Heat stress

📖 Overview

Heat stress is what happens when temperatures consistently exceed 32°C and your plants simply can't keep up with water loss through their leaves, even when soil moisture is plentiful. The plant's cooling system—transpiration—becomes overwhelmed, and dehydration sets in from the inside out. This is one of the most common environmental challenges home gardeners face during European and North American summers, particularly affecting cool-season crops like lettuce and spinach, but also reducing fruit set in tomatoes and other heat-sensitive vegetables.

Why this matters depends on what you're growing. Lettuce and leafy greens bolt (flower prematurely) and turn bitter within days of sustained heat, making them inedible. Tomatoes, peppers, and beans drop their flowers when temperatures spike, which means little to no harvest despite perfectly healthy plants. The window of danger is typically July through mid-August in temperate zones, though it can start as early as late June during warm springs and extend into September in cool regions.

The first sign is often wilting leaves even though you've watered—they look droopy and dull, sometimes taking on a slightly bleached or grayish tone. This is different from underwatering, where the soil itself is dry; with heat stress, the soil stays moist but the plant simply can't absorb water fast enough to match evaporation. You might also notice flower drop on tomatoes or beans, or sudden bolting in lettuce during a heat wave. The key tell is that the problem appears during the hottest part of the day and sometimes partially recovers at night, whereas true drought stress doesn't improve much even after evening.