Gadenin日本語Get the appSun and heat drive the flowering
Petunias want six to eight hours of direct sun. In four hours they keep growing but the stems lengthen between the leaves, the flowers come at half the rate, and the whole plant tilts towards the brightest side of the balcony. A hanging basket that flowers thickly on the sunny face and is bare on the wall side has not got a disease; it has an even light problem, and turning the basket a quarter turn every few days fixes it within a fortnight.
Heat is a different matter from light. Petunias flower hardest between 65F and 80F (18C and 27C). Above about 90F (32C) the plant keeps its leaves but slows the production of new buds, and the flowers that do open are smaller and fade faster. That midsummer lull is normal and temporary, and it is the same lull that leaves the plant looking bare in the middle, which is dealt with by cutting back rather than by feeding harder.
Judge water by the weight of the pot, not the surface
A full basket of petunias in high summer transpires enough to need water once a day, and in a hot dry wind twice a day. The top of the compost dries within an hour of watering and tells you nothing. Lift the basket instead, or tip it up on its bracket with one hand. A watered basket is heavy and unmistakable, a dry one is light enough to swing.
Water until it runs freely out of the base, then stop. Half watering a basket wets the top two inches and leaves the root ball at the bottom permanently dry, and the plant behaves as if it were in drought no matter how often you visit it. If a basket has gone bone dry and water now runs straight through the gap between compost and pot, stand the whole thing in a bucket for twenty minutes to rewet it properly.
Petunias recover from a mild wilt without lasting damage, so erring dry is safer than keeping the compost sodden. What they do not survive is standing water. A pot without free drainage, or one sitting in a full saucer through a wet spell, rots at the collar, and the first symptom is a whole stem going limp and staying limp after watering.
| Situation | How often | What to check |
|---|---|---|
| Hanging basket, high summer | Daily, twice in hot wind | Lift the basket to judge weight |
| Large container on the ground | Every one to two days | Push a finger two inches (5 cm) down |
| In the ground, established | A deep soak once a week in dry spells | Midday wilt that recovers by evening is normal |
| Cool wet spell | Stop watering entirely | Empty saucers; look for limp stems that do not recover |
| Just planted out | Every two or three days for two weeks | Keep the original root ball damp, not just the soil around it |
Feed a basket hard and keep feeding it
This is where most baskets fail. Petunias are among the hungriest bedding plants grown, and the compost in a basket is exhausted about six weeks after planting. From that point the plant is living entirely on what you give it. Feed a liquid fertilizer at full strength once a week, or at quarter strength with every watering, and do not stop when the plant looks good, because it looks good because you are feeding it.
Use a feed higher in potassium than nitrogen once the plants are established, on the pattern of a tomato feed. High nitrogen early on builds the framework you want, but continued through summer it gives long soft growth, wide gaps between the leaves and fewer flowers. If the plant is running to leaf with few buds, switch feeds rather than reducing water.
Yellowing between the veins on the newest leaves at the shoot tips, with the veins staying green, is iron shortage rather than hunger. It shows up in hard tap water areas because the compost drifts alkaline over the season and locks up the iron that is already there. A feed containing chelated iron, or a sequestered iron tonic, greens the plant up in ten days. Feeding more nitrogen at this point makes it worse, because you are asking for growth the plant cannot color.
Write down the week you started feeding and the week the plants first looked pale. Across two seasons that pair of dates tells you exactly when your own compost runs out, which is worth more than any interval printed on a bottle.
Deadheading depends on which sort you are growing
The older large flowered and bedding types set seed readily, and every seed pod left on the plant pulls it away from making new buds. These need deadheading. The trailing spreading types sold for baskets are largely sterile, drop their own spent flowers, and need no deadheading at all. Working out which you have takes one look: if faded blooms hang on as sticky brown rags and swell at the base, deadhead; if they fall off cleanly and the plant stays tidy on its own, leave it.
Deadhead properly by following the spent flower back to where its short stalk meets the stem and pinching there, taking the swelling green ovary with it. Pulling off just the limp petal trumpet leaves the seed pod behind, which is the part that actually costs you flowering. The job is unpleasant because petunia stems and calyxes are covered in sticky glandular hairs that blacken your fingers within minutes, and doing it in the morning while the plants are turgid is markedly easier than in the afternoon heat.
Even a self-cleaning trailing type benefits from having whole stems shortened through the season, which is a different job from deadheading. That is covered under cutting back.
- Go over the plants twice a week Ten minutes twice a week keeps ahead of it. Once a fortnight means you are removing swollen pods rather than spent flowers, and the plant has already slowed.
- Pinch behind the seed pod Take the flower stalk and the green swelling at its base, back to the joint with the main stem. Leaving the pod behind defeats the whole exercise.
- Clear the fallen petals off the foliage Sticky spent blooms that lodge in the crown stay wet after rain and start the gray mold that then spreads into healthy stems.
Rain, wind and where you hang the basket
The very large flowered types have thin wide petals that a heavy shower flattens into a translucent rag, and they do not recover. A stretch of wet weather can strip the display off a plant that was faultless the day before, while the small flowered and trailing types in the next basket shrug the same rain off entirely. This is the single strongest argument for putting the big flowered sorts under an eave, a porch roof or a balcony overhang and keeping the small flowered ones out in the open.
Wind does a slower version of the same damage, drying the petal edges to brown paper and shredding them. A basket on an exposed corner needs more water than the same basket six feet away against a wall, and it will look ragged all season no matter how well you feed it.
After a bad storm, go over the plants and strip off every mashed flower rather than leaving them to rot in place. The plant recovers in about a week, faster if you shorten the worst affected stems at the same time.
Note in your records which basket positions took storm damage and which stayed clean. Position matters more than variety choice, and the same two hooks behave the same way every year.