Gadenin日本語Get the appWhy the same seed behaves differently in spring and autumn
Bok choy bolts from cold and from heat, which is unusual and is the source of most confusion about it. A young plant that is past the seedling stage and then sits through a week below about 50F (10C) has been vernalised. It has been told that winter happened, and it will send up a flower stem as soon as the days lengthen and it warms up. That is the spring failure.
The summer failure is a different mechanism. Above about 82F (28C) the plant grows fast and thin, the leaves turn peppery, and heat with long days pushes it into flower anyway. The ideal is therefore mild: air between 59 and 77F (15 and 25C), which is exactly what late summer and autumn deliver, and which is why the autumn sowing is the one that behaves.
| Sowing | Main risk | How it usually ends |
|---|---|---|
| Early spring, outdoors | Cold check, then lengthening days | Bolts at five to six weeks with a small head |
| Late spring | Rising heat late in the crop | Usable if you plan to cut baby heads |
| Midsummer | Heat above 82F (28C) | Peppery leaves, thin ribs, early flowering |
| Late summer into autumn | Very little. This is the window | Full heads and the best quality of the year |
| Late autumn under cover | Slow growth only | Fine, but allow half as long again to reach size |
The season by season run
The following is a sequence rather than a set of dates, so it reads the same in either hemisphere. Anchor it to your last spring frost and your first autumn frost, and to the morning soil temperature rather than to the month.
| Stage | When | What to do |
|---|---|---|
| Bed preparation | Two to three weeks before any sowing | Compost dug in, pH checked towards 6.5 to 7.0 |
| First sowing | Once morning soil at 4 in (10 cm) holds above 54F (12C) | Sow under row cover and mesh. Plan to cut as baby heads |
| Spring succession | Every two weeks until the heat arrives | Short rows only. Do not commit the whole bed |
| Midsummer | Through the hottest weeks | Skip it, or sow under shade cloth for baby heads |
| Main sowing | Six to ten weeks before the first autumn frost | This is the crop to take to full size. Mesh from day one |
| Autumn succession | Every two weeks until four weeks before frost | Put the last sowings under a cloche |
| Winter holding | Once nights sit below 41F (5C) | Hold mature plants under cover and cut them as needed |
Succession is the whole technique
A bok choy plant goes from ready to over in about ten days in warm weather. Sowing a whole bed at once therefore gives you far more than you can eat, then a month with none, then a bed full of flower stems. Sow a short row every two weeks instead and the supply becomes continuous.
A practical unit is one row of about ten plants per two weeks per person, which lands as a mixture of baby heads pulled early and full heads at the end. If one sowing bolts you have lost a fortnight of supply rather than the season, and that resilience is really what succession buys you.
Protecting the spring crop from the cold check
If you want an early crop, the job is to stop the plant ever meeting a week of cold once it has true leaves. Warm the soil under clear plastic or a cloche for two weeks before sowing, sow into the warmed bed, and keep row cover over it at night until the nights are reliably above 50F (10C).
Lower your target as well. An early sowing grown for baby heads and cut at three to four weeks is off the bed before a bolt could show, so the cold check never gets time to cost you anything. Save the full-size ambition for the autumn sowing, when the weather is working with you rather than against you.
Do not pull an early sowing the moment you see a flower stem. Bolting shoots caught before the buds open are the sweetest eating on the plant, so treat a bolted spring crop as a choy sum harvest instead of a loss.
What to record through the year
Write down four things for every sowing: the date, the morning soil temperature at four inches (10 cm), how many nights afterwards fell below 50F (10C), and the date the crop was cut or bolted. Note which bed it stood in as well, for the sake of the brassica rotation.
After two seasons that record answers the question no general calendar can. It tells you the earliest sowing your particular garden gets away with, and the exact week in late summer that produces the best full heads. Everything else in this guide is a starting point that your own notes should eventually replace.