Tatsoi Growing Calendar: Autumn Sowing, Winter Harvest and Spring Bolting

The shape of the tatsoi year

Tatsoi is a cool season crop that grows best between about 40F and 70F (4C to 21C) and is exceptionally cold hardy. Established plants take frost without protection and survive down to around 15F (minus 9C) in the open, and under a cloche or low tunnel they will keep making usable leaves under snow. Very few salad greens are still worth walking out to in the middle of winter; this is one of them.

Heat, not cold, is what limits it. Above roughly 77F (25C) the rosette lifts off the ground, the leaves lengthen and go thin, and the plant begins preparing to flower. That gives a year with one main sowing window in late summer and autumn, a long harvest through winter, a short second window in late winter, and an end in spring when the days lengthen.

Nothing here is tied to a calendar date. Work from soil temperature, from your own frost dates and from day length, and the same sequence applies in either hemisphere.

Autumn is the crop that matters

Start sowing as soon as the worst of the summer heat has broken and keep going every two to three weeks until about eight weeks before your first hard frost. Plants from these sowings grow into cooling weather and shortening days, which is precisely what pushes the rosette flat and keeps it from flowering, and the leaves that result are sweeter and denser than anything a spring sowing produces.

The eight week cut-off matters more with tatsoi than with faster greens, because you want the plant at full size before growth stops. Once daylight drops below about ten hours, plants hold rather than grow whatever the temperature does, and you are harvesting from what is already there. A half-grown rosette caught by that will sit at half size until light returns.

Feed while the plants are actively growing, then stop about four weeks before hard frost so the growth hardens off. At the same time, clear all debris from under and around the plants, because whatever is on the ground when you cover the crop will be there all winter.

Winter: the season this crop exists for

Frost improves tatsoi. Cold converts starch in the leaves to sugar, and a rosette picked after several hard frosts is sweeter and rounder in flavor than the same plant in mild autumn weather. Plants look flattened and sorry at dawn in freezing weather and are perfectly fine by midday, so do not judge the crop before the sun has been on it.

A cover takes the crop from merely surviving to actually producing. A cloche, low tunnel or double layer of row cover raises the temperature enough that the plants continue to make usable leaves through most of the winter in a mild region, and it also keeps rain and snow melt off the flat rosettes, which is half the benefit.

1. Cover before the first hard frost, not after Put the cloche or row cover on while the plants are still in good condition. Covering a crop that has already been battered by freeze and thaw only shelters damaged tissue and encourages rot.
2. Ventilate whenever it is mild Open covers on mild dry days. Saturated still air under a cloche in winter causes far more damage than the cold outside it would, and it is where downy mildew and slug problems build.
3. Pick outer leaves only, at midday In deep winter the plant cannot replace what you take, so never remove more than a third of the leaves at a visit. Wait for the leaves to thaw in the sun before cutting; frozen leaves bruise and go to mush.

Spring ends it, and there is no fighting that

Tatsoi bolts in response to lengthening days as much as to warmth. Overwintered plants will send up a flower stem in spring even if the weather is still cold, because it is day length that triggers it, and no amount of watering, feeding or shading will hold them once that starts. This surprises people who expect it to behave like a heat-triggered crop.

The right response is to harvest hard rather than to hope. When the center of a rosette starts to thicken and the inner leaves begin to stand upright and lengthen, cut the plant and use it. Within a week or two after that the older leaves will be tough and the crop is over.

A late winter sowing is still worth making, but treat it as a baby leaf crop rather than as an attempt at full rosettes. Sow thickly under a cloche, shear it twice, and expect it to bolt while it is still small. Full-size plants from a late winter sowing are unusual outside a cold region with a long slow spring.

A bolting plant is not wasted. The flowering shoots are sweeter than the mature leaves and excellent raw or briefly cooked, and they are best picked while the buds are still tight. Leave one plant to set seed and it will give you more tatsoi seed than you can use.

The dates that build your own calendar

Five dates per season are enough to plan properly. Record the sowing date of each row, the date of the first pick from it, the date you covered the crop, the date growth stopped in winter, and the date the overwintered plants bolted in spring. Add the position of each row and its spacing.

Those numbers turn general guidance into a calendar for your own garden. You will know the last autumn sowing date that still gives a full rosette before the light goes, how long a covered plant keeps producing where you live, and exactly how much spring you have left once the days start lengthening. That last date in particular is worth more than anything a packet can tell you, because it sets when the bed has to be cleared and what goes in next.