What causes bolting?
This is an updated version of the section I included last week, followed by the all-important section on how to stop or at least delay, plants from bolting.
Bolting is the term for plants going to seed. Rather than grow more leaves and bigger roots, the plants develop stems, fewer and skinnier leaves, then flowers and seeds. The plants are switching their energy to survival of the species in response to the conditions.
When a plant starts to bolt, it is usually a sign to expect a poor harvest. It is also an indication that the plant will decline in terms of flavor. Lettuces become bitter. As long as you can harvest leaves or roots that are not too woody, you can eat bolting plants. But they do become too tough and inedible at some point.
Although bolting is usually seen on crops approaching maturity, it is initiated much earlier, by plant hormones called gibberellins. Bolting then can be triggered by one or more factors, sometimes acting in concert.
- · 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 to get a harvest before the plants bolt.
- · High soil temperatures: As soil temperatures increase in summer, annual plants are triggered to begin seed and flower production. This isn’t a problem late in the crop’s life, 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. Cool weather crops like lettuce and spinach will bolt in spells of hot dry weather.
- · Cold temperatures: A sudden cold snap in spring can signal to biennials (such as onions, leeks, beets and carrots) 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 young plants to cold weather, priming the plants to develop flower stems as soon as the weather warms up again.
- · Plant size: larger plants are more likely than small ones to bolt when a trigger such as cold temperatures strikes.
- · 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 are sensitive to daylength, and will start making flowers as the daylength (and temperature) increase. Many annuals are crops where we eat the fruit or seeds and bolting is not an issue.
- · 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. Bolting usually occurs after a prolonged cold spell, especially with immature plants. Brassicas started in cool conditions, and grown on in warmer conditions, are primed to bolt. Tatsoi, for example, bolts readily in these conditions.
- · 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 grower 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.
Avoiding or postponing bolting
- · Investigate, record and follow local last planting dates for early spring crops, and first planting dates for fall crops.
- · For some crops there are varieties that are resistant to bolting, such as ‘Boltardy’ beets. If you have had repeated trouble with a particular crop bolting, look for bolt-resistant varieties. Florence fennel is particularly prone to bolting so try ‘Amigo’, ‘Victorio’ and ‘Pronto’ varieties and sow in summer for fall harvests. 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 (exposure to high temperatures suppresses flower-bud formation)
- · Try to 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, avoiding shock.
- · Cover plants in the event of a cold spell, which can keep them from being directly exposed to cold temperatures, rain, or snow.
- · Use seaweed or kelp liquid fertilizer, which help plants handle stress better.
- · 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 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. If broccoli and cauliflower plants are stressed into flowering while young, the small plants will only produce small heads.
- · 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.
- · Cover near-mature bulbing onions during cold spells to protect them from bolting.
- · Use shadecloth to keep greens and lettuce cool as the season warms, or plant them in the shade of other plants
- · Consider the timing of sowing. Grow storage carrots and beets in the fall, not the spring.
- · Many cool-season crops mature better before temperatures get to 80°F (27°C), so plan accordingly. If your springs usually heat up fast as ours do, start earlier, or plant in late summer, fall, or even winter, depending on your climate, and grow these crops in reliably cool weather.
- · Plant later 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).
- · Once cold-hardy plants are big, they can endure cold winter temperatures. They will not bolt until the day length 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.
- · In central Virginia for outdoors in spring, the only Asian green we grow is senposai, which we have found to withstand bolting until a bit later than others. There may be other loose-leaf Asian greens we could grow, but spring can go from too cold to too hot very quickly here.
- · To prevent bolting in Asian greens, sow these crops from July onwards. Asian greens bolt as nights become warmer – on average above 50-55°F (10-13°C)
- · Winter radishes will only form a good root if they are planted in late summer or fall as the days get shorter.
- · 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 can grow chard all summer as a fresh cooking green, and it will not bolt no matter how hot it gets. We cannot keep kale and collards producing all summer as gardeners in cooler climates do.
- · 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. To avoid this, provide ample water.
- · If the compost is not nutritious enough, top-dress with more compost.
- · 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.
Some specific examples
Lettuce is one of the vegetables most frequently seen bolting. I’ve even seen photos of bolting lettuce in garden magazines, with no acknowledgement that the plants are only fit for the compost heap.
The bitter taste of bolting lettuce is caused by a rapid buildup of compounds called ‘sesquiterpene lactones’. Plants manufacture these compounds to provide resistance to leaf-eating insect pests. The plants also speed up their production of seeds to grow the next generation.
Onions (excerpted from Sustainable Market Farming)
Onions are a biennial plant. When onion plants experience an extended period of cooling temperatures, such as winter, they go dormant. When temperatures rise, they start growing again. After being exposed to cold temperatures, smaller seedlings with a diameter less than pencil thickness (3/8” or 1 cm), and fewer than six leaves will resume growth and not usually bolt. Over-large transplants are more likely to bolt. If seedlings are becoming thicker than a pencil before you can set them out, undercut 2″ (5 cm) below the surface to reduce the growth rate.
The trigger for the transition from bulbing to flowering is temperatures below 50°F (10°C) for 3-4 weeks, after the plants have six leaves or more. This can happen, if you are unlucky, after an unexpected cold period in spring. Avoid bolting because the bulbs start to disappear to feed the growing flower stems. Bolted onions will not dry down to have tight necks and so will not store.
It is possible in our climate (and perhaps yours) to sow onions in the fall and plant the seedlings out in the early spring, for bigger vegetative growth and therefore the chance of bigger bulbs. The temperature-and-size trigger limits how early in the fall seeds can be sown – if the seedlings have made lower stems larger than a pencil in diameter when winter closes in, the plants are likely to bloom in the spring rather than forming bulbs. A few onion plants will likely always bolt, especially if the spring is long with alternating warm and cool spells.
Starting seedlings in a hoophouse in early November works well for us. Previously we sowed outdoors in late September and protected the plants with row cover and cold frames, a system that would work fine somewhere warmer than zone 7. The hoophouse works well for us because the plants get much better air flow, are protected from very cold temperatures, and can be easily seen and cared for. The plants grow faster in the hoophouse than outdoors, so we must start them later. Outdoor sowings tend to suffer some winter killing and varying degrees of mold. The colder the temperatures the plants experience, the more likely it is for the larger ones to bolt before growing large bulbs. Hence a more moderate microclimate, such as a hoophouse, reduces the rate of bolting. In colder zones, a slightly heated greenhouse might work better for over-wintering.
Spinach grows best in temperatures from 35-75°F (1-23°C). Spinach will begin to flower once spring days are longer than 14 hours and temperatures get above 75°F (23°C). The leaves become pointed (arrowhead shape), less fleshy, and the plants get taller, and develop a flower head in warmer weather. 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. You can also plant seed in a cold frame in fall or cover late season plants with rowcover, for harvests during the winter or the following early spring. In warm weather grow chard or leaf beet instead of spinach.
Change your attitude
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. According to some excellent cooks, flowering bok choy stems are tender and sweet and make a great addition to stir-fry and salads.
Learn to appreciate peppery arugula or slightly bitter lettuce (mixed in with other salad greens). It can be more enjoyable than the bland tasting lettuce available in stores.
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.