Bolting refers to when a plant rather rudely decides to enter its flowering stage before we would like it to. Essentially, it has come to the conclusion that it is time to produce seed, and energy previously flowing into leaf growth is diverted to this task. As a result, the leafy part of the plant, which we were more than likely planning on eating, is lost to woody, not so tasty, and sometimes more bitter material. Bolting is essentially a survival mechanism which is triggered for a number of reasons. The factors which contribute to a plants susceptibility to bolting come from two powerful forces - those of Genetics and the Environment.
Plants, like most forces of nature, are genetically programmed to reproduce. It is worth taking a look at a plant's typical life cycle and where it came from to appreciate the conditions which can stimulate its premature flowering.
Annuals prone to bolting
Annuals such as spinach and lettuce which have a propensity for bolting are a long way from home here in temperate Ireland. They originated from the Fertile Crescent in the Middle East where the region is characterised by intensely hot and arid summers and are still largely influenced by this heritage. In climates like this, annual plants evolved to germinate in a cool autumn, grow slowly through a winter of cold short days before steady spring growth and a reproductive phase that sets seed before the return of the summer drought. So, contrary to native Irish plants, which try to set seed to endure a cold dark winter; plants from hot regions tend to produce seeds to survive an inhospitable summer of long blistering days. Knowing this we can see how the lengthening days of summer can trigger these latent ‘desert’ genes to spring into action. The plant rushes to flower and produce seeds before the desert summer it’s expecting. It’s safe to assume that our spinach is as disappointed as us when it discovers what an Irish summer actually resembles.
In these annuals, flowering is generally triggered by the photoperiod, that is, how many hours of daylight the plant receives. They are ‘long-day’ plants which flower when day length increases. This is why flowering occurs earlier in higher latitudes such as where Ireland is located and summer days are long.
Plants adapt slowly to changing landscapes and climates as they migrate naturally. This natural genetic adaptation has been steered by farmers for millennia by how they manage and save seed. More recently, plant breeding has become itself a profession and crops have changed radically in the last number of decades. One result of this is the development of varieties which are much more resistant to bolting as the day lengths required to trigger the process are stretched out. For instance, modern spinach varieties are much more bolt resistant than those developed before the 90’s.
What we can do
Knowing that bolting has a genetic basis, we can think about how to minimise it happening to our crops. Annuals are expected to flower in their first year but the conditions should be good so as a rapid growth of the usable portion of the plant (such as a lettuce head) develops before this happens.
- Timing - Choose your window. It is a natural progression for some spring-sown annuals to run to seed as summer progresses so timing of sowing is one of the most important factors. Regardless of bolt resistance, the long days we get at this latitude in the mid summer will make any spinach variety bolt. For this reason, spinach is grown in Ireland either early or late in the season to minimise exposure to long days. Alternatively, it is overwintered under cover which is popular amongst Irish growers in order to provide early greens. The same applies to many of the brassica salads, such as rocket and mizuna, which are also very prone to bolting in summer. So bolting can be prevented by planting bolt-prone plants very early in the spring or late in the summer to grow on during shorter cooler days.
- Moisture - Using the same logic, we can see why allowing plants that are this way inclined to dry out might not be the best idea. They will similarly be deluded into the mythical prospect of a hot Irish summer being close at hand and run to seed. It is vital not to let these plants dry out. Some sort of irrigation system can be useful in dry spells and mulching can help prevent the soil from drying out.
- Varieties –Different varieties will have different susceptibilities to bolting. Many seed companies will tout 'bolt-resistant' on their packets or varieties. However, this has to be taken with a pinch of salt and the surest way is to trial a few different alleged bolt resistant varieties and determine for yourself which performs the best in your conditions. In the case of lettuce, the cultivar is important as types such as butterhead generally don't do too well in summer whereas loose leaved types can be more successful.
- Other Stresses - Poor nutrition can also be stressful to a plant and makes for an increased likelihood of bolting. As with growing in general, it is important to take care of the soil to insure it is rich in nutrients and biological activity. Restricting the roots of plants will also quickly lead to bolting as anyone who has left plug plants sitting around in a tray for too long will have witnessed.
- Seed Saving - If you are saving seeds from plants which have a tendency to bolt it is important not to collect the seed from the plants which go to the flower first, as you are essentially selecting for this habit then.
In contrast to annuals, some biennial crops (those that normally grow in the first year and flower in the second) like onions, leeks, chard and cabbages can also bolt in the first year. These crops have evolved to produce a lot of energy in a storage organ in the first year. This could be a big taproot in the case of carrots, an onion’s bulb or the head in a cabbage. The plant puts this extra food source to work in the second year to produce its seeds. Generally this increased energy source allows the plants to produce vastly more seed than their annual counterparts and this is probably the main advantage with this strategy. As such, biennials need to feel they’ve gone through a winter to initiate flowering.
This gives us more than a hint as to why unsettled weather is the main culprit behind bolting biennials. This is usually a problem early in the season where cold spells and late frosts can manifest suddenly. A good rule of thumb is to not start susceptible crops too early. Another strategy is to protect plants from early frost with crop covers. As with annuals, biennials should be free from stresses such as drying out and lacking nourishment. Sometimes it is possible to halt the bolting process by nipping off the flower buds. This can be done with onions and works particularly well with basil which usually goes back to producing leaves for a while.
Let it go!
A final option against bolting is to just let it off. It can be interesting to watch plants flower that we would not normally see otherwise. Bees will be forever in your debt, and also, the flower shoots of some crops can be a tasty treat, With Tuscan kale, the second year's flower shoots can be as delightful a harvest as the previous year's leaves.