How extreme heat affects your crops

The weather in July has been truly extraordinary with many climate records broken. England saw its driest July on record, with rainfall 92% down on average and many counties receiving no rain at all. UK temperatures were also the second warmest on record (1). 

Most of our food crops have some degree of resilience, but when they reach these extremes, many plants show abnormal behaviour as they go into survival mode. Here are some examples of what we might expect to see in common crops:

• Carrots show reduced germination above 25C, and under high temperatures the seed beds dry out rapidly, so the crop gets off to a poor start (2). 
  Heat hack: Shade the plot or use the age-old trick of a plank of untreated wood placed on the row until the first sight of germination. This will ensure seeds remain in contact with the soil and prevent drying out.
• Runner beans are most susceptible to high temperatures when they are flowering, and any temperatures above 26C will reduce pod set (3), especially as insects are involved in the pollination stage. 
  Heat hack: You should have more success with self-pollinating varieties such as ‘Firestorm’ but also consider growing French beans, which are better at self-pollinating and less affected by high temperatures.
• Lettuces are induced to bolt at temperature above 25C. This has shown to be dependent on complex cycles governed by plant hormones and genetics (4). 
  Heat hack: Choose loose leaf and romaine varieties over whole head varieties (such as iceberg), and consider growing under shade netting.
• Brassicas show reduced growth at temperatures above 30C (5) and are frequently attacked by pests such as cabbage aphids and flea beetles under the hot dry conditions. 
  Heat hack: Many brassicas such as the kales and oriental brassicas will be more productive grown as a winter crop, so we may have to change when we grow things.
• Tomatoes show poor fruit set when daytime temperature exceed 29C, mainly due to poor pollen viability (6). High temperatures above 30C also inhibit the production of lycopene, the compound that develops the red colour in tomatoes, so ripening can be inhibited (7). 
  Heat hack: If you’re growing in the glasshouse, try hanging shade netting on south-facing glass to reduce temperatures. 
   

We’d love to hear if you’ve seen any interesting changes to your crops this year. Email our editor.