How to Prevent Cool Season Crops from Bolting

6 March 2009, written by Jeremy Dore

One of the most confusing terms that new vegetable gardeners come across is ‘bolting’. Also known as ‘running to seed’ this is where a plant suddenly, often in a matter of a few days, starts to grow flower stems, simultaneously stopping all useful growth of the vegetable itself. It is not just a problem for people new to growing edible plants – experienced gardeners also have to manage this problem as I found out last week when several salad plants I have been carefully over-wintering in my greenhouse suddenly shot up central stems. Once the flower shoots form not only is growth slowed as the plants put all their energy into reproducing but they can rapidly become unusable in the kitchen as well. Lettuce, for example, becomes bitter tasting and the leaves are less tender once the plant has bolted. So what can be done to prevent it?

The first step towards managing this problem is to understand what causes it. We need to realise that to some extent gardening is all about working against nature’s natural tendencies. The whole life-cycle of salad crops like lettuce is geared to lead up to the production of seeds in order to perpetuate the species. Running to seed, or bolting, is a natural part of this life cycle. Ending up on a plate is not! The gardeners job, therefore, is to persuade the plant to put off flowering for as long as possible so that a good leafy crop can be obtained.

Various crops are affected by this problem but they fall into a few main categories (worst affected listed first) and are often referred to as ‘cool season’ crops:

Salad and leafy plants such as lettuce
Members of the beet family: beetroot, spinach. (Although Swiss chard is part of this family it is less often affected, particularly if several sowings are made.)
Brassicas such as cabbages, radish, turnip, rocket (arugula), pak choi etc.

Some biennial crops, such as onion, leeks, carrots (and beetroot mentioned above). These crops are less susceptible to this problem, since we harvest them in the first year and they would normally only go on and produce seed in the second year. However, early bolting can be triggered by abnormal weather conditions or by leaving them in the ground over winter followed by a mild spring.

Although we usually think of bolting as being caused by a sudden warm spell, such as my salad experienced last week, for many plants (particularly biennials) the trigger is actually the cold weather they experienced much earlier on. This ‘primes’ the plants to produce flower stems as soon as the weather warms up. The other important factor which affects many annuals is significant increases in day length. These extra light-hours trigger the plant to run to seed as summer approaches.

So what can be done to prevent this phenomenon, or at least put it off as long as possible? There are several helpful strategies:

For annuals, the key is to not stress the plant. Poor growing conditions such as dry soil or lack of nutrients from compost do not help, so it is good to mix in some quality compost and keep the soil relatively moist. If a cold spell occurs, then some protection will be beneficial, such as covering with horticultural fleece or moving them inside a cold frame if they are still in pots.
For other plants, selecting varieties which are known to resist bolting is the best option. A well-known example of this is ‘Boltardy’ beetroot and I have certainly never had this run to seed whenever I have grown it.
Some plants just don’t do well sown in spring. Many salad brassicas fall into this category: Chinese cabbage (pak choi), mizuna and rocket (arugula) are good examples which are all best sown in mid to late summer.

Lettuces can be kept from bolting by regularly picking the outer leaves, keeping them from maturing properly. This ‘cut and come again’ approach to harvesting can extend the time they produce for up to 10 weeks.
The most general advice, applicable across the board, is to make regular sowings of plants. Weather is by very nature always unpredictable, so sowing a few plants every two weeks or so will guarantee that some of them should do well, whilst still giving you some early harvests before they bolt.

As for my salad... actually I quite enjoy a bit of peppery rocket (arugula) or even slightly bitter lettuce when mixed in with other leaves. It is a bit tougher than usual and won’t be to everyone’s taste but at this time of year it gives a welcome change to the bland tasting lettuce available in shops. There is little left in my garden now from winter, so I am not so fussy. Meanwhile, I am busy making regular sowings of other spring plants under cover which should start to fill the gap in a month or two. Bolting may be an inevitable outcome of longer spring days but I am determined to use every bit of ingenuity to minimise its effect and get a great range of early salads. I’m sure there are other approaches too, so please do share any tips you have on stopping plants bolting by adding a comment below...