Gadenin日本語Get the appLook at the plant properly once a fortnight
Almost every serious infestation on a houseplant is a small one that nobody looked at for two months. An arrowhead plant makes this easy to get wrong, because the trouble hides in exactly the places you do not see from across the room: the undersides of the leaves, the tight joint where a leaf stalk meets the stem, and the compost surface under the lowest foliage.
Take the pot to a window every couple of weeks, turn a few leaves over, and run your eye down the stems and into the joints. Two minutes is enough, and it turns a problem that would have needed weeks of treatment into one wiped off with a cloth.
The four things you are likely to meet
Each of these gives itself away in a different way, and telling them apart matters because the treatments are not interchangeable. Work through the visible signs before you spray anything.
| Problem | First sign | Where to look | What to do |
|---|---|---|---|
| Spider mites | Fine pale speckling across the blade, leaves dulling | Undersides, and fine webbing in the leaf joints | Rinse under a shower, raise humidity, treat weekly for a month |
| Mealybugs | White fluffy specks like cotton wool | Leaf stalk joints, new growth, under the pot rim | Dab each one with alcohol on a cotton bud, repeat every 5 days |
| Thrips | Silvery streaks and tiny black dots of frass | Along the veins on the upper surface | Remove worst leaves, treat repeatedly, isolate the plant |
| Fungus gnats | Small flies rising when the pot is disturbed | The top inch of permanently damp compost | Let the surface dry properly between waterings |
Isolate any affected plant from your others straight away. Mites and thrips move between touching leaves faster than any treatment works, and a plant you have separated is also a plant you will remember to check.
Treating pests without wrecking the leaves
The blades are thin and they mark easily, so heavy or repeated spraying does visible damage of its own. Physical removal first, then a mild treatment repeated on a schedule, beats one strong application every time.
- Wash it down first Take the plant to a sink or shower and rinse both sides of every leaf with tepid water. This alone removes most of a spider mite population and all of the dust they hide in.
- Deal with what is left by hand Wipe mealybugs off with a cotton bud dipped in rubbing alcohol, checking every leaf joint. Cut off any leaf that is more than half damaged rather than trying to save it.
- Treat on a repeat cycle Use insecticidal soap or a diluted horticultural oil, covering the undersides thoroughly, and repeat every five to seven days for at least three rounds. Eggs hatch after the first treatment, which is why single applications fail.
- Test before you soak it Spray one lower leaf and leave it two days before treating the whole plant. Do it out of direct sun, never on a plant that is dry at the roots, and never on a hot afternoon.
The problems that are not pests
Most calls for help about an arrowhead plant turn out to have nothing living behind them. Match the pattern before you reach for a spray, because a treatment aimed at a pest that is not there just adds stress.
Lower leaves yellowing one at a time while the top grows on is usually a wet pot; check the weight and the drainage. Brown crisp edges on otherwise healthy leaves point at dry air or heat from a radiator. Bleached pale patches on the side facing a window are sun scorch. Long bare stems with small leaves are low light. A whole plant that has flopped with dry, light compost is simply thirsty and will recover overnight.
One yellow lower leaf every few weeks on a plant that is otherwise growing well is just age. The plant sheds its oldest foliage as it climbs, and nothing needs doing about it.
Root rot, and when to cut your losses
Root rot is the one problem that kills. It starts in compost that never dries, and by the time the plant wilts the roots have already gone. If the pot is heavy, the base of the stems is soft and the lower leaves are limp and yellowing, take the plant out of the pot straight away.
Firm pale roots are fine. Brown, hollow roots that slide apart between your fingers are dead; cut all of them away, wash the rest, reduce the top growth by about the same proportion you removed below, and repot into fresh airy mix in a pot no bigger than the remaining roots need. Water sparingly until new growth shows.
If almost nothing firm is left, do not spend a month nursing the stump. Cut the healthiest tops into cuttings, drop them in a glass of water, and start again from those. Arrowhead plants root so readily that a jar of cuttings is often a faster route back to a good plant than the rescue is, and you will have a plant in fresh compost with none of the problem left in it.