Gadenin日本語Get the appWhat pinching does to a cosmos
A young cosmos grows as a single leading stem because the growing tip releases a hormone that keeps the buds in the leaf joints below dormant. Take the tip off and that suppression stops. The two or three pairs of side buds nearest the cut all break within about a week, and each of those shoots goes on to carry its own flowers and its own side branches.
The result is a shorter, wider, considerably sturdier plant with far more flowers. With cosmos the sturdiness matters as much as the flower count, because an unpinched tall type is a single brittle stem with all its weight at the top and it is the first thing in the garden to go over in wind.
The cost is time. Pinching delays the first flower by roughly ten days to two weeks. That is almost always worth paying, because the plant then flowers for far longer and from far more points.
Pinch at eight to twelve inches tall
The window is when the plant stands eight to twelve inches (20 to 30 cm) tall with three or four pairs of true leaves, which is usually a week or two after potting up or a few weeks after direct sowing. Pinch much earlier and there is not enough leaf left below to power the regrowth, so the plant sits still for a fortnight. Pinch much later and you are cutting away stem the plant has already paid for.
Take the top three to four inches (8 to 10 cm), cutting just above a pair of leaves. Use your thumbnail on soft growth or clean snips on anything firmer. Cut a few millimeters above the leaf joint rather than through it, because a long stub above the joint dies back and can rot down into live stem.
Never pinch below the second pair of true leaves. A cosmos left with only its seed leaves has no engine to regrow with and will either sulk for a month or die outright.
- Count the true leaf pairs Ignore the two narrow seed leaves at the very bottom. Count only the finely divided true leaves, and pinch once there are three or four pairs of them and the plant is at least eight inches tall.
- Cut just above a leaf joint Two or three millimeters above the joint is right. Cutting through the joint removes the very buds you are trying to release, and leaving a long stub invites rot.
- Pinch on a dry morning The wound seals within a few hours in dry air. Pinching in wet weather leaves an open cut for gray mold, which is the only real risk in the whole operation.
- Ease off the water for a few days The plant has just lost leaf area and is transpiring less. Keeping it wet while it has no growing tip is how young cosmos rot at the base.
A second pinch, and when to stop
Two or three weeks after the first pinch, the side shoots will be six to eight inches (15 to 20 cm) long. Pinching each of those tips again doubles the branch count and gives a dense, low, entirely self supporting plant. This is worth doing for containers, for the front of a border, and for anywhere windy, and it delays first flower by roughly another two weeks.
Stop pinching the moment buds are visible. From then on every tip you take is a flower you have already grown and thrown away, and the plant will branch perfectly well on its own through deadheading. As a working rule, finish all pinching before the plants go out, or within the first fortnight after planting.
| Plant stage | Action | Effect |
|---|---|---|
| 8 to 12 in (20 to 30 cm) tall, 3 to 4 true leaf pairs | First pinch, take 3 to 4 in | Two to four main branches, first flower about 10 days later |
| Side shoots 6 to 8 in long | Second pinch of each shoot | Dense self supporting plant, first flower about 2 weeks later still |
| Buds visible | Stop pinching | Further pinching only removes flowers you have grown |
| In flower | Deadhead instead | Cutting back to a bud does the same branching job |
| Leggy in late summer | Cut back by a third | One more strong flush before frost |
Which plants to pinch and which to leave
Pinch every tall type without hesitation. A four to six foot cosmos that has never been pinched is a liability: one stem, top heavy, and certain to need staking. Pinched once, the same plant becomes a wide bushy thing that often holds itself up.
Dwarf bushy types branch freely on their own and gain much less from pinching, though one pinch still tightens them up and does no harm. If you are short of time, pinch the tall ones and leave the dwarfs.
The one case for not pinching is cut flowers. If you want long single stems for a vase, leave the leading tip alone, stake the plant properly, and accept fewer, longer, straighter stems. You cannot have both on the same plant, so decide before you pinch, because it cannot be undone.
Note which plants you pinched, on what date, and roughly how many flowering stems each carried by midsummer. One season of that comparison across a few plants settles the question for your own conditions better than any general rule.
Rescuing a plant you never pinched
A leggy unpinched cosmos already heading for flower is not a lost cause. Cut the whole plant back by a third, taking every stem down to just above a leaf joint, and give it nothing to eat. It breaks from those joints within a week or so and flowers about three weeks later, bushier and much steadier than it was.
The same cut works in late summer on a plant that has gone thin, sprawling and bare in the middle. Cosmos takes hard cutting back better than most annuals because dormant buds sit at every leaf joint the whole length of the stem. The only cut that will not recover is one made below the lowest leaves, into bare stem with no buds left on it.