Gadenin日本語Get the appWhy the seedling stage decides the crop
Celery spends longer as a seedling than almost anything else in the vegetable garden. From sowing to a plant fit to go outside is ten to twelve weeks, which is two to three times what a tomato needs. Everything that goes wrong later in the season, stringy stalks, bitterness, a plant that runs to flower instead of building a head, traces back to a stall somewhere in those weeks.
The plant has no way to recover from a check. It does not put out fresh growth to replace what it lost, the way a bean or a squash will. It simply carries the damage forward as tough fiber in the stalks you eventually cut. That is why the seedling stage is worth more care than the whole rest of the season put together.
There are three things to get right and they are all decided before you sow: enough warmth for long enough, light on the seed rather than soil over it, and a compost that never dries. Miss any one and germination is patchy or the survivors are already compromised.
| Stage | How long it takes | What must not happen |
|---|---|---|
| Germination | Two to three weeks at 68F (20C) | The surface drying out even once |
| First true leaves | Two to three weeks after that | A stretch below 50F (10C) |
| Transplanting seedlings to planting size | Six to eight weeks | Running dry, or starving in a small cell |
| Hardening off | Ten to fourteen days | Going straight outside from a warm windowsill |
Count backwards from your last frost date. Celery wants to go outside once nights hold above 50F (10C), so sowing has to happen roughly ten weeks before that, which is usually late winter under cover.
Sowing: on the surface, not under it
Celery seed is tiny, roughly the size of a grain of coarse dust, and it germinates in response to light. Cover it and most of it will never come up. Sowing on the surface of a firmed, pre-watered tray is not a shortcut, it is the method.
Sow into a tray or pot rather than direct into the ground. Celery transplants well, which means you can hold it in ideal conditions for the weeks it needs rather than defending a patch of open soil.
- Fill and firm a seed tray Use a fine seed-starting mix, fill level and press it down so the surface is flat and firm. A loose surface swallows the seed and drops it below the light.
- Water before you sow, not after Soak the tray from below until the surface is damp through, then let it drain. Watering after sowing washes surface seed into cracks and corners, and you lose the even spacing you just set up.
- Scatter the seed thinly on top Aim for seed roughly a quarter of an inch (5 mm) apart. Mix it with a pinch of dry sand if you cannot see what you are dropping. Leave it uncovered, or dust the very lightest sieving of vermiculite over it if the air is dry.
- Cover the tray, not the seed A clear lid, a propagator top or a sheet of glass holds humidity without blocking light. This is what keeps the surface damp for three weeks, which no watering can can manage on its own.
- Hold it at 68F (20C) Germination is reliable between 59F and 70F (15C and 21C) and drops away sharply outside that. Above 77F (25C) celery seed can go into a dormancy that no amount of patience breaks. A warm indoor room beats a cold greenhouse.
- Wait three weeks before you give up Fourteen to twenty-one days is normal and stragglers appear later still. People resow at day ten and end up with two ages of seedling in one tray.
Cold at the seedling stage is what makes celery bolt
Celery is biennial. It is built to grow leaves in its first year, sit through a winter, and flower in its second. A young plant reads a run of cold as that winter, and once it has read it, it will flower the moment the days lengthen, no matter how well you feed it afterwards.
The trigger is roughly ten days or more below 50F (10C) on a plant that already has true leaves. Very small seedlings are less sensitive than ones the size of your palm, which is exactly backwards from what most people assume, so the risky moment is the week you feel confident enough to move the tray somewhere cooler.
In practice this means two rules. Do not move seedlings to an unheated greenhouse or a cold frame too early in spring, and do not plant out during a cold snap just because the calendar says it is time. Waiting a fortnight costs you nothing. Bolting costs you the plant.
A plant that has bolted sends up a hard, round flowering stem from the center of the crown and the stalks turn woody within a week or two. There is no fix. Cut what is still tender for the pot and use the space for something else.
Transplanting seedlings and growing on
Once each seedling has one or two true leaves it needs its own space. Left crowded in the seed tray they stretch, the stems thin out, and thin stems never thicken up again into good stalks.
Handle them by a leaf, never the stem. Celery seedling stems are thin enough to crush between finger and thumb without you noticing, and a crushed stem rots at the base a few days later.
- Move to cells or small pots Two inch (5 cm) cells or three inch (7 cm) pots, one seedling each, in a general purpose compost. Set them at the same depth they were growing. Burying the growing point rots the crown.
- Keep them at 59F to 68F (15C to 20C) Warm enough to keep growing steadily, but not so warm that they stretch towards the light. A cool bright spot is better than a warm dim one.
- Never let a cell dry out Celery is a marsh plant by origin and has a shallow, fine root system with no reserves. Check every day. A cell that dries once will still produce a plant, but that plant will carry fiber in its stalks.
- Start liquid feeding at four weeks A balanced liquid feed at half strength every ten days once the cell is filling with root. Hungry seedlings sit still, and sitting still in a small cell is itself a check.
- Harden off over two weeks Increase outdoor time gradually and bring the plants in on cold nights. Rushing this is one of the commonest routes to a bolted crop, because a couple of cold nights outdoors reads exactly like winter.
What to write down this year
Celery is a crop where notes pay for themselves quickly, because the timings are long and the failures show up months after the cause. Record the sowing date, the date the first seedling appeared and the date the last one did, since that gap tells you whether your germination conditions were steady or marginal.
Also record the date you moved the tray out of the warm room, the coldest night the seedlings saw, and the planting-out date. If a batch bolts, those three numbers between them will almost always show you where the cold came from. Note which end of the bed each batch went into as well, so that next year you can compare a sheltered position against an exposed one on real evidence rather than memory.