Why Lettuce Has Flowers: Tips For Preventing Bolting Lettuce Plants

Why Lettuce Has Flowers: Tips For Preventing Bolting Lettuce Plants

By: Susan Patterson, Master Gardener

Interestingly enough, flowering and bolting are the same thing. For some reason, when we don’t want vegetable plants to flower, such as lettuce or other greens, we call it bolting instead of flowering. “Bolting” conjures up a slightly negative thought, as opposed to the “flowering”. When our lettuce is flowering, for instance, we are unlikely to say that it’s so pretty. We’re more likely to be aggravated that we did not get it out of the ground soon enough.

Why Lettuce Has Flowers

Cool season annual vegetables, such as spinach and lettuce, bolt when chilly spring days turn into warm spring days. Bolting lettuce plants become bitter and sharp in taste as they shoot towards the sky. Other crops that are sensitive to bolting include Chinese cabbage and mustard greens.

Lettuce bolt will occur when daytime temperatures go above 75 F. (24 C.) and nighttime temperatures above 60 F. (16 C.). In addition, an internal clock inside lettuce keeps track of the number of daylight hours that the plant receives. This limit varies from cultivar to cultivar; however, once the limit is reached, the plant will send up a flower stalk with reproduction in mind.

Lettuce bolting to seed cannot be reversed, and when it happens it’s time to replace the cool season vegetables with more heat tolerant plants.

How to Delay Bolting Lettuce Plants

Gardeners who wish to keep bolting at bay can do so in a number of ways.

  • Starting lettuce indoors under lights and placing them outside while it is still nippy gives them a head start and may reduce the tendency to bolt.
  • Row covers can be used to extend the season in both the spring and the fall. If you plant lettuce late and wish to avoid premature lettuce bolt, try using a shade cloth over the row to reduce the intensity of the light.
  • Additionally, it is essential to fertilize new plants with a 10-10-10 fertilizer. Make sure the plants receive plenty of moisture.

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How to Avoid Growing Bitter Tasting Lettuce in Your Garden

I often hear people say that lettuce can only be grown in the spring or fall because it would become bitter in the heat of the summer. This isn’t true at all! You can actually grow lettuce all summer long and I have completed some research and experiments of my own with great success!

So how do you avoid growing bitter tasting lettuce in your garden? Keep your lettuce in a cool place, out of the hot sun to prevent it from bolting quickly and becoming bitter.

This is a very straightforward and general answer but I have a lot of tips to follow and methods that I use in my own garden to have fresh lettuce all summer long! First, let’s learn some causes of lettuce bolting and becoming bitter and then we will learn some ways to prevent it.

Why does lettuce bolt?

Bolting is a survival mechanism through which plants try to generate offsprings. When a plant faces environmental conditions that are not conducive to its growth, it tries to create seeds so that the next generation can kick in. It is nature's way of continuing the lifecycle of a plant. Iceberg lettuce thrives best in a cold climate and needs low exposure to sunlight. Thus, an increase in temperature or long exposure to daylight may cause iceberg lettuce to bolt. Another factor that can cause iceberg lettuce to bolt is using the wrong kind of manure or nutrition for the plant. Bolting is equivalent to fruiting. If one ends up giving a fruiting nutrient/manure to lettuce, it can also lead to bolting. Lettuce also bolts under drought-like conditions. Water stress accounts for unfavourable surviving conditions leading the flower to reproduce. Now, we do have a little understanding of what caused lettuce in our garden to bolt.

What do you do when your lettuce bolts?

With the advent of consistently warmer to hot weather in much of Southern California, many of the cool season leafy green vegetables, such as lettuce, will have flowered and gone to seed, or bolted. Those gardeners who live in Southern California areas with significant coastal influence may still be able to grow these leafy green vegetables throughout the summer.

Lettuce often bolts as temperatures rise. GardenZeus recommends using shadecloth or providing shade to lettuce when daytime temperatures rise above about 78° F. For specific information on shading, see GardenZeus Tips for Shading Vegetables During Hot Weather. At 85° F to 90° F temperatures and above, virtually all varieties of lettuce will suffer, become bitter, or bolt in from a few days to 2 to 3 weeks. There is nothing that can be done to reverse or stop bolting once it begins.

Leaves may begin to turn bitter as soon as lettuce starts to flower. GardenZeus recommends tasting leaves for quality and if still reasonable, harvesting the majority of leaves or whole plants at the first sign of bolting, unless you want to save seeds. As lettuce plants bolt or begin to grow bitter, soak plants or leaves in cool water overnight to draw out white sap that is produced in leaves and stems as plants prepare to set seeds and which causes bitter flavor.

Treat other greens such as arugula, parsley, mache and spinach in the same manner: harvest the majority of leaves or whole plants at the first sign of bolting, unless you want to save seeds.

Gardeners with bolting leafy greens can use this as anopportunity to engage in sustainable gardening practices.

Sustainable Landscaping Tip 1: when your parsley begins to bolt, leave the parsley in place. Flowering parsley provides food for many beneficial insects, so bolting might not be a problem in the larger plans of an insect-friendly gardener.

Sustainable Landscaping Tip 2: when your arugula begins to bolt, leave the arugula in place. Arugula will naturalize in many parts of California and you may be rewarded next year with effortless supply.

To view customized information for growing lettuce, arugula, spinach and mache, go to GardenZeus and then to the appropriate plant.

How to Delay Vegetable Crops from Bolting

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. U nsettled 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.

S tresses 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.

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 t o 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 Read all of Pam’s MOTHER EARTH NEWS posts here.

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