Gadenin日本語Get the appHow a head actually forms
Napa cabbage grows in two distinct phases. First it builds an open rosette of large outer leaves lying almost flat, and that phase runs on warmth. Then, once the plant has roughly 15 to 20 outer leaves and nights start dropping toward 50F to 60F (10C to 15C), the newer leaves stop opening outward. They stay upright, overlap, and the head fills from the inside as those inner leaves keep expanding in the enclosed space.
That means the head is not something you can encourage directly. Its size was decided by how big a frame the plant built beforehand. If the outer rosette is small when the cool weather arrives, the head will be small too, and nothing you do in the last three weeks will change it. Everything in this crop points back to the first month.
Reading the plant week by week
You can tell well in advance whether a head is coming. Watch the newest leaves at the center. While the plant is still in the frame-building phase they open out and lie back. When heading begins, the youngest leaves start standing up and curving in at the tips, and the center of the plant looks like a loose cup. From that point it usually takes three to four weeks to a firm head.
Squeeze the head gently with an open hand every few days once it starts to close. It goes from airy, to springy, to firm. Firm is the point of harvest. If the plant is well past its expected date and the center still lies open, it is not going to head this season, and the leaves are better used loose than left in the ground taking space.
| Stage | What the plant looks like | What to do |
|---|---|---|
| Frame building | Large outer leaves lying open and flat, center visible | Feed and water steadily, keep mesh on |
| Cupping | Youngest leaves upright, tips curving inward | Stop nitrogen, keep water even |
| Filling | Head closed but soft when squeezed | Water deeply once or twice a week, check inside for slugs |
| Firm | Head resists an open-handed squeeze | Harvest within one to two weeks |
| Overripe | Head very hard, outer leaves yellowing, splitting at the top | Cut immediately, quality is already dropping |
The five common reasons a head does not fill
Almost every loose or missing head traces to one of a short list of causes, and most of them are decided before the plant is halfway grown. Working through them honestly against your notes is more useful than changing variety, because the same cause usually repeats from year to year in the same garden.
The most frequent by far is sowing too late. A plant with only eight or ten outer leaves when the nights turn cool simply has nothing to fill a head with. It will make a small open rosette and sit there. Moving the sowing a week or two earlier next season fixes it outright, provided the pest protection goes on at the same time.
- Sown too late Too few outer leaves before the cool weather arrives. The plant stalls in an open rosette. Fix by sowing one to two weeks earlier, counting back from the first frost.
- Crowded Plants closer than about 12 inches (30 cm) shade each other and each builds a small frame. Fix by thinning early. Thinning after heading begins does not help the plants left behind.
- Short of water during frame building A dry fortnight in the middle of growth stops leaf expansion permanently. Fix by watering deeply twice a week in dry spells rather than lightly every day.
- Nitrogen running out early The leaves go pale and small from the outside inward around week four. Fix with a nitrogen feed at three and again at five weeks, stopping once cupping starts.
- Too much shade Napa cabbage wants six hours or more of direct sun. A row shaded by a tall summer crop that was still standing when you planted will not head properly. Fix by clearing the neighbors or moving the row.
Do not respond to a slow-heading plant by adding more nitrogen once the leaves have begun to cup. Late nitrogen produces soft inner leaves, a head that is loose rather than dense, and a much higher risk of soft rot in wet autumn weather.
Feeding for the frame, then stopping
The feeding pattern that produces a heavy head is generous early and nothing late. Give a nitrogen-rich side dressing about three weeks after planting, when the plant is expanding fast, and a second one two weeks after that if the leaves look at all pale. Water it in. That is the whole program. The aim is the largest possible frame of dark green outer leaves by the time the nights cool.
Once the youngest leaves stand up and start to cup, stop feeding nitrogen entirely and change nothing else about the watering. From here the plant is packing existing growth into a dense head, and the only input it needs is a steady water supply. This is also the moment when uneven watering causes tip burn inside the head, so keep it consistent rather than heavy.
Tying the head, and when it is worth it
In cold districts gardeners sometimes gather the outer leaves up over the head and tie them loosely with soft twine once the head is firm. It gives a few degrees of frost protection and keeps rain out of the top of the head, which slows the soft rot that starts where water sits. Tie loosely, and only when the leaves are dry.
It is not a way to make a loose head firm up. Tying a head that has not filled just traps damp air around leaves that are still expanding, and it is a reliable way to produce slug damage and rot. Treat it as short-term frost insurance on a finished head, and note in your records which plants you tied and how they kept, because the benefit varies a lot with how wet your autumn is.