By Kelly Miller
Since modern global temperature tracking began in 1880, the hottest 10 years on record have all occurred in the past 10 years, with 2024 being the hottest year ever recorded. For gardeners, growing plants in high heat is increasingly a topic of interest. Prolonged heat waves and higher temperature spikes will strain plants, but there are plenty of ways to beat the heat.
Know Your Local Climate
Most gardening discussion about extreme temperatures is focused on cold weather, since many plants can’t survive freezing temperatures. Warmer weather, meanwhile, is generally helpful for plants.
Heat speeds up a plant’s biological processes and promotes growth. However, when it comes to heat, you can have too much of a good thing. When temperatures reach 85°F (29°) and higher, heat begins to shift from a benefit to a hazard.
Most gardeners are familiar with the USDA Hardiness Zone Map that’s used for analyzing where various plants can survive the winter. For heat, a lesser-known map exists: the American Horticultural Society’s Heat Zone Map. This map indicates the average number of days with temperatures over 86°F (30°C) that an area experiences each year.
Although the Heat Zone Map hasn’t been updated since 1997 it can still provide a useful reference. If your area has more than a few “heat days” annually, then you should plan on taking extra steps to protect sensitive plants during the summer.
How Heat Stress Affects Plants
In high heat, photosynthesis slows down and respiration speeds up. In this condition, known as heat stress, plants deplete their food reserves, weaken, and grow more slowly. Meanwhile, both the plant tissue and the soil lose water rapidly, increasing the risk of dehydration. Visible symptoms of heat stress on plants include the following:
Wilting
Wilting is one of the most common effects of heat stress. Wilting occurs when a plant’s leaves and flowers lose their rigidity due to a lack of water pressure. Wilting foliage may turn yellow or brown or fall from the stems.
Short-term wilting is usually reversed when a plant is watered, but prolonged wilting can be fatal.
Bolting
For cool-season veggie plants, such as spinach and broccoli, another common problem when growing plants in high heat is bolting. Bolting is when a plant grows a tall, flowering stalk. When a plant bolts, it mostly stops producing edible foliage or fruits. This is a natural seasonal process, but can happen prematurely if a plant is overheated or overexposed to sunlight.
Blossom Drop and Sterile Pollen
Two of our favorite garden vegetables – tomatoes and peppers – can suffer from blossom drop when subjected to prolonged high temperatures. The plants go into survival mode, sacrificing flowers and fruit to focus energy into supporting the main plant.
Tomatoes may stop blooming or drop flowers when temperatures linger in the range of 85°F to 90°F (29-32°C). Peppers can drop flowers when temperatures are at 90°F (32°C) or higher. The flower pollen of tomatoes and peppers may also become sterile in high temperatures, impacting fruit production. Both plants should resume flowering and fruit production when temperatures cool down again.
Protecting Plants from High Heat
With extra care, you can avoid damage to your plants during heat waves. Here are a few strategies for protecting your plants during periods of high heat.
Grow Heat-Tolerant Plants
To improve your landscape’s hot-weather tolerance, swap out sensitive species for ones that are more tolerant of high temperatures. Consider cacti and other succulents, hardy shrubs, and native groundcover plants that evolved to handle your region’s full range of environmental conditions.
In hot, arid climates, xeriscaping is a smart approach for the sunniest parts of a property. Xeriscaping involves replacing irrigated groundcover with drought-tolerant plants along with elements like rocks, gravel, and mulch.
In a vegetable garden, cold-season crops like spinach, carrots, and broccoli are the most likely to struggle in high heat. In hot climates, time your plantings for these veggies to enjoy cooler harvest weather in the spring or fall. For bolting veggies, look for “bolt-resistant” varieties, such as ‘Corvair’ spinach or ‘Slow Bolt’ cilantro.
Meanwhile, the most heat-tolerant veggies include:
For growing in hot weather or in the hottest part of your garden, look for varieties labeled “heat-tolerant” or “drought-tolerant.” Choose varieties that were developed specifically for heat tolerance. For example, ‘Heatmaster’ tomatoes were bred to withstand high heat and humidity, and testing has proven that they’ll outperform other tomato plants in these conditions.
Water Regularly and Efficiently
In hot weather, plants release more water through their foliage. They also lose access to water in the soil due to increased evaporation. Thus, for most plants, it’s important to increase watering during high heat.
To make sure your plants don’t get dangerously dry, check the soil moisture daily and water whenever its dry. Remember that potted plants and young plants are most vulnerable to drying out and should be watered more frequently.
Water in the morning, helping plants hydrate before the afternoon heat sets in.
Sprinkle watering is inefficient. Most plants benefit more from a heavy soaking at the base of the stem, rather than a misty spray over the foliage.
When growing plants in high heat, it’s also important to conserve water, both indoors and out. There are lots of ways to save or reuse water so that you have all you need for your garden. Practices to consider include rainwater harvesting, reusing graywater for toilet flushing, and reusing cooking water for watering plants.
Provide Shade During Hot Periods
Shade can be a lifesaver for plants that are exposed to extreme sun and heat. With potted plants, this may be easy: find shadier spots. For garden beds, you have to bring the shade to the plants.
Shade cloth is a light-weight netting material that comes in large sheets. Use posts to hang shade cloth next to or over a garden bed, where it will reduce the intensity of the sunlight that strikes the plants. Shade cloth comes in various density ratings that indicate the percentage of blocked light. For veggie plants, a 30–50% density is usually appropriate for over-sunned areas.
You can also take a DIY approach to shading, such as with mounted umbrellas or other creative pole-mounted fabric arrangements. Just make sure that your shade material isn’t touching the plants, as this can burn the foliage or knock them over when the wind hits the cloth.
Mulch to Keep Plant Roots Cool
Adding a layer of mulch to soil helps protect plants against extreme heat. Mulch forms a barrier between the plant’s roots and the external environment. This barrier prevents the soil from overheating. Mulch also reduces evaporation, helping the plant to stay hydrated and reducing the amount of needed watering.
Good summer mulch options include shredded leaves, pine needles, and grass clippings. These fine materials form dense layers that help the soil retain as much moisture as possible.
Address Factors that Make Heat Damage Worse
Several factors can make plants more vulnerable to heat damage.
Fertilizer can do more harm than good for heat-stressed plants. The nutrient load may encourage new growth when a plant would be better off conserving its strength.
During a heat wave, it’s also wise to prevent dogs from urinating on sensitive plants. The salt and nitrogen in the urine may dry out plants that are already struggling to stay hydrated.
Additionally, consider a plant’s surroundings. Plants next to walls or concrete driveways may suffer from extra heat radiating off these surfaces. Provide extra shade to offset these effects, or relocate sensitive plants.
Safe Gardening in Hot, Dry Climates
Keeping plants alive in high heat is a challenge. In some regions, the risk of wildfire makes a dried-out garden a serious safety hazard. If fire safety is a concern in your garden, find helpful info find helpful info in our articles, Landscaping for Wildfire Protection and Flammable Plants – How to Use them Safely in Landscaping.
0 Comments