As St. Louis and much of the surrounding area experience another round of triple-digit temperatures, the plants around us are feeling it too. Hot, sunny weather will dry out soils quickly. With less water available for the roots to absorb, leaves and stems may start to wilt and flag. An occasional deep, slow watering should help prevent this in established plantings. But even with adequate soil moisture, extreme heat can cause damage to landscape plants.
How Plants Beat the Heat
Different species can tolerate different temperature extremes, but, in general, once air temperatures rise above 90°F, most plants will start to experience heat-related stress. At these temperatures, the physical and chemical processes related to photosynthesis begin to slow down, resulting in less food being available to the plant. Growth may slow or stop entirely. In certain conditions, plants may experience temperatures even higher than the ambient air temperature. Plants located near hardscaping such as driveways, stone paths, sidewalks, and roads will receive extra heat radiating off of these surfaces. This can lead to leaf scorch and dieback. In dark-colored pots, especially thin, plastic nursery pots, plant roots can easily be exposed to scorching temperatures of over 125°F.
Some plants, such as the dusty miller and Artemisia pictured here, have hairy or fuzzy leaves that help to reduce the intensity of sunlight on their foliage. Photo by Tom Incrocci.
Some plants have evolved mechanisms to better withstand extended periods of hot, dry weather. Succulent plants can store extra water when it is available and use it during periods of drought. Many native prairie plants grow deep, extensive root systems to tap into water resources that other plants might not be able to. Other plants have waxy or hairy leaves to help reduce the intensity of the sunlight hitting the foliage.
With extreme temperatures expected to become more common, here are a few recommendations from the William T. Kemper Center for Home Gardening for plants that can stand up to excessive heat.
Hesperaloe parviflora ‘Coral Glow’, redflower false yucca
A relative of the yucca, this Texas native can take the heat while producing a profusion of coral-colored blooms on gently arching stalks.
Redflower false yucca is easily grown in dry, sandy, sharply-drained soils in full sun. It thrives in hot, dry, desert conditions but is also surprisingly winter hardy to USDA Zone 5.
Pentas lanceolata, Egyptian star flower
Well-known in more southern climates, these plants deserve more usage in St. Louis gardens as bedding annuals or in container plantings. Egyptian star flower is available in a wide range of colors, including white, red, pink, and purple.
Egyptian star flower thrives in full sun and can even be kept as a houseplant.
Baptisia australis, blue false indigo
The deep root system and blue-green, somewhat waxy leaves of this and other false indigos help it to tolerate both heat and drought. Native to the eastern United States, blue false indigo is long-lived but can be difficult to transplant, so choose its permanent location wisely.
The common name “blue false indigo” refers to the use of certain native Baptisia species by early American colonists as substitute for true indigo (genus Indigofera) in making dyes.
Lagerstroemia ‘Pocomoke’, crape myrtle
While not known for their cold-hardiness, crape myrtles perform very well in hot and humid St. Louis summers. Dwarf cultivars such as ‘Pocomoke’ are perfect for climates where crape myrtles are already limited in size due to winter temperatures.
Crape myrtles may sustain damage during harsh winters in the St. Louis region, but they thrive in the summer and produce lovely, showy blooms.
Public Information Coordinator