Managing the Garden in Extreme Heat

Care and How To

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.  Below are a few tips to protect your lawn, garden, and landscape when temperatures soar.

A newly planted maple tree showing signs of drought stress
A newly planted maple tree showing signs of drought stress.

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.  There is less water lost to evaporation in the cooler early part of the day, and it gives plants an opportunity 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 of the 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 it would at 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.

Add Mulch

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 amount of 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 also 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. 

The leaves on this variegated dogwood show damage from dry conditions and intense light
The leaves on this variegated dogwood show damage from dry conditions and intense light.

Don't Fertilize

In extreme heat, plant processes slow down.  Adding fertilizer promotes growth that the plant cannot support, leading to additional stress on the plant.

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.

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 are at risk of 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. 

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.
Last Reviewed: 
August, 2022