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The pungency in mustard comes from compounds the plant makes when a leaf is damaged, and the plant makes more of them under stress. Drought is the stress that matters most in a garden. A bed that dries out between waterings produces leaves you can barely eat raw, and the difference is not subtle: the same plant, watered properly, is pleasant in a salad, and left dry for a week becomes something you have to cook and dilute.
Aim for soil that stays evenly damp two inches (5 cm) down through the whole growing period. That usually means an inch (25 mm) of water a week in cool weather and closer to an inch and a half (38 mm) in warm, windy conditions, delivered in one or two soakings rather than a daily splash. Shallow daily watering keeps the roots near the surface where they are most vulnerable to the next hot afternoon.
Mulch is the cheapest way to hold that consistency. An inch or two (2 to 5 cm) of compost or straw, laid once the seedlings are through and thinned, evens out the swings and keeps the soil surface cool. Cooler soil directly delays bolting, which buys you weeks of picking on a spring crop.
The one exception is the last few days before a cut you intend to eat raw. Water well two days before picking and the leaves take up water, thin the flavor slightly and lose the harsh edge.
| Condition | Watering | Effect on the leaf |
|---|---|---|
| Cool, damp weather | 1 in (25 mm) per week | Broad, tender, mildest flavor |
| Warm and breezy | 1.5 in (38 mm) per week, two soakings | Good quality if kept up; harsh if missed |
| Hot and dry | Deep soak every 3 days plus mulch | Sharper even when watered; cook rather than eat raw |
| Allowed to wilt once | Recovery watering | Flavor stays hot and the plant is primed to bolt |
Feed for leaf, and feed early
Mustard is a leaf crop with a short life, so almost all the feeding value has to be in place before or just after sowing. Fork two to three inches (5 to 8 cm) of well-rotted compost into the top six inches (15 cm) of the bed before you sow. That alone will carry a baby leaf crop from start to finish.
Full-size plants benefit from one liquid feed high in nitrogen when they are about a third grown, roughly three weeks after emergence, and a second three weeks later if the crop is going to run past two months. Feeding late does more harm than good: soft, sappy growth heading into cold weather is the growth that rots, and soft growth in warm weather is what flea beetles find first.
Do not overdo it. Excess nitrogen gives huge pale leaves with poor texture, and this family accumulates nitrate in the leaf when overfed and grown in low light. If your soil is already rich from a previous heavy-fed crop, sow mustard into it with no extra feeding at all.
Cool the bed in warm weather
Once daytime temperatures are regularly above 75F (24C), everything you do should be aimed at keeping the plants cool. Shade cloth of about 30 percent over the row through the afternoon lowers leaf temperature enough to slow bolting noticeably, and it does not cost you meaningful growth because mustard does not need full sun to make leaf.
Position helps too. A bed on the east side of a taller crop, or one that a fence shades from mid-afternoon, will hold a spring sowing one to two weeks longer than an open bed in the same garden. That is often the difference between a crop worth picking and one you compost.
In cold weather the opposite applies and you want everything you can get. Full sun, a cloche or low tunnel over the row, and no mulch tight against the crowns where it holds wet against the stems.
- Pick the outer leaves regularly On full-size plants, taking two or three outer leaves per plant every few days keeps the center producing and stops the outer ring from ageing into leather. A plant left uncut slows down.
- Clear yellowing lower leaves Old leaves lying on damp soil are where rot and slugs start. Snap them off at the base and take them out of the bed rather than leaving them as mulch.
- Check the growing point weekly Look into the center of each plant. A crown that has stopped making flat leaves and is starting to elongate is bolting, and you have about a week of good picking left.
Keep a note of which beds hold moisture and which dry out, and pair that against how hot each cut tasted. After one season you will know precisely which part of your garden gives you salad mustard and which gives you braising mustard.
Growing in containers
Mustard grows well in a pot as long as the pot is not shallow. Baby leaf needs six inches (15 cm) of depth and can be sown thickly across the whole surface. Full-size plants need eight to ten inches (20 to 25 cm) of depth and one plant per eight-inch (20 cm) pot, or three across a standard window box.
Containers dry faster and heat faster than open ground, which pushes flavor towards the hot end and brings bolting forward. Use a large container rather than several small ones, keep it off hot paving, and check the moisture daily in warm weather. A pot that has dried to the point of wilting once will give you leaves you can only cook.