Gadenin日本語Get the appYellow leaves: two opposite causes, one symptom
Yellowing and dropping leaves is by far the most reported bougainvillea problem, and it is caused both by too little water and by too much. Since the usual response to a distressed plant is to water it, and since overwatering is the more dangerous of the two, a great many plants are killed by the attempt to save them.
Drought yellowing works from the oldest leaves inward. The plant sheds its lower, inner foliage to protect the growing tips, the leaves go a uniform pale yellow and fall cleanly, and the shoot tips stay firm. This is normal behavior and is largely how the plant survives a dry season. A soak fixes it, and some leaf loss during a hard dry cycle is acceptable.
Overwatering yellowing is different. Leaves yellow anywhere on the plant rather than starting at the base, they often go limp or blotchy before they drop, the compost stays dark and heavy days after watering, and there may be a sour smell at the drainage hole. Shoot tips blacken and go soft. Watering more makes it worse within days.
The test is your finger, not the leaves. Push it two inches (5 cm) into the compost. If it comes out damp and the plant is yellowing, the problem is water, and the treatment is to stop, move the pot somewhere warm and bright, and let it dry right out before anything else.
| Symptom | Too dry | Too wet |
|---|---|---|
| Which leaves yellow | Oldest and lowest first | Anywhere, often scattered |
| How they fall | Dry and clean | Limp, sometimes with dark blotches |
| Shoot tips | Firm, growth paused | Soft, blackening |
| Compost two inches down | Dust dry | Still damp days later |
| What to do | One deep soak | Stop watering, improve drainage, wait |
Root rot is the one that kills
Almost every dead bougainvillea died at the roots, in winter, in compost that never dried out. The plant is adapted to seasonal drought and has no tolerance for cold, wet, airless soil. Combine a heavy compost, a saucer that holds water, a shaded position and cool weather, and the fine roots die within a fortnight.
You rarely see it coming, because the top of the plant looks unchanged for weeks while the roots go. The first visible sign is usually sudden, wholesale leaf drop with no drought signs, followed by stems that shrivel from the tip down and stay dry and brittle rather than shooting again.
Catch it early and there is a chance. Take the plant out of its pot, cut away any roots that are black, soft or smell sour, and repot into fresh gritty potting mix in a container no bigger than what is left of the root system. Cut the top growth back hard to reduce the demand on the remaining roots. Then keep it barely moist and warm, and be patient.
Prevention is trivial by comparison. Never leave a pot standing in water. Raise containers on feet. Use at least one third grit in the compost. Water less in cool weather, and never water a plant whose compost is still damp.
Bougainvillea is a plant you should be comfortable underwatering. In cool or dull conditions, a fortnight without water does no harm at all. The same fortnight of weekly watering can be fatal.
The pests worth knowing
Outdoors in a sunny spot, bougainvillea is largely untroubled. Problems concentrate on plants kept under cover, particularly through winter, where warmth, still air and low light suit pests far better than they suit the plant.
Check the undersides of leaves and the soft growing tips whenever you are handling the plant, and do it with gloves on. Because the thorns make close inspection awkward, infestations on bougainvillea are often found later than they would be on anything else, by which point the population is large.
| Pest | What you notice first | What to do |
|---|---|---|
| Aphids | Sticky shoot tips, distorted new growth | Wash off with water, repeat every few days |
| Spider mite | Dull stippled leaves, fine webbing at the tips | Raise humidity, wash foliage, isolate the plant |
| Mealybug | White fluff in leaf joints and under bracts | Dab each one with alcohol on a cotton bud |
| Whitefly | Small white insects flying up when disturbed | Yellow sticky traps and repeated washing |
| Caterpillars | Leaves chewed to the veins overnight | Pick off by hand, check the undersides |
| Scale | Brown bumps along stems, sticky leaves | Scrape off, prune out the worst stems |
The thorns are a genuine hazard
Bougainvillea thorns deserve treating as a safety matter rather than an inconvenience. They are hard, sharp, curved backwards and positioned at the leaf joints where your hand naturally goes when you pull a stem into position. They catch on the way out, so a shallow contact becomes a tear.
Scratches from bougainvillea are notorious for becoming inflamed. The thorn tip breaks off and stays in the wound, and what should be a scratch turns into a swollen, itchy lump that takes weeks to settle. Wash any puncture properly and look for a fragment left behind.
Wear thick leather gauntlet gloves that cover the forearm, and long sleeves, for pruning, tying in or repotting. Cotton gardening gloves are useless; the thorns go straight through. Keep an eye on where the plant sits too, since a large specimen overhanging a path at head height is a hazard for anyone walking past.
Dispose of prunings carefully. The cut stems are just as sharp as the living plant, they hide in a compost pile, and they will go through a shoe sole. Bag them rather than piling them loose.
Keeping a record that actually helps
Bougainvillea problems are slow and repetitive, and they recur in the same season each year. A short log turns them from mysteries into a pattern you can head off.
Note the date of every watering through the cool months. This one habit prevents more losses than anything else, because root rot almost always follows a run of waterings that felt reasonable individually. Seeing them listed makes the frequency obvious.
Record the date you first find each pest and where the plant was standing at the time. Spider mite and mealybug reappear at the same point in the year on plants kept in the same place, so a note from last winter tells you when to start checking this one. If you have to cut back rotted roots, write down how much you removed and how long recovery took, since that is the only way to judge whether a struggling plant is on track.