Gadenin日本語Get the appThe short list of what goes wrong
A kalanchoe has a small and predictable set of problems, and most of them come from the grower rather than from anything arriving on the wind. Before you do anything else, push a finger into the compost, because damp compost and dry compost point at completely different answers to almost identical-looking symptoms.
Sort the problem by how fast it is moving. Stem rot needs action the same day. Everything else on this list gives you a week or two to work with, and a fortnight of doing nothing has never made mildew or mealybugs materially worse.
| Problem | First sign | How fast it moves |
|---|---|---|
| Stem and root rot | The stem just above the compost goes soft and brown | Days. Act immediately |
| Powdery mildew | Gray-white dusty patches on the leaf faces | Weeks, faster in still humid air |
| Mealybugs | White woolly specks in the leaf joints | Weeks, but they breed out of sight |
| Aphids | Green or black clusters on the flower stems | Days on soft new growth in spring |
| Botrytis on the flowers | Brown mushy flower heads with a gray fuzz | Days once the blooms have been wetted |
| No flowers on a healthy plant | Nothing wrong except the absence of buds | A whole season. It is a light problem |
Stem rot: the one that kills the plant
Almost every kalanchoe that dies indoors dies of this. The stem just above the compost turns soft, brown and slightly translucent, the plant starts leaning, and within a week or two the whole top falls over because there is nothing solid holding it up. It is caused by compost that stays wet, and cold makes it far worse, which is why most losses happen in late autumn and winter.
The good news is that the top of the plant is usually still perfectly healthy when you notice, and a kalanchoe roots so readily from stem cuttings that you can save the plant even when the base is beyond help. Do not spend a week trying to nurse a rotting base; cut above it and start again.
- Feel the stem at compost level Squeeze the stem gently just above the compost. Firm means the rot has not reached it. Soft, squashy or hollow means it has, and the base cannot be saved.
- Cut well above the damage With a clean blade, cut the stem two inches (5 cm) above the highest point of soft tissue. Look at the cut face: it must be uniformly pale and firm. Any brown streak in it means cutting higher.
- Take the healthy tops as cuttings Strip the lower leaves, dry the cut ends in bright shade for two days, then push them into fresh gritty potting mix. They will root in two to three weeks and you have the plant back.
- Discard the base and the compost There is no saving a rotted stem base. Throw away the old compost with it and wash the pot thoroughly before reusing it, because the organisms that did the damage are still in there.
- Fix the cause before you restart Repot into free-draining gritty potting mix in a pot only just big enough, stop using a saucer, move it somewhere brighter with air moving, and stretch the winter interval. Otherwise the new plant follows the old one.
If only the roots have gone but the stem is still firm, you can lift the plant, cut off every soft root, dry it for two days and repot it into fresh dry compost. Do not water for a week afterwards.
Powdery mildew and flower botrytis
Powdery mildew shows as gray-white dusty patches on the upper leaf surfaces, and it is a still-air problem rather than a wet-leaf problem. It appears on plants crammed against a window with other pots, in rooms that are never ventilated, and it spreads across a plant over a few weeks rather than overnight. Spacing the plants out and getting some air moving does more than any spray.
Botrytis is the gray fuzz that follows water sitting on the flower heads. Brown mushy blooms with a dusty gray coating spread to the flowers below them within days, and the rot can run back down into the shoot. It is entirely preventable by watering at the compost and never over the blooms.
| Problem | What causes it | What to do |
|---|---|---|
| Powdery mildew on the leaves | Still, humid air and crowded plants | Remove the worst leaves, space the pots out, ventilate the room |
| Mildew returning every year | The position, not the plant | Move it somewhere with air movement and stronger light |
| Gray fuzz on the flower heads | Water splashed on the blooms | Remove every affected head, water at the compost only |
| Rot running down into the shoot | A mushy flower head left in place | Cut the shoot back to firm tissue and improve airflow |
Mealybugs, aphids and scale
Three insects find a kalanchoe indoors. Mealybugs are the most persistent because they hide in the leaf joints and in the crown where the leaves are packed together. Aphids are the most predictable: they arrive in spring on soft new shoots and on the flower stems, and a plant that has been outside in summer often comes back in with them. Scale is occasional and shows as brown limpet-like bumps on the older stems.
Check the plant whenever you water. The leaf joints and the undersides of the lowest leaves are where all three start, and catching a mealybug colony while it is still on one shoot takes five minutes instead of six weeks.
| Pest | Where to look | What you see | What to do |
|---|---|---|---|
| Mealybugs | Leaf joints, the crown, and roots at the pot rim | White woolly blobs and a sticky film below them | Lift each one off with a cotton bud dipped in rubbing alcohol, then recheck weekly for a month |
| Aphids | Flower stems and the tips of new shoots | Dense green or black clusters, sticky leaves below | Wipe or rinse them off, then check every few days through spring |
| Scale | Older stems and the undersides of lower leaves | Brown or fawn bumps that do not brush off | Scrape off with a thumbnail or an old toothbrush, then wipe with alcohol |
| Spider mites | Leaf undersides in a dry heated room | Fine pale stippling and, when heavy, faint webbing | Rinse the foliage, raise the humidity around the plant, repeat weekly |
Isolate any plant you are treating. Mealybugs in particular walk between pots that are touching, and treating one plant on a crowded windowsill simply moves the problem next door.
The healthy plant that will not flower
This is the most common complaint of all, and it is not a disease. A kalanchoe only sets flower buds after about six weeks of fourteen-hour nights, and in an ordinary room the evening lighting cuts the dark period to nine or ten hours every night of the year. The plant reads that as permanent summer and simply keeps making leaves.
Work through the possible causes in order, because they are not equally likely. Nine times out of ten it is the light at night, and no amount of feeding, repotting or moving to a sunnier window will substitute for the darkness.
| Cause | How to recognize it | The fix |
|---|---|---|
| Nights too short | Any room where a lamp goes on in the evening | Six weeks of fourteen-hour nights under a light-tight cover |
| Light leaking into the cover | Treatment done, but no buds at all afterwards | A weighted box or a dark cupboard, and repeat the six weeks |
| Nights too warm during treatment | A living room that stays above 75F (24C) overnight | Move it to a cooler room at 59 to 64F (15 to 18C) |
| Too little light in the day | Long bare stem between the leaf pairs | The brightest window you have for the ten light hours |
| Too much nitrogen | Large soft deep green leaves, vigorous growth | Switch to a high potassium feed at half strength, or stop |
| The plant is too young or just cut back | Short shoots made in the last few weeks | Let the shoots mature over summer, treat the following autumn |
What to write down
Kalanchoe problems repeat on an annual cycle, and two lines in a record turn that repetition into something you can act on before the damage starts rather than after. Note the date you last watered before any soft stem appeared, the room the plant was standing in, and the date you first saw the symptom.
For the flowering question, record the start and end dates of the dark treatment, the room you did it in and the night temperature there, and the date the first bud appeared. If a season produces no buds, those four figures tell you which of the causes above was responsible, and one year of your own numbers is worth more than any general timing advice.
- Log the last watering before any symptom For rot, the useful number is how many days of damp compost preceded the first soft patch. Two seasons of that figure tell you exactly how far to stretch your own winter intervals.
- Log the first date you saw each pest Aphids and mealybugs return at roughly the same point each year. Knowing the date means you inspect a fortnight earlier next year and catch them while they are still on one shoot.
- Log the dark treatment dates and the room Start date, end date, room and night temperature. If buds appeared, you have a repeatable recipe. If they did not, you have the evidence for which variable to change.