Raising Brussels Sprout Seedlings and Planting Them Out Firm

A long season plant grown from a small module

Brussels sprouts occupy the ground for most of a year, which is longer than anything else in a normal vegetable garden. That length is the reason the seedling stage matters so much: an error made in a three inch pot is still visible eight months later as a stem full of loose, open sprouts.

Sow in modules or small pots rather than directly in the bed. You need the plants to spend four to six weeks somewhere you can watch them, and you need the final bed free for something else while they do it. Two seeds per cell, thinned to the stronger one, is the normal way.

The window is late winter under cover for an early crop, or spring outdoors in a nursery bed for the main crop. Judge it by the soil rather than the calendar: seed germinates in four to seven days once soil is above 54F (12C), and sits and rots below 45F (7C).

Grow them hard, not fast

The single most common seedling fault is a long, pale, floppy stem. A drawn seedling never fully recovers: it goes into the ground already unstable, rocks in wind all summer, and rocking is what produces open sprouts. You want a plant that is shorter than it looks like it should be, with a stem you can feel between finger and thumb.

Three things cause stretching, and all three are easy to remove. Too little light, too much warmth at night, and too much water. Sprout seedlings are happiest cool and slightly hungry.

1. Give them the brightest place you have Full sun from the moment the seed leaves open. A windowsill that gets light from one side only produces a leaning plant, so turn the tray daily if that is all you have.
2. Drop the temperature after germination Once they are through, move them somewhere cooler, around 59F (15C) by day and below 50F (10C) at night. Germinating warm and growing on cool is the standard method and it makes a noticeably stockier plant.
3. Water in the morning only Let the surface of the module dry between waterings and water early so the compost is not wet overnight. Constantly saturated modules give soft stems and invite damping off.
4. Thin to one at the first true leaf Cut the weaker seedling off at compost level with scissors rather than pulling it, so you do not disturb the roots of the one you are keeping.
5. Harden off for a week Stand the tray outside for progressively longer each day for about a week before planting. A plant that goes straight from shelter to open ground checks badly and sulks for a fortnight.

If the seedlings have already stretched, plant them deeper than normal so the stem is buried up to the seed leaves. Brassicas root from the buried stem and this partly rescues a drawn plant.

Preparing ground that will hold a five foot plant

A finished sprout plant is three to four feet (90-120 cm) tall, sometimes five, carrying a heavy load high up and catching every autumn gale. The ground has to be able to anchor it. Soft, fluffy, recently dug ground cannot, and the plant leans, the stem works loose, and the sprouts blow open.

Prepare the bed several weeks before planting so it has time to settle. Work in compost, add lime if your soil is acidic, and then leave it alone. Sprouts want a pH of 6.5 to 7.5, which is higher than most vegetables, and firm, settled, limed ground is also the best defense against clubroot.

Planting out and firming

Plant when the seedling has four or five true leaves and the module is filled with root but not yet spiralling round the bottom. Later than that and the check to growth costs you weeks at the end of the season.

The firming is not optional and it is not gentle. Standard practice is to plant, water, and then tread the soil around the stem with your heel until you can pull a leaf and tear the leaf rather than move the plant. Gardeners who have never grown sprouts before almost always underdo this, and it is the single most common cause of loose sprouts in a first attempt.

1. Make a deep hole Dig a hole deep enough to set the plant with its lowest leaves an inch or two (3-5 cm) above the soil. Burying part of the stem gives extra rooting and extra stability.
2. Water the hole, not the plant Fill the hole with water and let it drain before setting the plant in. This puts water where the roots will go instead of on the surface where it evaporates.
3. Backfill and tread hard Push the soil back and tread it down firmly all round the stem with your full weight. The surface should be visibly compressed. Then loosen only the top half inch with a hand fork to stop it capping.
4. Net immediately Cover with insect mesh the same day. Cabbage root fly and cabbage white butterflies find a newly planted brassica within hours, and a young plant has no reserve to lose leaves.
5. Check the plant does not move A week later, take hold of a lower leaf and pull sideways. The plant should not rock in the soil at all. If it does, tread it again.

Record the planting date and the position of each plant in the bed. Sprouts should not go on the same ground for at least three years because of clubroot, and by next spring you will not remember which end of the bed they were on.

The first month in the ground

For the first two weeks the plant is repairing roots, not growing. Water thoroughly two or three times in that period rather than a little every day, then leave it to send roots down. A shallow daily sprinkle produces a shallow root plate, which is another route to a plant that rocks.

Do not feed at planting. The compost dug in beforehand is enough to get them going, and nitrogen applied now pushes soft leaf growth at exactly the wrong stage. The feeding that matters comes later, in the middle of the season, and stops well before the sprouts start forming.