As temperatures soar, many gardeners are discovering that their leafy crops are suddenly producing flowers instead of fresh leaves. This process, known as bolting, is a plant's natural response to stress and can quickly bring harvests to an early end. The good news is that understanding what causes bolting – and how to reduce heat and water stress – can help keep crops productive for longer.
'It's a natural part of a plant's life cycle, but heat, drought and other forms of stress can trigger it weeks earlier than normal' says Didrik Dimmen, gardening expert, CEO and co-founder of indoor smart garden company Auk.
'My simple rule is: keep roots cool, keep moisture consistent and harvest little and often'.
Didrik explains how extreme weather can accelerate the process dramatically.
Top 3 plants most likely to bolt in a heatwave
- Coriander
- Lettuce
- Spinach
Coriander, lettuce and spinach are among the plants most prone to bolting during periods of extreme heat. These crops prefer cooler growing conditions, so when temperatures suddenly rise into the high twenties and thirties, they can rapidly begin flowering and setting seed.
Leafy crops are generally far more susceptible to bolting than plants such as tomatoes chillies and peppers, which are naturally better suited to warmer conditions.
'Coriander is the worst offender for bolting and can seem to go from healthy growth to flowering almost overnight during a spell of very hot weather' states Didrik. 'When temperatures rise significantly, they read it as a signal that the summer is over and begin focusing on flowering and reproduction rather than producing leaves'.
Can you still eat the leaves once a plant has started to bolt?
'Leaves from a bolting plant are usually still perfectly safe to eat, but the flavour and texture take the hit,' Didrik warns. 'Lettuce, spinach and rocket can become noticeably more bitter once flowering begins, while leaves may also become tougher.'
How to protect plants from bolting during a heatwave
One of the most effective ways to reduce the risk of bolting during a heatwave is to minimise stress on the plant wherever possible.
'The biggest mistakes gardeners make during hot weather is watering little and often in the middle of the afternoon' says Didrik. 'In very hot weather, most of that moisture evaporates before it can reach the roots.'
Instead, water deeply first thing in the morning. 'The best time to water during a heatwave is early morning, every time.' says Didrik. The soil is cooler, plants can absorb moisture before temperatures peak and less water is lost through evaporation. 'If you only dampen the surface, roots stay shallow and plants become more vulnerable to heat stress,' Didrik continues.
Beyond watering, keeping the soil cool is equally important. 'Adding a layer of bark, compost, leaf mould or even grass clippings around the base of plants helps the soil retain moisture for much longer and keeps roots cooler in extreme temperatures,' Didrik adds.
Providing light shade in the afternoon can also help protect more vulnerable crops.
Should I cut off the flowers if my herbs start bolting?
'Yes. Once herbs begin flowering, they naturally start directing more energy into producing seeds rather than fresh leaves' explains Didrik.
Removing flower stems as soon as they emerge can help extend the harvesting period and encourage continued leaf growth.
'For herbs such as basil, coriander and parsley, pinching out flower spikes can often buy home growers valuable extra harvesting time. It won't completely stop the plant's natural response to heat, but it can slow the process and help maintain better-quality leaves for longer'.
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