Gadenin日本語Get the appChrysanthemums count the nights, not the days
A chrysanthemum is a short day plant. It does not flower because autumn is cool, and it does not flower on a fixed date. It measures the length of the uninterrupted dark period each night, and when that dark period passes roughly ten hours the growing tips switch from making leaves to making flower buds. Everything about the plant's autumn timing follows from that one measurement.
The measurement is broken by very little light. A street lamp over the fence, a porch light left on, a security light, a lit conservatory window or even a bright neighboring window can be enough to keep the night reading as short. The plant then carries on making leaves. What you see is a big, healthy, entirely flowerless mound in mid autumn, or a plant that finally opens a few blooms weeks after everything else and gets caught by frost.
There is no feed, no pruning and no watering regime that fixes this, because nothing is wrong with the plant. The cures are to move the plant to a genuinely dark part of the garden, to turn the light off, or to shield the bed from it. If you cannot do any of those, grow the plant in a pot and stand it somewhere dark from late summer onwards, when bud initiation is happening.
This is also why one plant in a row can behave completely differently from its neighbors. Light falls off sharply with distance, so the end of a bed nearest a lamp may never flower properly while the far end flowers on time in the same soil, with the same feeding and the same watering.
| Light source | Effect on bud set | What to do |
|---|---|---|
| Street lamp shining directly on the bed | Flowering delayed for weeks or prevented | Move the plants, or screen the bed with a solid panel |
| Porch or patio light left on all evening | Delayed, and worst on the nearest plants | Switch it off from late summer, or fit a short timer |
| Motion sensor security light | Usually harmless, the flashes are too brief | Nothing, unless it is triggering repeatedly all night |
| Bright window or conservatory nearby | Delays plants within a few feet of the glass | Draw a curtain, or plant further out from the house |
| Indoor room light on a windowsill plant | Reliably prevents flowering altogether | Move to an unused room or a dark porch at night |
| Full moon and ordinary night sky | None, far too dim to register | Nothing |
Water at the base, steadily, and mulch
Chrysanthemum roots are fibrous and shallow, mostly in the top 8 in (20 cm) of soil, which is the layer that dries first. That makes the plant quick to wilt in a dry spell and quick to recover, but repeated wilting during the weeks when buds are forming costs you flowers permanently. The buds that abort do not come back.
Soak the soil to 8 in (20 cm) once or twice a week in warm weather rather than sprinkling daily. Check by pushing a trowel in and looking at the side of the slot, because most people badly overestimate how far a watering can reaches. In hot spells a plant in full growth in a container may need water every single day, and a container mum that dries out once often drops its lower leaves for the rest of the season.
Water the soil, not the plant. Wet foliage overnight is how chrysanthemum leaf spot spreads up a plant, and the lower leaves are already the most vulnerable part. Two inches (5 cm) of mulch laid over damp soil, kept clear of the stems, halves the watering and stops rain splashing spores from the ground onto the lowest leaves.
Wilting at midday that recovers by evening is a heat response and needs nothing. Wilting that is still there after dark means the root zone is genuinely dry, and that is when to soak it.
Feed for growth first, then stop
Through spring and early summer the plant is building the frame that will carry the flowers, and it wants nitrogen for that. A balanced liquid feed every two weeks from the time growth is properly away until the middle of summer is plenty on a decent soil. On a poor sandy soil, feed weekly at half strength instead.
Then stop the nitrogen. From late summer, when the plant is preparing to initiate buds, nitrogen produces soft leafy growth at exactly the wrong moment: it gives you a taller, floppier plant, more aphids and fewer flowers. Switch to a high potassium liquid feed, the sort sold for tomatoes, every two weeks until the buds show color, then stop feeding altogether.
A chrysanthemum that is all leaf and no bud in autumn is usually being told either that the nights are not dark enough or that there is still too much nitrogen in the soil. Rule out the light first, because it is the more common cause and the easier one to check.
- Balanced feed every two weeks through early summer This is the growth phase. The plant is making the branch structure that will carry the flowers, and it will not add stems later.
- Stop nitrogen by late summer Soft nitrogen fed growth going into the bud initiation period gives leggy stems and fewer flowers. This one stop matters more than any single feed you apply.
- Switch to high potassium fortnightly A tomato type feed at label strength, applied to already damp soil. It firms the growth and improves both flower count and color.
- Stop all feeding once buds show color From here the plant is spending reserves, not building. Late feeding only produces soft shoots that will be blackened by the first hard frost.
Support, airflow and what to record
A well pinched plant mostly holds itself up, but a mound loaded with wet flowers in autumn rain is heavy and splays open from the center. Push three or four canes in around each clump in midsummer and run soft twine around the outside at 12 in (30 cm) and 24 in (60 cm). Doing it in midsummer takes two minutes; doing it after the plant has splayed never really works.
Keep the base of the plant open. Strip any leaves touching the soil, keep the mulch clear of the stems and resist the urge to plant something at the foot of the clump. Most chrysanthemum leaf problems start in the still, humid air in the bottom 6 in (15 cm) of the plant.
Each autumn, write down the date the first flower opened on each plant and how dark its position is at night. Two or three years of that record tells you far more about your own garden than any general guidance can: it shows which corners flower on time, which ones run late every year, and therefore where to put the plants when you next divide them.