The greatest disappointment any farmer may encounter is when his leafy vegetables prematurely flower. Instead of using its energy reserves to make the leaves you want, it starts to flower and produce seeds that eventually dry off. This is what is referred to as “bolting”.
BOLTING is the premature production of a flowering stem or stems on a plant before it can be harvested. It is a term applied to vegetables when they prematurely go to seed usually making them unusable. It can affect a wide range of vegetables e.g. annual and biennial vegetables. It is usually seen on crops which are approaching maturity. It is the plant’s natural attempt to produce seeds so it can reproduce.
Crops inclined to bolt include lettuce, basil, leeks, carrots, cabbage, turnip, arugula, beetroot, brassicas, spinach, celery, and onion. As a farmer it is helpful to learn popular phrases that go along with the activity. Knowing what is happening to your plants and how to name is an important education. It is also helpful if you seek out agronomical support. A farmer should be able to learn and understand the plant’s language.
Plants do communicate with us in various ways and we need to understand the signs. Take for instance when a plant is bolting. If you observe your spinach or kales growing a tall flower stalk in a very short period of time you know there is a problem. This means lower yields as a result of early aging of the crop. So, what do you do?
Photo courtesy: Bolted Lettuce.
During bolting, plants typically redirect resources to producing leaves, roots or other unedible parts which results in a poor quality harvest. Bolting is induced by plant hormones called, “gibberellin” and when a plant bolts, it is often a sign that you are in for a poor harvest. It is also an indication that it will decline in terms of flavor.
So what causes this bolting to seed, and how can you prevent it?
Plants often flower in response to stress. It’s their way of ensuring reproduction in the face of an uncertain future. Environmental factors that stress plants include pests and diseases (although succumbing to these can also be an indicator of other stressors); too much nearby competition for water, nutrients and sunlight; or too many days of high temperatures or sunlight. Once a plant is on the flowering path, in most plants there’s nothing you can do to stop it; cutting off the flowering heads will not work to return it to leafy growth. One exception seems to be basil which can be turned back to leafy growth.
Causes of bolting
- Bolting is triggered either by cold spells or by the changes in day length through the seasons. It may occur after a prolonged cold spell. Cold nights, hot days and late frosts may also contribute to premature initiation of flowering.
- Changes in the length of the day effects crops e.g. lettuce, some radish cultivars and spinach are very sensitive to the number of hours of daylight received.
- Stressors such as not getting enough water or minerals can also trigger bolting.
Photo courtesy: Bolted broccoli.
How to prevent bolting.
- Try to avoid plant stress- if the soil is too dry or the compost is not nutritious enough, give it more water or fresh soil or compost. Cover plants in the event of a cold spell to keep them from being directly exposed to cold temperatures, rain or snow.
- Choose varieties that can resist bolting; plant breeders have introduced cultivars of “bolt-proof” crops that are less prone to the condition, such as Boltardy beetroot. These vareties are bred to withstand higher temperatures. Red onions seem to be more prone to bolting than white or brown types. If bolting in onions is a recurring problem, plant heat treated sets in early spring (exposure to high temperatures suppresses flower bud formation)
- Change your growing season: sowing can be delayed for cold sensitive plants until temperatures are more stable. Successional sowings will also help to achieve a constant harvestable supply if the season is changeable.
- Harvest the outer leaves off early, keeping them from maturing. If you keep cutting off growth from plants like lettuce, spinach, kales and broccoli, it stimulates the plant to replace it.
- Sow plants regularly. Sowing plants at different intervals can give you more of a chance of producing at least some that do not bolt. This is termed as succession sowing.
- Soil conditions. Provide good growing soil conditions for the plant. This will encourage rapid growth and formation of usable portions of edible parts before flower formation.
- Dry soil can encourage bolting, particularly with cauliflower and spinach. Careful watering to keep the soil moist can prevent this.
- Bolting can also be suppressed by topdressing with 70-100g per sq m. of nitrogen rich fertilizer.
- Use the right fertiliser. Fertilisers for vegetables are not a one-size-fits-all. You will be growing some plants precisely because you want them to flower and then set fruit or seed. Others are being grown for their leaves and stems. If you use a fertiliser meant for a fruiting plant on your leafy greens, the nutrient mix will encourage them to flower. Look for fertilisers made for growing greens, which will be high in nitrogen (N).
- Good crop spacing. Proper spacing will help the plant to establish itself through optimal use of sunlight, ground nutrients, water and efficient gaseous exchange. Proper spacing will also eliminate the plants’ need to compete with other plants for these resources.
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