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Mustard has two windows and one dead zone. The spring window opens as soon as the soil is workable and above 45F (7C) and closes when daytime temperatures start reaching 75F (24C) regularly. The autumn window opens when the summer heat breaks and runs until about eight weeks before your first hard frost, and it is by a wide margin the better of the two. Between them sits high summer, when sowing this crop mostly wastes seed.
Everything else follows from that. Spring sowings should be small, frequent and picked young, because they are working against the clock. Autumn sowings can be spaced wide and allowed to size up, because nothing is pushing them to flower and the cold is going to improve them.
The table below is the whole calendar in outline. Read the seasons rather than the months so it works wherever you garden.
| Season | Conditions | What to do |
|---|---|---|
| Late winter | Soil workable, above 45F (7C) | First sowing under cloche or row cover; prepare beds with compost |
| Spring | Soil 50F to 65F (10C to 18C) | Short sowings every two weeks; thin early; mesh against flea beetle |
| Late spring | Days lengthening, warm afternoons | Last worthwhile sowing; pick hard; expect bolting |
| Summer | Above 80F (27C) | Do not sow; clear beds, add compost, prepare for autumn |
| Late summer | Heat breaking | Main sowing season begins; sow every two to three weeks |
| Autumn | Cooling nights, shortening days | Best growth of the year; thin to full spacing; harvest steadily |
| Winter | Frosts arriving | Cover with cloche or row cover; harvest through cold; flavor improves |
The spring crop is a race
Sow the first short row as soon as the soil is workable, under a cloche or a doubled layer of row cover. Growth is slow at first but the plants establish while flea beetle activity is still low, and the cover brings the first cut forward by two or three weeks. Keep sowing two feet (60 cm) of row every fortnight while daytime temperatures stay below 70F (21C).
Thin at the first true leaf and again two weeks later. Mulch the moment the second thinning is done, and keep the bed evenly damp from then on. Every check to growth in spring costs you a week of crop at the end.
When daytime temperatures start reaching 75F (24C), stop sowing and switch to salvage mode: cut everything usable while the leaves are still good rather than waiting for the plants to get bigger. Spring mustard goes from perfect to bolted in about a week, so the harvest is a decision you make once.
- Cover the first sowing Row cover or a cloche over the earliest row warms the soil, speeds germination and excludes the first flea beetles. It is the highest-value ten minutes of the spring.
- Sow small and often Two feet (60 cm) of row every two weeks gives a steady supply and limits the damage if one sowing runs into a warm spell or a beetle wave.
- Take the whole crop when bolting starts Once the center of the plants goes narrow and upright, cut the lot within a few days. Leaves picked after that are hot, bitter and stringy.
The autumn and winter crop is the real one
Start sowing again as soon as the worst of the summer heat has passed, and keep going every two to three weeks until about eight weeks before your first hard frost. These plants grow into cooling weather and shortening days, which suppresses flowering completely, so you can space them wide, let them reach full size, and pick from them for months.
Frost makes them better. After a hard freeze the plant converts starch to sugar in its leaves, and mustard that was fiercely hot in autumn becomes noticeably rounder and sweeter. Full-size leaves that would have needed long cooking become good with a quick wilt. A stand that looks flattened and glassy at dawn will usually be upright and perfectly usable by midday.
Growth slows to a halt once day length drops below about ten hours, whatever the temperature. From that point the plants hold rather than grow, and you are harvesting from what is already there. Plan for it by having the plants at full size before the light goes rather than half-grown.
- Stop feeding a month before hard frost Soft nitrogen-fed growth going into a freeze is the growth that rots at the crown. Let the plants harden and they will take far more cold.
- Get cover on before the first hard frost A low tunnel, cloche or double row cover takes the crop from merely surviving to still producing. In a mild region it keeps usable leaves coming most of the winter.
- Pick at midday in freezing weather Frozen leaves bruise and turn to mush when handled. Wait until they have thawed in the sun and they cut crisp.
Keep four dates per season: each sowing date, the first cut from it, the date the spring stand bolted, and the date winter growth stopped. Those four numbers turn general advice into your own calendar within two years.
Summer, and the self-seeding decision
High summer is a gap, and the best use of it is preparation. Clear the spring beds, dig in two to three inches (5 to 8 cm) of compost, and have the ground ready so the autumn sowings can go in the day the weather turns. If you must sow through the heat, give the row afternoon shade, water the drill before sowing, and pick everything as baby leaf within a month.
This is also when you decide about seed. Mustard self-seeds readily and generously, and a single plant left to ripen will scatter hundreds of seeds that come up as volunteers across the bed and the paths for years afterwards. Those volunteers are free food, but they appear where and when they choose, and in a small garden they become a weed you spend time pulling.
If you want them, leave one plant to ripen and let it drop. If you do not, pull bolted plants before the pods turn from green to tan, because once the pods dry they split at a touch and the decision is made for you.