When a summer heat wave arrives, it can be stressful for plants and gardeners alike. The plants of your landscape will require a little more TLC to make it through periods of extreme heat.
How Plants Respond to High Temperatures
As temperatures increase, so too does the growth rate of plants. Once temperatures exceed 86°F, the growth rate of most plants begins to slow. This happens because the rate of photosynthesis (the process in which plants use the sun's energy to create carbohydrates as a food source) reduces when temperatures soar into the 90s and 100s. In contrast, the rate of respiration (the process by which plants use carbohydrates to grow and develop) continues day and night, even at higher temperatures. This depletes the food reserves of the plant. If extreme heat continues for weeks at a time, plants can actually die from a depletion of their food reserves.
High temperatures can also cause severe water loss (desiccation) when transpiration (the process by which leaves release water vapor to the atmosphere) exceeds moisture absorption by the roots. Evaporation of water from the soil can further reduce the amount of water available to plants. As the water content in leaves decreases, leaves wilt, slowing the rate of water loss, but this causes leaf temperatures to increase because of reduced evaporative cooling. If high temperatures persist, this cycle can worsen so that a portion or all of the leaf can be killed.
What to Do in the Garden in Extreme Heat
Below are a few tips to protect your lawn, garden, and landscape when temperatures soar.
Change Watering Practices
With higher temperatures comes more water loss both through the leaves of the plant (transpiration) and through evaporation from the soil surface.
- Check soil moisture at least daily. If soil is dry at a finger's depth or more, it's time to water.
- Water in the morning. Less water is lost to evaporation in the cooler early part of the day, and it allows plants to fully hydrate before the heat of the day sets in.
- Apply water efficiently. High temperatures mean a high potential for water to evaporate before it ever reaches the roots. Avoid sprinklers and instead use spot watering at the base of the plant, soaker hoses, or drip irrigation systems to provide water efficiently.
- Plants in containers and hanging baskets may need water twice a day. Especially when windy and hot, containers dry out very quickly. Check often, as most will need water at least once a day, and some may need water in the morning and the afternoon!
- Pay close attention to recently planted plants. Perennials, trees, and shrubs planted earlier in the year have not yet rooted in fully. With most roots still in the original root ball, that root ball can quickly dry out. Check newly planted plants frequently and water both the original root ball and the surrounding soil if either is dry. The same consideration is important for those plants planted within the last three years. These plants still do not have extensive root systems and may need extra water during stressful times.
- Remember to always check soil moisture before watering. The metabolic processes of plants slow down in extremely hot temperatures. While plant processes like respiration and transpiration increase as temperature goes up, there comes a spot where these processes plateau or slow down. This means the plant may use less water at 95ºF than 85ºF (assuming water is not lost to evaporation).
More tips on watering can be found in this article: Watering Tips for the Garden, Lawn, and Landscape.
Covering the soil with an organic mulch such as woodchips, cocoa hulls, compost, or leaf mold can help hold water in the soil by reducing evaporation. It has the added benefit of shading the soil to keep the root zone cooler.
Learn more about the many benefits of mulch in this article: Using Mulch in the Garden.
Provide Some Shade
For plants in containers, move them to a shady spot during the heat wave. This reduces the light that hits the plant and container, keeps temperatures lower, and reduces water loss.
If you cannot take the plant to the shade, then bring the shade to the plant. Utilize shade cloth, screening, or even white sheets to reduce the amount of light hitting the plants and the surrounding soil. Secure shade covers on a few posts or a wire hoop frame several inches over the plant. Putting up some type of shade cover helps the soil and plants stay a few degrees cooler while reducing the amount of moisture lost. Shade cloth comes in various densities that can block different levels of light. For many plants, start with 30 to 50 percent shade cloth. Different densities can be layered to provide even more shade. Any shade covering should stay above the plant and not come in direct contact to avoid retaining heat and potential leaf burn.
In extreme heat, plant processes slow down. Adding fertilizer promotes growth that the plant cannot support, leading to additional stress.
Wait to Transplant or Propagate
Planting, transplanting, and propagating are stressful on plants at any time of the year. Don't add to this stress by doing it during extreme heat. Wait until the heat wave subsides to plant or propagate.
Learn more about transplanting and dividing plants in this article: How to Divide and Transplant Perennials.
Many plants will stop blooming during extreme heat. Removing the spent flowers can improve the plant's appearance, keep plants from spending precious resources on fruit and seed development, and set the plant up to bloom again once the heat wave passes.
Learn more about deadheading in this article: Deadheading Herbaceous Ornamentals and Roses.
Allow the Lawn to Go Dormant
If supplemental irrigation is not provided for the lawn during hot, dry weather, the cool-season grasses commonly grown in Iowa, such as Kentucky bluegrass, can survive by going dormant. In these conditions, the shoots of the turfgrass plants stop growing. While the leaves have turned brown and died, the turfgrass roots and crowns remain alive. Generally, Kentucky bluegrass and other cool-season grasses can remain dormant for four to six weeks without suffering significant damage.
If the heat wave lasts longer than six weeks, dormant cool-season grasses risk dying. To ensure the survival of dormant grass, it’s best to water lawns that have been dormant for six weeks. Apply 1 to 1 ½ inches of water in a single application. Water again seven days later. The grass should begin to green up after the second application.
Learn more about summer lawn care in this article: Summer Lawn Care
Don't Forget About the Gardener!
Of course, hot weather is just as hard on you as it is on the plants you're growing.
- Work in the garden early in the morning when temperatures are cooler.
- Drink plenty of water. Carry a water bottle with you as you work.
- Wear sunscreen and a large hat to protect your skin.
- Take frequent breaks to cool down.
- Learn the signs of heat exhaustion and heat stroke and cool off indoors or get help should they start to develop.
Learn more about heat-related illness in this article from the CDC: Heat Stress - Heat-Related Illness.