Gadenin日本語Get the appSow into cells, not into a flat tray
Broccoli hates having its roots torn. Sow two seeds per cell into a tray of 2 inch (5 cm) cells, half an inch (1 cm) deep, and thin to the stronger one once the first true leaf shows. Seedlings transplanted of a shared tray sulk for a week and often bolt later, because every check to growth in the seedling stage counts against you.
Seed germinates over a wide range, from about 45F (7C) to 85F (29C), and comes up in four to seven days at 68F (20C). Above 85F (29C) germination drops off sharply, so for a crop that will head in cool weather, sow in a shaded spot outdoors rather than in a hot greenhouse.
- Two seeds per cell Drop two seeds into each cell and cover with half an inch (1 cm) of compost. Firm the surface lightly so the seed sits in contact with damp compost rather than in an air pocket.
- Thin with scissors When the first true leaf appears, snip the weaker seedling off at compost level. Pulling it out drags the roots of the one you are keeping.
- Move to bright light immediately The moment the seed leaves break the surface, give the tray the brightest position you have. Broccoli goes leggy within 48 hours in dim light and never fully recovers.
Seed keeps well. Stored cool and dry, broccoli seed still germinates strongly at three years old, so a packet is worth sowing in small batches two or three weeks apart rather than all at once.
What a plant ready to go out looks like
The right transplant has four to five true leaves, a stem about as thick as a pencil, and short gaps between the leaf joints. That plant is roughly four to six weeks from sowing. Younger than four leaves and it has no reserves to push through transplant shock; older than six leaves in a small cell and it is already root bound.
Slide one out of its cell and look. You want white roots working around the outside of the compost, holding it together when you lift it. A solid brown mat wrapped around the base means it has sat too long. That plant will head early and small, a habit gardeners call buttoning, and no amount of feeding afterwards fixes it.
| Stage | What you see | Verdict |
|---|---|---|
| Two to three true leaves | Compost falls away from the rootball | Too young. Give it another week |
| Four to five true leaves | White roots just reaching the sides | Plant it out now |
| Six to seven true leaves | Roots circling, lower leaves yellowing | Plant today and expect a smaller head |
| Any size, stem thin and stretched | Long bare stem below the leaves | Grown too dark. Bury the stem when planting |
Cold at the seedling stage causes bolting later
Broccoli reads cold as a signal that winter has passed. Once a plant has a stem thicker than about a quarter of an inch (6 mm), a spell of ten days below 50F (10C) can flip it into flowering mode, and it will then throw a tiny, loose head as soon as the weather warms. This is the single most common reason a spring crop fails.
The practical rule is to keep young plants above 50F (10C) until they have five or six leaves. For a spring crop that means raising the seedlings under cover and planting out under row cover. Seedlings that are still very small, under a quarter inch of stem, are much less sensitive, which is why an early sowing that stays tiny through a cold snap often does better than one that grew well.
- Keep spring seedlings above 50F (10C) Raise them on a windowsill, in a cold frame with a cover, or in a heated propagator. A few cold nights are survivable; ten days of them are not.
- Harden off over a week Stand the tray outdoors for two hours on the first day and build up to a full day and night by the seventh. Skip this and the leaves bleach white in the first strong sun.
- Cover the bed after planting Row cover or a low tunnel over a spring planting holds a few degrees overnight and keeps cabbage root fly and pigeons off at the same time.
Watering seedlings and the two things that kill them
Cells dry out fast and broccoli seedlings wilt quickly, but the bigger risk is the opposite. Compost kept sodden brings on damping off, where the stem collapses at soil level overnight and the seedling topples. Water in the morning, let the surface dry between waterings, and give the tray air movement.
The other killer is flea beetle. On a warm day it shreds the seed leaves of a whole tray in an afternoon, leaving them peppered with tiny round holes. A seedling with half its leaf area gone will not catch up. Fine insect mesh over the tray from the day you sow is far more effective than anything you spray afterwards.
- Water from below when you can Stand the tray in an inch (2 cm) of water for ten minutes, then lift it out. This keeps the surface drier and cuts damping off.
- Mesh the tray from day one Lay fine insect mesh over the tray as soon as it is sown. Flea beetle finds a tray of brassica seedlings within days of them emerging.
- Give the tray a light feed at four leaves If the leaves are pale and the plant is not yet ready to go out, a half strength liquid feed holds it without pushing soft growth.
Write down the sowing date and the leaf count
Broccoli is timed backwards from the weather you want the head to form in, so the useful record is not just when you sowed but how long the plant took to reach four or five leaves. That number changes with temperature by two weeks or more between an early and a late sowing, and it is the figure that tells you when to start next season.
Note the sowing date, the date the plant reached four true leaves, the planting out date, and where in the bed each batch went. If one sowing buttons and another heads perfectly, the record shows you which cold spell caught which batch, and you will move the sowing accordingly next year.