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Leaf mustard (Brassica juncea var. integrifolia) is the large-leaved, broad-stemmed mustard green, grown either as cut-and-come-again baby leaf or grown out into a whole head the size of a small cabbage. It germinates anywhere between 45F and 85F (7C to 29C) and pushes through in four to seven days when the soil sits near 60F (15C). That willingness is exactly what catches people out, because a seed that sprouts happily in early summer heat will still bolt six weeks later at half size.
There are two usable windows. The first is late winter into early spring, as soon as the soil is workable and above 45F (7C), and it is a race: you are trying to get leaves off the plant before lengthening days and rising temperatures send it to flower. The second window is the one that actually produces heads, running from the point where summer heat breaks until roughly eight to ten weeks before your first hard frost. Plants growing into shortening days and cooling nights build broad, heavy, mild leaves and show no interest in flowering.
Soil temperature is the number to go on rather than any calendar. Push a probe two inches (5 cm) down in the morning for three days running and use the average. Below 45F (7C) germination stalls and the seed sits and rots; above 85F (29C) germination is patchy and whatever comes up is primed to run to seed.
| Soil temperature | Days to emerge | What to expect |
|---|---|---|
| 45F (7C) | 10 to 14 | Slow, uneven, some seed lost |
| 55F (13C) | 6 to 8 | Reliable; ideal for a spring sowing |
| 65F (18C) | 4 to 5 | Fastest clean germination |
| 80F (27C) and above | 3 to 4 | Fast but thin stands; high bolting risk |
Decide the picking style before the seed goes in
The same seed gives two completely different crops depending on spacing, so make the decision at sowing rather than trying to correct it later. For baby leaf, sow thickly in bands or broadcast across a block, aiming for a seed every half inch (1 cm) and rows four to six inches (10 to 15 cm) apart. You will cut this at four to five inches (10 to 13 cm) tall and never thin it beyond removing the obvious crowds.
For full-size plants, the ones that make a broad-leaved head with thick pale midribs, you need far more room than most people give them. Set final spacing at twelve to eighteen inches (30 to 45 cm) between plants and eighteen to twenty-four inches (45 to 60 cm) between rows. A mature plant can spread two feet (60 cm) across, and crowded plants stay small, hold damp air in the crown and give you rot at the base of the outer leaves.
Sow depth is the same for both: a quarter to half an inch (6 to 12 mm), firmed down and watered in. Deeper than that and emergence gets slow and ragged in cool soil. A useful middle path is to sow the full-size spacing but drop three seeds at each station, then eat the two thinnings as baby leaf and let the strongest one grow on.
| Picking style | Sowing rate | Final spacing | Ready in |
|---|---|---|---|
| Baby leaf, cut and come again | Thick bands | No thinning, 1 in (2.5 cm) apart | 21 to 30 days |
| Medium leaves for cooking | Every 2 in (5 cm) | Thin to 6 in (15 cm) | 35 to 45 days |
| Full heads | 3 seeds per station | 12 to 18 in (30 to 45 cm) | 60 to 80 days |
Cover the row on the day you sow
Flea beetles find mustard seedlings within days of emergence and can shred a stand before it has four true leaves. The single most effective thing you can do is lay lightweight floating row cover over the bed at sowing, before anything is up, and seal the edges with soil or boards. Covering after the beetles arrive traps them in with the crop and makes the problem worse.
Leave the cover slack rather than tight so the leaves have room to lift it, and keep it on until the plants have six or seven true leaves and tough enough foliage to shrug off feeding damage. Mustard needs no insect pollination to produce a leaf crop, so there is no reason to uncover it at all if you are growing purely for greens.
In a spring sowing the cover does double duty by holding a few degrees of warmth over cold nights. In a late-summer sowing, choose the lightest weight cloth you can find or use insect mesh instead, because a heavy cover over warm soil raises temperatures enough to bring bolting forward.
- Prepare a fine, firm seedbed Rake out clods to a crumb structure in the top two inches (5 cm) and firm the surface with the back of the rake. Mustard seed is small and will vanish into a loose, fluffy bed and germinate at uneven depths.
- Water the drill, then sow In warm weather, run water into the open drill and let it drain before dropping seed in and covering dry. This puts moisture where the seed is without crusting the surface, which matters when soil is above 75F (24C).
- Thin in two passes First thin at the two-true-leaf stage to roughly half the final spacing, then again three weeks later to the full spacing. The second round of thinnings is a usable baby leaf harvest rather than waste.
Write down the sowing date, the soil temperature you measured, and which end of the bed you used. Leaf mustard self-seeds freely, so knowing exactly where you sowed makes it easy to tell a deliberate row from last year's volunteers when both come up together.