Gadenin日本語Get the appDiagnose before you treat
A kentia sends the same signal for very different problems: a frond goes off color. Because the plant grows slowly, that signal arrives weeks or months after the cause, so treating on a guess usually means treating the wrong thing. Spend five minutes with a hand lens on the underside of the leaflets and a finger in the compost before you reach for anything.
Check three places every time. The undersides of leaflets, where mites and their webbing live. The midribs and the leaf bases, where scale hides in the grooves. And the compost, two inches (5 cm) down, which tells you whether the roots are drowning.
| What you see | Where to look | Likely cause |
|---|---|---|
| Fine pale stippling, dull bronze cast | Leaflet undersides, fine webbing | Spider mite |
| Sticky film, sooty black patches | Midribs and stems | Scale or mealybug honeydew |
| Brown oval bumps that scrape off | Along the midrib grooves | Scale insects |
| White cottony fluff in the crown | Leaf bases and new spears | Mealybug |
| Lower fronds yellow one by one | Compost is wet and heavy | Root rot |
| Dry papery tips with a yellow band | Frond tips only | Salt or hard water build-up |
| Dark spots with a yellow halo | Older fronds, damp still air | Fungal leaf spot |
Spider mite, the one that actually kills
Spider mite is the most serious pest of indoor palms and it thrives exactly where a kentia often stands: a warm room with dry central heating and still air. The mites are barely visible, so you find them by the damage first. Hold a frond up to the light and look for fine pale dots across the leaflets, then check underneath for a dusty look and, later, for fine webbing between the leaflets and along the midrib.
Treat as soon as you see stippling, because a heavy infestation on a slow-growing palm removes fronds faster than the plant can replace them. Take the plant to a shower or outside and hose both surfaces of every frond hard, paying attention to the undersides. Repeat that rinse every five days for three weeks, which breaks the egg cycle, and follow up with insecticidal soap or a light horticultural oil on the undersides. Raising the humidity around the plant makes conditions much less favorable for them.
One rinse never works. Mite eggs hatch in days and a single treatment leaves the next generation untouched. Three treatments spaced five days apart is the minimum that clears an outbreak.
Scale and mealybug
Scale insects sit still in the grooves along the midribs and leaf bases, looking like small brown or tan oval bumps that you can scrape off with a thumbnail. Mealybug looks like specks of cotton wool tucked into the crown and around new spears. Both feed on sap and excrete honeydew, so the first thing most people notice is a sticky floor under the plant or a black sooty film on the fronds below.
- Isolate the plant Move it away from your other houseplants immediately. Both pests crawl and both spread readily to anything within touching distance.
- Remove what you can see by hand Wipe scale off midribs with a cloth, and dab mealybug with a cotton bud dipped in rubbing alcohol. Work along every midrib and into every leaf base.
- Treat with horticultural oil Spray thoroughly, covering both surfaces and the stems. Oil works by smothering, so incomplete coverage means incomplete control. Do not spray a plant standing in direct sun.
- Repeat every two weeks, three times Crawlers hatch continuously, and one application only kills what is exposed at that moment. Check the midribs again a month after the last treatment.
- Wipe the honeydew off Sponge the sticky residue and sooty mold off with plain water once the pests are gone. Left in place it blocks light and the fronds stay dingy.
Root rot and tip burn
Root rot is the leading cause of death in indoor kentias and it is not really a disease, it is a watering habit. Compost that stays wet excludes air, the fine roots die, fungi move into the dead tissue, and the plant then shows drought symptoms because dead roots cannot lift water. The tell is a wet, heavy pot and a sour smell at the drainage holes. Unpot it, cut away every black hollow root, repot into a gritty mix in a pot no bigger than the surviving roots need, and water sparingly for two months.
Tip burn is far less serious and almost always chemical. Dry papery brown tips with a thin yellow band between the dead and living tissue mean dissolved salts, from hard tap water or from fertilizer that has accumulated in the pot. Flush the pot with four times its volume of clean water, switch to rainwater or filtered water, and feed at half strength no more than three times over the growing season. If you trim the tips for looks, cut just inside the dead tissue, leaving a hair of brown, because cutting into green tissue creates a fresh brown edge.
Prevention and what to log
Most of the prevention is environmental. Keep humidity moderate rather than desert-dry, give the plant a shower and a wipe every couple of months so pests never establish, keep air moving gently, and inspect any new plant for a fortnight before it joins the others. Never leave the pot standing in a full saucer.
Record the date you first see any pest, the date of each treatment and the date the last live insect disappears. Indoor pests recur seasonally, and after two years those dates show you which month your mite outbreaks start, which is when the heating comes on and the air dries out. Knowing that lets you raise humidity before the outbreak rather than after it.