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 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 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 rocket, coriander and pak choi, 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.
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, rocket 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 sweetcorn. 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 rocket.
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.