How to Prevent Bolting in Vegetable Crops

, written by Benedict Vanheems gb flag

How to stop vegetable crops from bolting

One of the biggest nuisances in the summer vegetable garden is bolting - when crops put on a vertical growth spurt to flower and set seed before the vegetables are ready for harvest. The result is inedible, bitter-tasting leaves or poor-quality produce with little that can be salvaged.

Bolting is of course perfectly natural, after all it's simply marks the point at which a plant reaches maturity and produces the seeds for the next generation. It's the coming-before-the-harvest bit that makes is so irritating for us gardeners! The trick, then, is to coax vegetables to maturity quickly and efficiently so they're ready to eat before the plants have a chance to flower.

Lettuce that has bolted

Bolting Lettuces

Lettuce is one of the most frequent-to-bolt vegetables. And it's sneaky too, seemingly running to seed within a matter of days; how many times have you gone away for the weekend only to return to drawn out plants teetering like the salad equivalent of the Leaning Tower of Pisa? By this stage the plants are good for nothing but the compost heap.

The wince-inducing bitter taste that accompanies this bolting is the result of a rapid accumulation of compounds called 'sesquiterpene lactones'. Never heard of them? Neither had I till I did a bit of investigation. It turns out that plants manufacture these compounds to give themselves better resistance to pests such as burrowing insects and hungry leaf-strippers such as locusts. These clever plants are literally arming themselves against attack so that they can cross the finish line and produce the seeds of the next generation. It's a really rather remarkable stroke of evolutionary genius.

What causes lettuce to bolt?

What Triggers Bolting?

Bolting in lettuces is well studied with research going back close to a century. German scientists confirmed a link between increasing day length and the switch from leaf production to flowering as early as 1931. Many leafy salads and herbs exhibit this same response, including arugula, cilantro and pak choi (bok choy), to name but a few. Incredibly these plants 'count' the hours of daylight then blossom once a specific trigger point is reached. This photoperiodism works both ways, with many ornamental plants flowering once day length begins to shorten.

While day length is undoubtedly the biggest influence on bolting, heat and/or water stress can speed the time to bolting still further. If a plant is having a tough time of it, it stands to reason that it's going to want to hasten it's purpose in life - to reproduce - before it's time's up. Cold-season plants such as those mentioned above plus the likes of radish and spinach simply don't like hot temperatures, so long spells of hot, dry weather are a surefire catalyst to premature flowering.

Chard that has bolted

Other vegetables bolt in response to cold weather. A sudden cold snap in spring can signal to biennials (plants such as onions and carrots that complete their lifecycle over two years) that winter has been and gone and now's the time to prepare the seeds for the next season. Start these plants off too early in the year and you risk exposing young, sensitive plants to cold weather and initiating flower formation in the process.

How to Avoid Bolting

So how, then, to avoid or at least slow bolting? Here are some steps you can take to fend off the prospect of a bolting crop.

Bolt-resistant varieties: Some varieties, for instance beetroot 'Boltardy', are specifically bred to be resistant to bolting - so this is a logical place to start! Use bolt-resistant varieties for the earliest sowings of annual vegetables that respond to increasing day length: spinach, lettuce, beetroot, arugula etc. Resistant varieties are also a good option for biennials like onions and carrots that are sown very late in winter/early in spring. Onion lovers should also seek out heat-treated onion sets, which are exposed to high temperature, a process that dramatically impedes flower bud formation.

Time sowings: Biennial vegetables sensitive to cold snaps can also be started off within a greenhouse before planting out under cloches once the weather has improved. For annual vegetables, don't forget to sow little and often to ensure a steady supply of quick-growers such as lettuces - this way you can pick leaves in good time, before they become too old and more likely to bolt. Oriental leaves such as pak choi and mustards are best sown a couple of weeks after the summer solstice, once day lengths are visibly beginning to shorten.

Shade cool-season crops: Relentlessly hot, dry summers are rare where I live - for me it's more about maximising light levels and willing plants such as tomatoes to produce their fruits before the nights turn cold! But if you are growing in a hot part of the world then offering shade for cool-season crops is a must. Grow the likes of lettuce and spinach in the shadow of taller plants such as climbing beans or corn. Shade cloth can also be deployed and makes a dramatic difference to plants that would otherwise wither and collapse in full view of the sun.

Maximise soil health: Healthy soil with plenty of nutrients and balanced moisture levels will, of course, encourage the quickest growth. Every gardener should aim for this ideal, but particularly those growing in hotter climates where there's a race to get in leafier salads and vegetables before the hottest months hamper progress. Or simply wait until the weather cools off a little in late summer. Target your watering to those plants that need it most. Cool-season crops will benefit from moist soil the most, while dry soil also plays havoc with brassicas such as cauliflower and arugula.

How to avoid your plants bolting

What To Do if Plants Bolt

If a few plants do bolt it isn't the end of the world. You may be able to salvage some leaves to mix in with other salads. Root vegetables can be chopped up and used in stews. And it's easy enough to cut out the usable parts of onions and leeks left around the hardened central flower stem.

Or be pragmatic. Your bolted vegetables are a boon for pollinating insects such as bees. Think of it as an added feast for them and a floriferous touch of beauty for you: sprays of yellow blooms courtesy of brassicas; bobbing globes of alliums; or a lace-like carrot umbel as dreamy as anything in the flower border.

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Show Comments


"Bolting truly isn't the end of the world. If it happens-it happens, you'll have to react at the moment. There is no certain prevention, but keeping the soil in perfect condition will drastically minimize the the chance your plants start to bolt. Audrey"
AudreyF on Thursday 7 July 2016
"My tomatoes grow and grow and never flower, help"
Cody on Friday 8 July 2016
"Hi Cody. The fact they aren't flowering could mean a number of things. You may find that feeding your plants with a tomato feed (one that is high in potash) will provide the right nutrients to encourage flower production. If the tomatoes are in a greenhouse, sometimes very hot, dry air can cause flowers to drop off, in which case watering the paths of the greenhouse to raise humidity (known as 'damping down') may help. They need lots of sun - so if the plants are in a cooler, shadier part of the garden this could possibly be the reason. Or it may simply be that they're not quite developed enough to flower yet."
Ben Vanheems on Monday 11 July 2016
"My cilantro does this and it's so frustrating!"
Lisa on Thursday 4 August 2016
"Try sowing your cilantro after the longest day of the year, so July onwards if you're in North America. Cilantro often bolts as days get longer but is less likely to do so as day length shortens."
Ben Vanheems on Thursday 4 August 2016
"Short lived annuals are often best sown directly in place rather than started indoors, stressed by low light levels or artificially warm temperatures. Some plants don't like being transplanted: peas, cilantro, root crops. Wait till the ground is warm enough and sow directly in place in your garden or large container. Then, make small sowings MORE OFTEN is the ticket."
sue on Friday 12 August 2016
"Hi Sue, thanks for your advice. I haven't had problems transplanting peas, but definitely accept that direct sowing is preferred for many crops."
Ben Vanheems on Saturday 13 August 2016
"What about small tomatoes, chillies plants bolting? Should I throw them?"
Anil on Saturday 15 April 2017
"My small plants of tomatoes and chilly are bolting. Should Ithrow them?"
Anil on Saturday 15 April 2017
"Hi Anil. Bolting just means coming into flower. You would want your tomatoes and chilies to flower so that they can then go on to produce their fruits. If they are very young, just make sure they are kept fed and watered and have plenty of direct sunlight. They will still put on plenty of leafy growth and expand as a plant, while the flowers will keep coming - meaning more tomatoes and chilies for you to harvest."
Ben Vanheems on Tuesday 18 April 2017
"There is a technique to get the bitterness of kale that you would like to eat raw: you massage the leaves with a little olive oil. Next time, try the same thing with your bolted, bitter lettuces! You may be amased at how good they suddenly taste! "
Is on Tuesday 30 January 2018
"Will definitely try this - thanks for sharing!"
Ben Vanheems on Tuesday 30 January 2018
"Are the yellow flowers from the bolting green vegetables edible? Thank you!"
Yieteh on Monday 19 March 2018
"Most flowers from bolting vegetables such as lettuce, brassicas (cabbage etc.), arugula/rocket etc are perfectly edible, but they tend to have a very bitter taste, so don't make for good eating. But they aren't poisonous. As always, caution should be exercised, so if you're not 100% sure, best to leave it alone."
Ben Vanheems on Monday 19 March 2018
"I am just noticing that my bok choy is starting to bolt. Without thinking- I quickly snipped the tiny, tiny buds forming inside the stalk near the base. I thought that maybe (like basil) it would go back to producing leaves, but did I only harm my plant?"
Emily on Tuesday 29 May 2018
"Hi Emily. I think it's hard to stop plants like bok choy from bolting once their mind is set! The trouble is with brassica and a lot of Oriental leaves including bok choy, is that they respond to increasing daylength by bolting. Wait until the end of June (assuming you are in the Northern Hemisphere) and then re-sow - you'll have more success and far less risk of it all bolting."
Ben Vanheems on Monday 11 June 2018
"Will shading stop bolting?"
Capt Anil Kumar Gupta on Wednesday 20 May 2020
"In hot weather or climates, growing bolt-prone crops such as salads, spinach, chard etc in the shade will certainly reduce the chances of the plants bolting."
Ben Vanheems on Thursday 21 May 2020
"We have ear wigs and live in N Calif I have put empty tuna cans out with soy sauce snd old oil with some results Any ideas for natural remedies? Also, lots of rolls Polly bugs in cans "
Joy Bennett on Tuesday 22 June 2021
"Hi Joy. It seems you're doing the right thing in trying to trap the earwigs. It's also worth avoiding growing susceptible plants near ivy, which tend to harbour earwigs. Other than also covering over particularly susceptible plants with row cover, properly secured all along the ground, I'm not sure what else you could do. Some damage always seems inevitable."
Ben Vanheems on Wednesday 23 June 2021
"Nice series of articles on bolting. However, one fine point. Garner and Allard had their work on photoperiodism by 1920, long before 1931. There work is on the web. They also did some nice work demonstrating that the night length, rather than the day length, is what controls flowering in a lot of plants. Let's give credit where credit is due.."
Dan G on Sunday 10 July 2022
"Interesting Dan - thanks so much for sharing that, it's appreciated. "
Ben Vanheems on Thursday 14 July 2022

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