Gadenin日本語Get the appSoil temperature, not the date, decides when to sow
Komatsuna is a brassica grown for its leaves, and it germinates anywhere from about 41F (5C) to 95F (35C). The sweet spot is 68F to 77F (20C to 25C), where seed is up in two to three days. That range is so wide that in most temperate gardens there are only a few weeks in midwinter when the soil is too cold to bother, and a short spell in high summer when heat makes the seedlings stringy.
Push a soil thermometer two inches (5 cm) into the bed in the morning, before the sun has warmed the surface. Above 50F (10C) you will get a crop, though germination takes five to seven days and the stand will be patchy. Above 59F (15C) the row comes up evenly and you can start counting properly.
There is no transplanting stage worth bothering with. Komatsuna grows so fast that a plant raised in a cell and moved out has usually already checked its growth by the time it settles, and a checked brassica seedling runs to flower earlier. Sow where it is to grow.
Depth, spacing and how much seed to use
The seed is small and round, roughly the size of a pinhead. Sow it too deep and it exhausts itself before it reaches the light; sow it on the surface and the first hot afternoon dries the root out. Half an inch (1 cm) is the working depth in a garden bed, a little shallower in a fine seed-starting mix.
How you space it depends on the size of leaf you want. Baby leaf for salad is sown thickly and cut once. Full-size bunching plants with the thick pale stems need real room, and the difference between a four inch (10 cm) spacing and a six inch (15 cm) spacing is visible in the finished plant.
| Purpose | Sowing pattern | Final spacing | Days to cut |
|---|---|---|---|
| Baby leaf for salad | Broadcast or a wide band, seed about half an inch (1 cm) apart | No thinning, cut the lot | 20 to 25 |
| Medium bunching leaves | Drills 8 in (20 cm) apart, seed 1 in (2.5 cm) apart | 3 to 4 in (8 to 10 cm) | 30 to 35 |
| Full-size plants | Drills 12 in (30 cm) apart, seed 1 in (2.5 cm) apart | 5 to 6 in (12 to 15 cm) | 40 to 50 |
| Overwintering under cover | Drills 12 in (30 cm) apart, sown a little thicker | 6 in (15 cm) | 60 or more |
A packet goes a long way. For a bed three feet by three feet (about 1 m square) sown for baby leaf you need roughly a teaspoon of seed. Sowing heavier than that guarantees a thinning job you will resent.
Sowing a drill, start to finish
The whole operation takes about ten minutes for a short row. The one part people skip is firming the soil back down over the seed, and it is the part that decides whether the row comes up in one flush or in dribs and drabs over a week.
- Rake the bed level and take out the stones Break the top two inches (5 cm) down to a crumb no bigger than a pea. Komatsuna seedlings are small and a cloddy surface leaves half of them stranded in air pockets. Do not add fresh manure; it is not needed and it invites root maggots.
- Draw a shallow drill and water it before sowing Press the edge of a board or the back of a rake into the soil to make a flat-bottomed furrow half an inch (1 cm) deep. Water the furrow itself and let it drain. Watering the drill rather than the surface afterwards keeps moisture down where the seed sits and stops the surface capping.
- Space the seed along the bottom Aim for one seed per inch (2.5 cm) for plants you intend to thin, or a wider scattered band two to three inches (5 to 8 cm) across for baby leaf. Sowing evenly here saves far more time later than it costs now.
- Cover and firm Draw dry soil over the seed and press it down with the flat of your hand or the back of the rake. The seed needs contact with damp soil on all sides. Firming is what gets a row up together in three days rather than over a week.
- Cover the row before anything emerges Lay insect netting or fine mesh over hoops and pin the edges down to the soil. Flea beetles find komatsuna within a day or two of the seed leaves opening, and a cover put on after they arrive traps them in. This is the single most useful thing you can do for the crop.
Sowing in the heat and sowing in the cold
In hot weather the problem is not germination but what happens immediately after. Seedlings that come up at soil temperatures above 86F (30C) stretch, the stems go thin and pale, and they fall over. Sow late in the day, shade the row with a length of shade cloth or an old net curtain until the seed leaves are fully open, and water in the evening rather than at midday.
In cold weather the problem is the opposite: the seed sits. Below 50F (10C) germination drags out past a week and slugs get more of the crop than you do. Warm the soil first by covering the bed with clear plastic or a cloche for a week before sowing, then sow and put the cover straight back. Komatsuna itself tolerates frost happily once it has three or four true leaves, so the cold weather sowing is worth the trouble.
Komatsuna is a brassica, so it should not follow cabbage, kale, radish, turnip or mustard in the same ground. Two years between brassica crops in a bed is the working minimum if clubroot is anywhere in your garden.
When a sowing fails, it is usually one of four things
Gappy rows come from dry soil at seed depth, from sowing too deep, from a crust forming on the surface after heavy watering, or from slugs and flea beetles taking the seedlings before you noticed they were up. Dig up a short length of the row and look: seed that has swelled but not shot means cold or waterlogging, seed that has shot and then rotted means the ground stayed saturated, and no seed at all means something ate it or a bird took it.
Because the crop is so quick there is no sense nursing a bad stand. Rake it out and sow again. Losing a row costs you three weeks at most, which is why komatsuna is worth sowing in small repeated batches rather than one big bed.