As people hunker-in at home during the COVID crisis, it has been noted that there’s a developing interest in exploring old movies. In this blog post, as we take a close look at those plants that thrive in the summer heat, it triggers memories to the old classic comedy Some Like It Hot starring Marilyn Monroe. It is a fitting title for an entertaining story with the steamy legend. That’s about all the connections I can draw with heat-tolerant plants and Marilyn, but if you have a chance to watch the movie (before COVID 19 ends), some might agree both are worth a closer look.

Marilyn Monroe stamp

The heat of summer takes its toll on plants, and it is not uncommon during a garden stroll to see some of your favorite plants bending from the hot sun. Certain plants like Impatiens, Ligularia, Giant Butterbur, Summer Phlox, and many vining vegetables are some of the first to signal stress from excessive heat with a bow, bend, or eventual wilt. Just as these flowers falter, remarkably there are others that withstand the higher temperatures and truly take it as their time to perform.

Hot weather annuals worth planting:

  • Caladium – This foliage plant really likes it hot, but requires frequent watering.
  • Madagascar Periwinkle a.k.a. Annual Vinca (Catharanthus roseus) – This flower is native to India (and that speaks volumes). I have distinct memories of managing huge floral displays of annuals during a long drought which eventually led to watering (irrigation) bans by the city. Slowly, many of the flowers began to fade and wither. The Vinca plants, with their waxy leaves, never faltered and looked even better from the heat. 
  • Sweet Potato Vine – I recall walking through downtown Louisville, Kentucky on a blistering 96-degree day when the urban environment of asphalt, concrete, glass, and metal was absorbing and then reflecting the sun’s heat-causing this Michigander to physically wilt. Always on the watch for a good plant story and photo, I spotted a large beautiful container planter with at least 4-6 feet of numerous Sweet Potato Vines sprawling down out of the container onto the sidewalk and along the asphalt of the street. It was vigorous, vibrant, and thriving in the brutal heat.
  • Gaillardia – This plant species is sold both as an Annual and Perennial and continues to yield new flower color varieties and heat tolerance. To hear that it endures field trials being grown in sand dunes should be a valid indicator of its hot weather tolerance.

Qualities and characteristics that enable specific plants in our gardens and landscapes to survive in the heat:

  • Waxy leaves – This feature on some plants prevents moisture loss, as seen with Sedums and the BIG Series of Begonias (admired and hailed in an earlier blog post).
  • Spines, tiny hairs or fuzz on the leaves – Spines are obviously worn by Cacti, but the fuzzy hair is evident on Artemesia and some succulents that absorb some of the sun’s rays and shade the leaf surface.
  • The angle and orientation of the leaf – Leaves typically set horizontally receive the maximum brunt of the sun’s rays. As a plant lowers the leaf to a 45-degree angle or if necessary, a straight pitch down to a 90-degree angle, it effectively reduces much of the sun’s ultraviolet light.
  • Leaf loss – Two tree species, Sycamore and Tuliptree adjust to the potential water loss in high temperatures and actually drop or release some leaves. Since moisture loss occurs from evapotranspiration through stomates on the leaf surface, this survival adaptation effectively reduces water loss by discharging the mechanism where the loss occurs.
  • Dormancy – Some plants can slowly shut down or reduce growth and size to endure the hot times until cooler weather prevails. Pansies are an example of this feature. Well known for its ability to be an early spring bloomer and tolerate cold weather and even frost damage, when the summer heat comes along, Pansies can shrink back and wait until fall weather when they return to grow and bloom.

When the summer heat is bearing down and you are watching your flower gardens struggle, consider these species as tactical performers in your bloom sequence medley. Plants like these provide the assurance they’ll survive, so you can head inside, relax in the A/C and watch old films of Norma Jean.

-Rob McCartney, Horticulturist

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