Gadenin日本語Get the appHow the flowering actually works
Calibrachoa flowers on new growth, continuously, from every leaf joint along the trailing stems. It does not flower in flushes the way a rose does, and it does not need to be triggered by day length. As long as the plant is producing new shoot growth in good light, it is producing buds behind that growth. Everything you do to keep it flowering is therefore aimed at keeping new growth coming without letting that growth run soft and leafy.
A well-run basket carries flowers from late spring, once nights are reliably above 50F (10C), through to the first frost. Expect a slight dip at the very hottest part of summer, above about 90F (32C), when the plant keeps its leaves but slows bud production and the flowers that open are smaller and fade faster. That lull is normal and passes as soon as nights cool.
The four reasons it stops
When a healthy-looking green plant refuses to flower, the cause is almost always one of four things, and they are easy to tell apart by looking at the plant rather than guessing.
The commonest is not enough direct sun. Calibrachoa needs six to eight hours of it. In four hours the plant keeps growing and stays green, but the stems lengthen between the leaves, flowering drops by more than half, and the whole basket leans towards the brightest side. A basket that flowers thickly on the outer face and is bare against the wall has an even light problem, not a disease, and a quarter turn every few days fixes it within a fortnight.
| Symptom | Likely cause | Correction |
|---|---|---|
| Green, leafy, long gaps between leaves, few buds | Too much nitrogen | Switch to a high-potassium tomato-type feed |
| Leans to one side, flowers only on the bright face | Under six hours of direct sun | Move to full sun; turn the basket every few days |
| Pale yellow leaves and thin flowering together | Iron locked up by alkaline water | Chelated iron or ericaceous feed |
| Flowers only at the ends of long bare stems | Plant has run out of new growth | Cut everything back by a third, then feed |
| Smaller, faster-fading flowers in high summer | Heat above 90F (32C) | Nothing; keep it watered and it resumes as nights cool |
Feed for flowers, not for leaves
Nitrogen builds stems and leaves; potassium builds flowers. Once a basket has filled out, a general-purpose feed high in nitrogen will give you a magnificent green plant with disappointing bloom. Move to a feed with potassium clearly higher than nitrogen, the sort sold for tomatoes, and keep it going weekly. The change in bud count is visible within two to three weeks.
Do not read poor flowering as a sign to feed more of the same thing. More nitrogen on a plant that is already leafy makes the problem worse and softens the growth so it flops. Equally, do not stop feeding altogether in the hope of stressing the plant into flower; calibrachoa in an exhausted basket simply thins out and goes pale.
Leaf color and flower count are linked more tightly here than in most bedding plants. A basket whose newest leaves have gone yellow between the veins will also be flowering poorly, and both problems answer to the same iron feed. Fix the color and the flowers follow.
Cutting back to restart flowering
The most reliable way to restore a tired basket is a hard cutback. By midsummer many baskets carry flowers only in a ring at the ends of long, bare trailing stems, with nothing in the middle. No amount of feeding restores the center, because there is no growth there to flower on.
Shorten every stem by about a third to a half, cutting with scissors just above a pair of leaves. Feed and water immediately afterwards, and keep the compost evenly moist. New shoots break from the crown and from every remaining leaf joint within a week, and the basket is fuller and better balanced than before within three weeks. Done once in midsummer, this typically buys another two months of dense flowering into autumn.
- Choose the moment Cut back when the center has gone bare and flowering is thinning, not when the plant is at its peak. Avoid doing it during a heatwave; wait for a run of milder days so the regrowth is not scorched.
- Cut by a third Take every trailing stem back by a third to a half with clean scissors, cutting just above a leaf pair. Cut the longest stems hardest so the plant ends up an even dome rather than a ragged one.
- Feed straight away Water in a full-strength high-potassium feed the same day. The plant is about to build a lot of new shoots at once and this is the point at which running short of nutrients costs you the most.
- Keep it evenly watered Cut-back plants transpire less, so ease off slightly for the first few days, then return to the normal routine as new growth appears. Do not let it dry out during regrowth.
Late season and what to note down
Flowering continues until the first real frost, which blackens the plant overnight. Cold nights below about 40F (4C) slow it down well before that, and flower size shrinks noticeably as autumn advances even while the count stays high. There is nothing to be done about this and no reason to stop feeding until the plant is actually finished.
Record the week the first flower opened and the week flowering became continuous, along with the date and result of your midsummer cutback. Over two seasons these dates tell you how early your baskets can safely go outside and exactly how long the recovery from a cutback takes in your conditions, which lets you time the plant to be at its fullest for a particular week.