Gadenin日本語Get the appSow on the surface and give it heat
Celosia seed is about the size of a poppy seed and black, and it germinates in light. Scatter it on the surface of moist, fine compost and press it in with a flat piece of wood rather than burying it. If you want to cover it at all, use no more than a dusting, an eighth of an inch (3 mm) of sieved compost or vermiculite that light can still pass through. Seed pushed down half an inch will simply not come up.
Temperature does the rest. At 75 to 80F (24 to 27C) at the surface, seedlings appear in 7 to 10 days and come up evenly. At 65F (18C) they take three weeks, come up patchily, and half of them damp off before they get their first true leaves. A heat mat under the tray, or a spot on top of a fridge, is worth more here than any other piece of equipment.
Keep the surface just damp, never wet. Because the seed sits on top of the compost, it dries out in an afternoon in a warm room, and a single drying is enough to kill germinating seed outright. Water from below by standing the tray in shallow water, and cover with a clear lid or a sheet of glass until the first seedlings show. Take the cover off the day they appear.
Buy or save more seed than you think you need. Celosia seed loses viability quickly in a warm damp house, and seed two or three years old typically germinates at half the rate of fresh seed even when everything else is right.
The timing that matters is when it goes out, not when it goes in
Celosia is a hot weather plant that stalls below 60F (16C) and sulks at anything cooler. It goes into the ground after the last frost, once night temperatures stay above 55F (13C) and the soil has reached at least 60F (16C), and it grows twice as fast at 70F (21C). There is no advantage whatsoever in planting it out early. A celosia set into cold ground sits still for a month while a plant put out two weeks later overtakes it.
Work backwards from that date, not forwards from the calendar. Celosia needs only four to six weeks from sowing to the point where it should be planted out, which is far less than most bedding annuals. Sowing in late winter because that is when you sow everything else guarantees you a tray of plants that are ready long before the ground is, and holding them back is exactly what ruins them.
In a long hot summer you can skip the tray entirely and sow directly where the plants are to grow, once the soil is genuinely warm. Direct sown celosia never suffers a transplant check at all and often catches up with plants started six weeks earlier indoors. Thin to final spacing at two true leaves and do not attempt to move the thinnings.
| Stage | Timing | Conditions to hit |
|---|---|---|
| Sow indoors | 4 to 6 weeks before your last frost | 75 to 80F (24 to 27C) at the compost surface, uncovered seed |
| Germination | 7 to 10 days at the right heat | Surface just damp, cover removed the day seedlings show |
| Pot on or space out | At the first true leaves | One seedling per cell or small pot, never a shared tray |
| Plant out | After the last frost, at 3 to 4 true leaves | Soil above 60F (16C), nights above 55F (13C) |
| Direct sow outdoors | Two weeks after the last frost | Soil above 65F (18C), surface sown and pressed in |
| Thin direct sown seedlings | At two true leaves | Snip surplus at ground level, do not transplant thinnings |
One seed per cell, and out of it fast
The whole difficulty with celosia is that it decides to flower in response to stress, and the commonest stress a seedling meets is a pot that is too small. A plant that fills its cell and sits there for an extra fortnight will set a flower bud at four inches (10 cm) tall and never make a proper plant. It stays that size all summer, carrying one undersized head, and no amount of feeding or repotting undoes it.
Transplant or sow into individual cells at the first true leaves so there is no competition and no second move. Deep cells or small pots of about two and a half inches (6 cm) are ideal, big enough that the roots are not circling within a fortnight, small enough to warm up quickly. Handle seedlings by a leaf, never by the stem, because the stem crushes and the plant is finished.
Then get them out. A celosia at three or four true leaves with white roots just reaching the sides of the cell is the plant you want to be setting into the ground. If it already shows a bud in the pot, pinch that bud out at planting and accept that this plant will be a week or two behind the others.
- Sow thinly onto a firmed surface Fill and firm the tray, water it, then scatter seed sparsely and press it into contact with a flat board. Do not bury it. Light is part of the germination trigger.
- Put the tray somewhere genuinely warm Aim for 75 to 80F (24 to 27C). Below 70F (21C) germination is slow and uneven, and slow germination in damp compost is what causes damping off.
- Uncover the moment seedlings appear Leaving a lid on past emergence gives you drawn, floppy seedlings and stem rot within days. Move the tray to the brightest light you have as soon as it is uncovered.
- Move to individual cells at the first true leaves Lift with a dibber under the roots and hold the seedling by a seed leaf. One plant per cell, planted no deeper than it was growing.
- Harden off over about a week Increase outdoor time daily once nights are mild, but never leave them out below 50F (10C). Cold nights during hardening off cause the same early budding a cramped pot does.
- Plant out at 3 to 4 true leaves Set them into warm ground at the spacing you want and water them in. A slightly small plant going into warm soil beats a large one going into cold soil every time.
Record the date you sowed, the date the first seedling showed and the date you planted out. After one season those three numbers tell you exactly how many weeks your setup needs, and you can stop sowing too early.
Spacing decided at planting, not later
How far apart you set them depends on which form you are growing, and it is worth measuring rather than eyeing it. The dwarf bedding kinds branch into a low mound and want 8 to 10 in (20 to 25 cm). The tall cutting and drying kinds reach 24 to 40 in (60 to 100 cm) and want 12 to 18 in (30 to 45 cm), closer if you are growing them as a block for straight stems, wider if you want branched plants with many heads.
Crowded celosia is the reliable way to get thin stems, small heads and leaf disease in the second half of summer. Because the plants are stiff and upright they do not visibly complain about spacing the way a sprawling annual does, so it is easy to plant far too close and only find out in late summer when the lower leaves start spotting.
Set the plants slightly proud of the soil rather than in a dip. Celosia stems rot at the base in standing wet, and a shallow saucer of soil around the stem is enough to hold water against it after heavy rain. Water them in well at planting, then leave them alone for a few days rather than keeping the ground constantly saturated.