Gadenin日本語Get the appWhat a good seedling looks like
The first true leaves are small, narrow and fleshy, and they carry the same glistening surface as the adult plant, which under a hand lens looks like a scatter of tiny water droplets that do not wipe off. A good seedling is short, with almost no visible stem between the seed leaves and the first true leaves, and it sits low and compact in the cell. The color is a slightly blue or gray green rather than a soft bright green.
A bad seedling is easy to recognize and impossible to correct. It has a long pale stem below the seed leaves, it leans towards the light, and it falls over when you water it. That plant will never make a dense flowering mat, and the right response is to discard the tray and sow again rather than to nurse it along. Sowing again costs about two weeks and gives you a far better plant.
Between germination and planting out you are aiming for roughly six weeks of steady, unhurried growth. There is nothing to be gained by pushing them. A seedling grown hard and slow in strong light will establish faster and flower sooner than a large soft one, because it goes outdoors without a check.
Light, not warmth, is what they need
The seed needs warmth of 60 to 65F (15 to 18C) to germinate, but as soon as the seedlings are up the priority changes completely. Drop the temperature and put them in the brightest position you have. A cool bright greenhouse or a cold frame is ideal. A warm windowsill in late winter is the worst common option, because the warmth drives growth that the weak light cannot support.
The classic mistake is to leave the tray on a propagator or above a radiator after germination. Within a week the seedlings are twice the height they should be and permanently spoiled. If your only bright place is cold, use it; these seedlings tolerate cool conditions well as long as they are not frosted, and cool bright growing is exactly what produces the stocky plant you want.
Turn the tray every day or two if the light comes from one side only, so that the seedlings grow upright rather than leaning. If everything in the tray is leaning the same way and stretching regardless, the light is simply insufficient and the honest answer is a later sowing rather than a rescue.
Watering the tray without losing it
Water from below by standing the tray in shallow water until the surface darkens, then lift it out and let it drain completely. Overhead watering flattens these fine seedlings and, more importantly, keeps the stems wet at soil level, which is exactly where damping off attacks. Let the surface dry between waterings; the mix should never be continuously wet.
Keep air moving. A tray in a still, humid, closed propagator will damp off even if the watering is careful. Take the lid off as soon as germination is under way, and open a vent or a door in the daytime whenever the weather allows. Space is part of this too, which is why thin sowing matters so much with such fine seed.
A shallow dressing of fine grit or vermiculite over the surface after germination helps considerably. It keeps the vulnerable point of the stem dry, stops moss and liverwort colonising a damp surface, and does not interfere with the light the seedlings need.
Do not feed seedlings. A fed seedling grows soft and large and then sulks for a fortnight after planting into poor ground. Everything about raising this plant is aimed at producing something that is already used to lean conditions before it goes outside.
Transplanting seedlings and growing on
Transplant as soon as the seedlings are large enough to hold by a seed leaf, which is usually two to three weeks after germination. Handling them later is harder because the fine roots tangle together in the tray.
- Loosen rather than pull Slide a plant label or a dibber under a small group of seedlings and lift them together with their mix. Never pull a seedling out by the stem; a bruised stem at soil level almost always rots.
- Handle by a leaf only Separate the group gently and hold each seedling by a seed leaf. The stem at this stage is a thread and the plant can regrow a damaged leaf but not a crushed stem.
- Plant one to a cell in gritty mix Use a free-draining mix of about two parts compost to one part grit or perlite, in cells or small pots of two to three inches (5 to 8 cm). Set each seedling slightly deeper than it was, up to just below the seed leaves, and firm lightly.
- Water in once from below Stand the tray in shallow water until the mix darkens, then drain fully. Do not water again until the surface has dried, which in cool bright conditions may be four or five days.
- Grow on cool and bright Keep them at 50 to 60F (10 to 15C) in the strongest light available for about four weeks. Expect slow, hard, compact growth and do not be tempted to push them with warmth or feed.
Hardening off and planting out
Harden off over a week to ten days before the plants go out. Start with a couple of hours outdoors in a sheltered spot, build up to a full day, and finish with two or three nights outside once frost is genuinely past. Seedlings raised under cover have thinner leaves than the plant will make outdoors, and moving them straight into full sun scorches them badly enough to set them back for weeks.
Plant out only after the last frost and once the soil is above about 60F (15C). Knock some of the raising mix off the roots so that the plant meets the gritty ground it will live in, set it six to eight inches (15 to 20 cm) from its neighbors with the crown just proud of the surface, and water once. Then leave it alone, and protect it from slugs for the first three weeks, which is the only period when it is genuinely vulnerable.
Keep a short record of the sowing date, the date you transplanted, the date you planted out and the date of the first flower, along with a note on how much light the seedlings had. That sequence tells you next winter whether your sowing was too early for the light you can offer, which is by far the commonest reason for a poor batch of plants.