What Causes Bolting?
Bolting is the name for plants making flowers and seeds. When a plant starts to bolt, it is usually a sign to expect a poor harvest, and a decline in flavor. You can eat bolting plants, but they become too tough and woody at some point.
Factors that can Trigger Bolting
Annual plants (basil, lettuce, melons, peas) grow from seed, flower and set seed all in one year. That can be spring to winter or fall to summer. Annual crops start making flowers as the daylength and temperature increase. Some annuals are crops where we eat the fruit or seeds and bolting is not an issue (sweet corn, tomatoes).
Increased day length: Bolting can happen (especially with annual crops) when day length increases as summer approaches. This can be a problem if you planted your seeds too late in the spring for that crop to mature before the plants bolt.
High soil temperatures: As soil temperatures increase, annual plants begin flower and seed production. This isn’t a problem after bountiful harvests. But, when spring has unusually hot weather or if you plant crops too late into the growing season, your crop may bolt before any harvests.
Biennial plants (beets, brassicas (broccoli, cabbage, kale, turnips etc), carrots, celery, chard, leeks, onions, spinach) grow big the first year, then seed the second year, if we still have them. Many biennial food crops are grown as cool weather annuals. Unsettled weather (cold nights, hot days, late frosts) early in the season can cause biennials to bolt. Spinach grows best in temperatures from 35-75°F (1-23°C) and will begin to flower once spring days are longer than 14 hours and temperatures get above 75°F (23°C).
Cold temperatures: A prolonged cold spell in spring can signal to biennials (especially immature plants) that “winter” has happened and it’s time to develop seeds for the next season. If you start these crops too early in the calendar year, you risk exposing them to cold weather, priming them to develop flowers as soon as the weather warms up again. Brassicas started in cool conditions, and grown on in warmer conditions, are primed to bolt.
Plant size: larger biennial plants are more likely than small ones to bolt when a trigger such as cold temperatures strikes.
Root stress: Bolting caused by root stress typically happens when you disturb a plant's root system by transplanting, or if your plant runs out of growing space in a container that's too small, or because the rows did not get sufficiently thinned.
Stresses such as insufficient minerals or water: Healthy soil with plenty of nutrients and balanced moisture levels will encourage quick growth. Every gardener should aim for this balance, especially those in hotter climates where it's a race to plant leafy salads, cooking greens and root crops before the hot weather wins. High salt levels are another stressor, particularly in hoophouses.
Mustard bed bolting
25 Tips to Delay Vegetable Crops from Bolting
- Investigate, record and follow local last planting dates for early spring crops, and first planting dates for fall crops. Plant spinach 4-6 weeks before the average date of the last frost in your region. You can also sow 6-8 weeks before the first fall frost.
- For some crops there are varieties that are resistant to bolting. If you have had repeated trouble with a particular crop bolting, look for bolt-resistant varieties. White and brown onions are less prone to bolting than red varieties.
- Onions grown from sets (plants stymied in mid-growth) are prone to bolting. Grow onions from seed or plant heat-treated sets in early spring.
- Avoid stressing your plants.
- Direct sow. Plants prone to bolting due to root stress (beets, carrots, radishes, turnips, and many herbs) grow best when you direct sow them, rather than transplanting. This allows their root systems to develop without interruption.
- Transfer seedlings to a larger pot before the roots get crowded ("root bound")
- Harden off plants before transplanting. Get them used to outdoor conditions, avoid shock.
- Cover plants in the event of a cold spell, which can keep them from being directly exposed to cold temperatures, rain, or snow.
- To postpone bolting in spring, avoid chilling young brassica plants (above 5-8 true leaves, or with a stem diameter above a certain size), below 40°F (4.5°C) for a few days, or longer at 50°F (10°C). The interaction of plant size (age) and cold temperatures makes the plant flower. Older plants are more likely to bolt than young plants at the same cold temperature. Young hardened-off plants are very resistant to bolting.
- Coax your vegetables to maturity quickly and efficiently so they're ready to eat before the plants have a chance to flower.
- Mulch spring crops early to help keep the soil and roots cooler, extending the harvest. We have found this to be especially helpful with spring cabbage and broccoli.
- Use shadecloth to keep greens and lettuce cool as the season warms, or plant them in the shade of other plants
- Many cool-season crops mature better before temperatures get to 80°F (27°C), so plan accordingly. If your springs heat up fast as ours do, start earlier.
- Plant some annuals after the summer solstice to grow in the decreasing daylength without risk of bolting (unless another factor such as stress or temperature comes into play). Spring-sown Asian greens will bolt as nights become warmer – on average above 50-55°F (10-13°C). To prevent bolting in Asian greens, sow these crops from July onwards.
- Winter radishes will only form a good root if they are planted in late summer or fall as the days get shorter. Grow bulb fennel, storage carrots, beets in fall, not spring.
- Once cold-hardy plants are big, they can endure cold winter temperatures. They will not bolt until the daylight is lengthening again (after the Winter Solstice) and the temperature starts to rise.
- Brassica greens started in hot conditions do not usually bolt if they have enough water. I recommend both Tokyo bekana and Maruba Santoh (both "celery cabbage" types of Asian greens), for summer substitutes for lettuce. You do have to grow them fast, with plenty of water, and insect netting if you have brassica leaf pests.
- Sow quick-maturing plants like lettuce, cilantro, or radish regularly. Succession sowing can keep some plants always coming into maturity instead of relying on one sowing to last a long time without bolting in the garden.
- If you grow biennial plants and harvest them in the first year, they are unlikely to bolt. A few specimens may still do so. Chard is cold-sensitive, and by delaying sowing until April, we grow chard as a fresh cooking green all summer, and it will not bolt no matter how hot.
- For early harvests of biennials, start the plants in plug flats or soil blocks indoors, planting them out when the weather is more settled and avoiding cold stress.
- Dry soil can also encourage bolting, particularly with cabbages, cauliflower, arugula and spinach. Provide ample water.
- For over-wintered leeks and onions, bolting can be delayed by topdressing with 2-3oz per sq yd (70-100g per sq m) of nitrogen rich fertilizer very early in the new year
- Pick off the outer leaves from leafy crops such as lettuce, keeping the plants from maturing. As well as providing you with multiple harvests, this can extend the harvest period by as much as 10 weeks, although in hot weather the flavor may still become bitter, even without bolting. Grow Batavian varieties in hot weather.
- With some crops, like basil, if you catch a plant in the very early stages of bolting, you can temporarily reverse the process by snipping off the flowers and flower buds. The plant will go back to producing leaves and will stop bolting. In most plants (such as broccoli and lettuce) this only buys you a little extra time to harvest the crop.
- Cabbage wrangling: If a cabbage is mature and preparing to split open (a stage of bolting) before you are ready to harvest, you can get a firm hold on the head and give it a quarter turn. This will break some of the feeder roots and reduce the water uptake, delaying splitting.
Change Your Attitude about Bolting
You can’t control the climate, the weather or the daylength. If you’ve taken the steps listed above and your plants are still bolting, change your attitude! As soon as you see signs of your greens bolting, harvest the entire plant. Learn to appreciate peppery arugula or slightly bitter lettuce (mixed in with other salad greens). Bolted vegetables are food for pollinating insects such as bees. Enjoy the beauty of sprays of yellow brassica flowers, majestic globes of leeks and onions, and lacy carrot umbels.
Pam Dawling has worked at Twin Oaks Community in central Virginia for more than 27 years, growing vegetables for 100 people on 3.5 acres and training many members in sustainable vegetable production. She is the author of Sustainable Market Farming and The Year-Round Hoophouse. Pam often presents workshops at MOTHER EARTH NEWS FAIRs and at sustainable agriculture conferences. She is a contributing editor with Growing for Market magazine, and a weekly blogger on SustainableMarketFarming.com. Read all of Pam’s MOTHER EARTH NEWS posts here.
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