Growing Cauliflower Seedlings: Sowing to Planting Out

The head is decided before you plant out

Cauliflower is the least forgiving of the brassicas at the seedling stage. A plant that sat dry in a cell for two days, or spent a week starved in a pot that was too small, will still grow, still look green, and still make a head. It will simply make a head the size of a golf ball. That failure has a name, buttoning, and almost all of it is set before the plant ever reaches the bed.

The reason is that a cauliflower has only one growing point and one crop from it. A cabbage that is checked recovers and heads later; a cauliflower that is checked converts its single growing point to a curd early, on a plant that has nowhere near enough leaf to fill it. There is no second head and no way back.

So the whole aim of the seedling stage is a plant that has never once stopped growing. Not the fastest plant, not the biggest plant, but one with a completely uninterrupted history.

If you can only fix one thing about your cauliflowers, fix the seedling stage. Everything from planting out onwards is maintenance of a decision that was already made.

Sowing into modules

Sow into cells rather than a seed tray. Cauliflower resents having its roots torn apart, and a module raised plant goes into the bed without losing a single root hair. Use a cell of at least one and a half inches (4 cm) across, and be ready to pot on rather than let it fill.

1. Fill and firm the cells first Fill with a fine seed-starting mix, tap the tray down and level it off. Loose compost settles after watering and leaves the seed sitting in a hollow that stays wet.
2. Sow two seeds per cell, half an inch deep Sow at about a quarter to half an inch (5 to 10 mm) and cover. Two seeds per cell insures against a blank; you will remove one later.
3. Water from below Stand the tray in shallow water until the surface darkens, then lift it out. Watering from above shifts small seed and caps the surface.
4. Keep it at 68 to 77F (20 to 25C) to germinate At that range seedlings appear in four to six days. Below 59F (15C) it drags on for two weeks and losses rise. Above 86F (30C) germination falls off sharply.
5. Move into full light the day they emerge The instant the loops of the stems break the surface, get them into the brightest place you have. A single warm dim day gives you long stretched stems that never straighten.
6. Remove the weaker seedling at one true leaf Cut the loser at compost level with scissors. Pulling it out disturbs the roots of the one you are keeping, which is exactly the check you are trying to avoid.

Temperature and light while they grow on

Once they are up, cauliflower seedlings want to be cooler than they were for germination but never actually cold. Growing them warm and dim gives soft, drawn plants that collapse at planting. Growing them genuinely cold for a stretch of weeks is what starts a curd on a plant with six leaves.

The most useful number to hold on to is 50F (10C). A young cauliflower that spends a long run of days below that will start heading regardless of its size. That is why an early spring sowing left on a cold windowsill so often ends in buttons.

Long runs below 50F (10C) on a young plant are the classic trigger for premature heading. If a cold spell is forecast, bring the tray in at night rather than letting it stand it out.

Potting up before the roots circle

A cell that is full of roots is a plant that has stopped. Cauliflower shows it late, so by the time the leaves look pale the check has already happened. Judge by the root, not the leaf: tip a plant out of its cell once a week and look at the base of the plug.

If you sowed early and the weather has held you outside, potting up into a three inch (8 cm) pot buys you two or three weeks of unchecked growth. That is far better than planting into cold, wet soil on schedule.

1. Tip one out and look weekly Squeeze the cell and slide the plug out whole. White roots visible at the sides with compost still showing is right. A solid mat of root with no compost visible means you are already late.
2. Pot up into three inch (8 cm) pots Use a compost with some nutrition in it rather than seed-starting mix. Set the plant at the same depth it was, firm lightly, and water it in once.
3. Feed weakly if they stall A pale, slow plant in a full cell needs a half strength liquid feed while you find it a bigger pot. Feed does not substitute for root room, it only buys days.
4. Space the pots as the leaves meet When leaves from neighboring plants touch, spread the pots out. Crowded seedlings stretch towards each other and go soft at the stem.

Hardening off and planting out

Plant out at four to five true leaves, on a plant six to eight inches (15 to 20 cm) tall with a stem thick enough that it does not bend under its own leaves. Older than that and the roots are already restricted. Younger and it has not got the reserves to take a knock from wind or slugs.

Harden off over seven to ten days, but stop short of chilling them. The aim is to toughen the leaf cuticle against wind and sun, not to give them the long cold spell that triggers a premature curd.

1. Start with a few hours outside Two or three hours in a sheltered, part shaded spot on the first day, building up over a week to a full day out. Wind does more damage than cold at this stage.
2. Bring them in at night for the first week Nights below 41F (5C) on an unhardened plant are a real check. Once they have had a week out, leave them out unless frost is forecast.
3. Water the tray an hour before planting A dry plug crumbles as you lift it and half the roots tear off. Wet, it slides out whole.
4. Plant slightly deeper, up to the seed leaves Setting the stem an inch (2 to 3 cm) deeper anchors a plant that will later carry a heavy head and a large leaf area. Do not bury the growing point.
5. Firm hard and water in Press the soil down around the plug with your knuckles until the plant resists a gentle tug. A loose plant rocks in wind and never roots properly. Then water a pint or so into each planting hole.
6. Shade for two days in strong sun Lay a piece of netting or an upturned pot over each plant for the first two afternoons if it is hot and bright. Wilting on day one is a check like any other.

Space at 20 to 24 inches (50 to 60 cm) each way for a full sized summer or autumn crop, and 24 to 28 inches (60 to 70 cm) for an overwintered one. Crowding cauliflower costs head size directly.

What to write down at this stage

Cauliflower failures show up two or three months after their cause, which makes them almost impossible to diagnose from memory. A few lines noted at the seedling stage turn next year's guesswork into a decision.

Note the checks even when the plants recover and look fine. A recovered seedling is exactly the case you will not remember, and exactly the one that explains the small head.