop wetting the foliage in the evening, and improve the air movement in the room without putting the plant in a draft.
- Fungus gnats Small dark flies that run over the compost surface. Harmless as adults, but the larvae eat fine roots, which is a real problem on a shallow rooted plant. Let the top inch dry between waterings, add a half inch layer of grit on the surface, and use yellow sticky traps.
- Root rot Limp leaves with wet compost, and a sour smell from the pot. Slide the plant out and look. Firm white or cream roots are fine, brown mushy ones are not. Cut the dead ones off, repot into fresh free draining compost, and water very sparingly for a month.
Mealybugs, scale and thrips
These three appear far less often than spider mites but you should be able to recognize them, because each needs a different treatment.
| Pest | How you spot it | Treatment |
|---|---|---|
| Mealybugs | White fluffy specks in leaf joints and under the crown | Dab each one with a cotton bud dipped in alcohol, then spray with soap weekly |
| Scale | Small brown limpets on stalks and leaf veins, sticky residue | Scrape off by hand, then treat with horticultural oil three times |
| Thrips | Silvery streaks and tiny black dots of frass, distorted new leaves | Blue sticky traps plus a soap or oil spray every five days |
| Aphids | Clusters on new growth, usually after a spell outdoors | Wash off with water, repeat until they stop appearing |
Prevention and a record worth keeping
Almost all of this is prevented by three habits: look at the undersides every time you water, quarantine any new plant for three weeks before it joins the others, and keep the humidity above 50 percent in winter.
Keep a short pest log. Write down the date you first saw a problem, what you treated with, and the date of each repeat treatment. Spider mite control fails almost entirely because people lose track of the five to seven day interval, and a written date makes that impossible. Note the season too: after two years the log will show you exactly which weeks of the year you need to start checking.
Before you throw a badly infested plant away, take a healthy division from the outside of the clump, wash its roots clean, and grow it on in isolation. It is often the fastest route back to a clean plant.