How many times have you planted a herb like coriander/cilantro and had it go to seed, within a couple of weeks? This ‘bolting’ to seed uses up costly reserves that you’d rather your plant was turning into the leafy green bits or bulbs you want to harvest. So how can you stop it happening?
‘Bolting to seed’ happens when a plant, instead of using its energy reserves to make the leaves you want, starts to flower and produce seed. As this takes a large amount of energy, when a plant flowers, its vegetative (leaf) growth usually stops. When it sets seed, if it’s an annual plant that’s also a signal for the plant to begin dying.
Plants that often bolt to seed are herbs like coriander/cilantro, basil and dill, and vegetables like lettuce, celery, beetroot, cabbage, spinach, radish, bok choy, rocket/arugula) and onion family plants like garlic and leek. Instead of tight heads of crisp green leaves, or juicy stalks, or tightly packed bulbs, you get flowers. The plant also withdraws sugars and water from the leaves to ‘fund’ this extravagant flowering, so leaves change from being sweet and juicy to tough and bitter. Some, like lettuce, also produce bitter compounds like sesquiterpene lactones in their leaves which may be to deter foraging predators.
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.
1. Plant seedlings at a cooler time of year. Assess your climate zone, including your microclimate, for periods of prolonged heat. For example, in cooler but still subtropical zones like east coast NSW, there are often many hot days in November associated with the dry spring season, making an early-spring sowing vulnerable to bolting. Sow during autumn and winter instead. Spinach and broccoli will start to bolt after more than a few days at 24ºC (75ºF). Conversely in some very cold climates, you’ll need to delay sowing some of the summer-grown brassica plants like bok choy and mizuna until early summer. Plants like lettuce and Swiss chard if exposed to very cold temperatures early in their growth may have had flowering triggered which will then happen as soon as the weather warms up.
2. Check the sunlight hours the plant needs for optimum growth. Many plants, especially lettuce, spinach and radish, are programmed to respond to daylight hours as much as they are temperature. Don’t plant these longer-daylight flowering veggies too late in the spring. Shading these plants for part of the day will reduce their exposure and reduce bolting.
3. Give early bolting plants a little more shade as the weather warms. Although most vegetables are supposed to grow best in full sun, in a warmer climate many need some shade, especially to protect them from sunlight during the hottest part of the day, usually the afternoon. Plant them among taller-growing crops, or where they’ll be in some shade from about 2pm onwards, or rig up some shade cloth covers over bent PVC pipe.
4. Plant at the correct spacing. We all like to err on the side of ‘more is more’ and it’s easy to overplant when seeds are so tiny. Be rigorous about thinning out your seedlings to the correct spacing, or only planting as many as you really have room for.
5. Mulch heat-sensitive herbs and vegetables. Some plants like coriander/cilantro) and broccoli will bolt to seed if its roots get hot, so a mulch layer will keep it cooler and the desired heads forming .
6. Keep the water up to your plants during hotter weather. Moist soil stays cooler and even one hot day with dryish soil can be enough to trigger a flowering cycle in heat-sensitive plants like cauliflower and rocket/arugula.
7. Harvest the plants early and often that prefer cooler temperatures. If you keep cutting off growth from plants like lettuce, spinach and broccoli, it stimulates the plant to replace it.
8. Plant ‘slow bolting’ seed, or choose the right variety for your climate zone. Slow bolting seed really means a variety that’s been bred to withstand higher temperatures
9. Use the right fertiliser. Fertilisers for vegetables are not a one-size-fits-all. Some plants you will be growing precisely because you want them to flower and then set fruit or seed, others are there 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).
10. Eat the flowers! If all else fails, don’t forget you can often harvest the flowers instead and use them in stir fries and salads.