Gadenin日本語Get the appWhat actually triggers it
Two things push mustard into flower, and they usually arrive together. The first is warmth: sustained daytime temperatures above about 75F (24C), especially when night temperatures also stay above 60F (15C), tell the plant that its season is ending. The second is day length. As days lengthen past roughly thirteen hours, the plant shifts from making leaves to making a flower stem regardless of how comfortable the temperature is.
That combination is why spring crops are so short. In late spring both triggers ramp up week by week, and a plant that looked perfect on a Monday can have a visible flower spike by the following weekend. Autumn crops face the opposite: shortening days and cooling nights, so they simply do not bolt, and this is the main reason autumn is the better season for this crop.
A third factor sets the stage rather than pulling the trigger. Any serious check to growth primes a plant to run early. Drying out, root damage from careless thinning, a long spell of crowding, or transplanting a seedling that had already outgrown its cell will all cause a plant to bolt at a smaller size than an unstressed neighbor in the same row.
| Trigger | Threshold | How fast it acts |
|---|---|---|
| Daytime heat | Above 75F (24C) for several days | Flower spike visible within 7 to 14 days |
| Warm nights | Above 60F (15C) sustained | Accelerates whatever the daytime heat has started |
| Day length | Beyond about 13 hours | Gradual, but unstoppable once past |
| Drought check | One wilt is enough | Bolting at a much smaller plant size |
| Root disturbance | Transplanting or rough thinning | Bolts one to two weeks earlier than undisturbed plants |
What actually delays it
Nothing prevents bolting, so treat every measure as a way of buying weeks. The largest single gain comes from moving the sowing rather than fighting the weather: a crop sown after the summer heat breaks is not in a race at all, and will yield several times what a late spring sowing gives you for the same effort.
Within a spring crop, keeping the soil cool and evenly moist is worth the most. Mulch, deep watering, and afternoon shade all lower the plant's effective temperature. Thirty percent shade cloth over a spring row from midday onwards is a straightforward way to add one to two weeks.
Spacing matters more than people expect. Crowded plants compete, run short of water sooner, and bolt earlier than well-spaced neighbors in the same bed. Thinning to the proper spacing early is a bolting measure as much as a yield measure.
Finally, pick hard. Regular removal of outer leaves keeps a plant in a vegetative habit for longer than leaving it undisturbed. A plant you have stopped picking is a plant that will flower.
- Mulch as soon as thinning is done One to two inches (2 to 5 cm) of compost or straw over damp soil holds the moisture and keeps the root zone cool through the afternoon. Apply it while the soil is still cool, not after a hot spell has already warmed it.
- Shade from midday in warm weather Thirty percent shade cloth on hoops, or the shadow of a taller crop on the west side, cuts leaf temperature enough to matter. Full shade is not needed and reduces growth.
- Sow direct rather than transplanting in spring Mustard resents root disturbance and transplants bolt earlier than direct-sown plants. If you must raise seedlings, plant them out before they are three weeks old.
- Water deeply and never let it wilt A single wilt sets the plant on course to flower early even if you water perfectly afterwards. Check the soil two inches (5 cm) down rather than trusting the surface.
Recognizing it early enough to act
You can see bolting coming about a week before the flower stem is obvious, and that week is worth having. Look into the center of the plant. The first sign is the crown lifting slightly and the newest leaves emerging narrower and more upright than the ones below them, often with a paler midrib. The spacing between leaf joints stretches. Only after that does a recognizable spike appear.
Once you see the narrow upright leaves, the crop's remaining life is measured in days. That is the moment to cut everything usable in one go rather than continuing to pick a few leaves at a time. Waiting a week for the plants to get bigger is a trade you will lose.
The leaves also change as bolting begins. They get hotter and more bitter, the midribs toughen, and the texture becomes stringy. A leaf that would have been salad two weeks earlier now needs cooking.
Using the crop once it has bolted
A bolted plant has a second useful harvest in it. The unopened flower shoots are sweeter and better balanced than the leaves at that stage, and they crop like sprouting broccoli: snap the top four to six inches (10 to 15 cm) while the buds are tight, and side shoots follow for another two or three pickings over a fortnight.
Once the flowers open, stop. The stems go woody, the yield is negligible, and the plant is now working on seed. At that point you either clear the bed or make a deliberate choice about seed, which is a decision with consequences because mustard self-seeds enthusiastically and will populate the surrounding beds for years.
Record the date each spring sowing bolted, along with the sowing date. Two or three seasons of those pairs give you the last worthwhile spring sowing date for your own garden, which is the single most useful number you can have for this crop.