Most bolting is cold-induced – this is referred to as “vernalisation”. Seed producers and breeders are very conscious of this as it’s important for them to get all plants to bolt and set seed, for two reasons. First, bolting correlates to seed yield. Second, if all plants don’t bolt, and we take seed from those that do, we’ll be selecting for bolting. The most desirable plants to take seed from are those that didn’t bolt. If this isn’t watched, there will be a drift to bolting susceptibility.
Bolting is a problem with onions, as many producers try to plant early to get larger bulbs. The rule here is that the more mass the plant has when going into a cold period, the less cold is required for it to bolt. If onions bolt, the bulbs will be smaller. If the bolting stem is broken off, it will leave a hollow, tough tube in the centre of the onion, reducing the grade and quality.
There is a compromise for those who grow onions during winter. A few bolters will indicate they’re just about right for planting season, to produce the highest yield for the growing conditions. When there are no bolters, we might think we could have planted a little earlier for a higher yield. The more favourable the growing conditions, the later we would have to plant, as the crop would have a greater mass when going into the cold.
Varieties also play a role as some are bred to be more bolting resistant. As a rule of thumb, the warmer the weather, the earlier we can plant and the higher the yield potential. Young cabbage plants that have been subjected to cold are more prone to bolt. Most farmers know that certain varieties are more suited to growing out of cold due to greater bolting resistance. Although this year’s winter on the Highveld was very cold, it doesn’t look like there will be very much bolting. This seems to be related to the late onset of very cold weather. What happens at the end of winter also has a bearing on whether or not the plants will bolt.
When conditions are favourable, the plants may make a fair-sized head that’s more pointed or deeper than one grown in summer. When cut through, the growing tip will be far up the head and the heads will also weigh much less. You may even see buds developing at the growing point.Broccoli tends to make bigger beads and smaller main heads. The plant initiates many side shoots from an early stage of its development. For this reason, some varieties aren’t suited to go though to spring.
Cauliflower doesn’t have this problem as we eat the developing buds. Dedicated winter varieties usually don’t form proper heads in summer.Most modern carrot hybrids are now pretty bolt resistant. Old varieties like Cape Market and Brazilia are extremely susceptible and need very little cold to seed in early summer. But after maturity, don’t leave the carrots in the ground for long in early summer, or some varieties will start bolting sooner then others. Once bolting begins the roots start to deteriorate rapidly, even if well-formed.
Lettuce works the other way around and tends to bolt more readily in stressful, hot conditions. It’s even more susceptible to bolting when there’s excessive soil nitrogen, which can induce bolting even when it isn’t hot. Again, variety plays a role.You need to become aware of bolting requirements and risks for every crop to reduce possible hazards.