When and How to Sow Purple Choy Sum for Autumn and Winter Stems

Sow in late summer, not spring

The crop here is the flowering stem, cut while the buds are still shut and the skin is still deep purple. That means the sowing date exists for one reason: to line the plant up so it starts sending up stems as the weather cools, rather than in the heat. Sown in spring, the plant races straight to bud on a short, thin stem, throws almost no side shoots, and is finished in a fortnight. Sown in late summer it builds a wide rosette first, and every leaf joint on that rosette becomes a future stem.

The window opens when the worst of the summer heat breaks and closes about eight to ten weeks before your first hard frost. Inside that window, earlier is better for total yield and later is better for quality of the individual stems. A plant sown at the start of the window will be cutting within seven or eight weeks and keep going into hard weather. A plant sown at the very end of it will be small going into cold and will give you a handful of stems in late winter instead of a long autumn run.

Seed germinates from 45F to 85F (7C to 29C) and comes up in three to five days in late-summer soil around 70F (21C). Sow a quarter to a half inch (5 to 12 mm) deep. Any deeper and the seedlings come up stretched, and a stretched seedling makes a leggy crown that sits too high, rocks in wind, and never builds the tight cluster of leaf joints you want.

Spacing decides how long you keep cutting

Every purple choy sum plant gives one thick central stem and then a run of side shoots from the leaf joints below wherever you cut it. The number of usable joints is fixed by how big the rosette got before the plant switched to flowering, and that is almost entirely a spacing question. Crowded plants make a narrow crown with four or five joints. Plants with real room make a broad crown with twelve or more and stay in production a month longer.

For the main crop, give each plant ten to fourteen inches (25 to 35 cm) each way, with rows about eighteen inches (45 cm) apart so you have somewhere to stand. You will be walking this bed every two or three days for weeks, and treading on the outer leaves of a plant snaps the very joints you were hoping to harvest from.

There is a second way to grow it that is worth knowing about: sow thickly in a band, never thin, and take one slim stem from each plant as a single cut. That gives a fast, dense crop from a small patch, but there is no second flush and the bed is cleared in one go. Choose which of the two you are doing before the seed goes in, because you cannot convert a crowded band into a long-cropping bed later.

Direct sowing versus modules

Direct sowing gives the strongest taproot and the sturdiest crown, which is what you want from a plant that has to stand through wind and cold. Sow three seeds every twelve inches (30 cm) along the drill, then thin to the single best seedling at the first true leaf. Thin ruthlessly and thin early. Two plants left sharing one station will each make a thin, one-sided crown and neither will ever catch up.

Modules make sense when the bed is still under a summer crop. Sow two seeds per cell in a three-quarter inch to one and a half inch (2 to 4 cm) cell, thin to one, and plant out at three to four true leaves, which is fifteen to twenty days from sowing in warm weather. Do not hold them longer than that. Brassica seedlings that get root bound in a module carry a permanently checked crown, and a checked plant runs to bud early on a short stem, which is exactly the failure you sowed in late summer to avoid.

Whichever method you use, plant the crown at the same depth it grew, or a fraction deeper. Planted high, the crown wobbles in wind and the plant wastes weeks re-anchoring; planted deep enough to bury the lowest leaf joints, it will root from the stem and stand firm.

1. Prepare the ground before the heat breaks Fork in a bucket of well rotted compost per square yard (per square meter) and rake to a fine tilth. Water the drill and let it drain before sowing, rather than watering after, so the seed sits in moist soil and does not wash to one end.
2. Sow shallow and firm the drill Quarter to half an inch (5 to 12 mm) deep, then press the soil down with the back of a rake. Firm contact matters more than depth in warm dry soil; loose soil dries out around the seed within a day.
3. Shade and water for the first week A sheet of shade netting or a light straw mulch over the drill keeps the surface below 85F (29C) and stops the crust that stops emergence. Lift it the moment the first seedlings show, or they will stretch.
4. Thin at the first true leaf Pinch the surplus seedlings out at soil level rather than pulling them, so the roots of the one you are keeping stay undisturbed. Thin to final spacing in two passes a week apart if slugs are active.

Note the sowing date and the stand

Write down the sowing date, the soil temperature at sowing if you measured it, and the date you saw the first seedlings. Next year those three numbers tell you whether your window opened where you thought it did. Also note how many stations you sowed and how many plants actually stood at final spacing, because germination losses in hot soil are the usual reason a bed looks thin and it is easy to blame the seed a year later.

The single most useful record is the gap between sowing and the first cut. Once you have that figure for your own ground, you can count backwards from the date you want stems on the table and sow to hit it, which is far more reliable than any general guideline.

If a sowing gets caught by unexpected heat and comes up thin and stretched, clear it and sow again rather than nursing it. A resow at week two still beats a checked stand, because the checked plants will bolt short and finish before the good ones start.