Hot enough for ya? It’s only July, but we’ve had more than our share of relentless heat.
Have you ever wondered how plants cope with heat? It’s not as if they can turn on their air conditioners or pour themselves a cold one. In fact, plants have myriad adaptations that help them survive high temperatures. Some involve managing heat while others focus on conserving water.
- Many coping mechanisms are structural. Plants such as sedum have waxy leaves to conserve water. Fuzzy lamb’s ear has reflective leaf hairs.
- Ornamental grasses’ rolled leaves give them an advantage as does threadleaf coreopsis’ smaller, finer leaves. Less surface area means leaves lose less water.
- Lavender, Russian sage and other plants with bluish leaves are summer survivors, too.
- Plants with thick roots such as iris, peonies and daylilies store water better. And native plants’ deeper roots find water more easily.
Drought-resistant native plants like this butterfly weed (Aesclepias tuberosa) also attract pollinators such as this zebra swallowtail. Photo credit – Joan Willoughby 
Natives plants such as this bee balm (Monarda fistulosa) have deep roots that make them drought resistant.
All of these evolutionary adaptations help plants tolerate hot, dry conditions. We know that summer’s heat comes every year and that global warming is bringing more temperature extremes. So it makes sense to help our gardens adapt by incorporating plants with these characteristics.
Okay, science geeks. Here’s one for you. Did you know that some plants can make special “heat-shock proteins” to help them recover from heat stress? When you cook an egg, you are unfolding proteins. When you melt butter, you are disrupting cell membranes. These same disruptions can happen when plants get too hot. Cell membranes can literally melt, leaking plant’s vital fluids. Heat-shock proteins act like “molecular chaperones,” preventing these bad things from happening at a cellular level. They beef up membranes and collapsing proteins. Plants survive.
Iris’ fleshy rhizomes help it hold water and thrive in high temperatures. Photo credit – Marie Bikle, Washington County Master Gardener
But don’t make plants go it alone, relying only on their adaptations. Help them when it’s hot by watering them more often and deeply. Newer plantings of trees and shrubs need slow, deep soaks once a week. Use a hose on a trickle, a soaker hose, drip irrigation or a 5-gallon bucket with nail holes in the base.
Container plants heat up and dry out faster, so check and water them once or twice a day. Soak them until water runs out the drainage holes.
Keep plants mulched to conserve moisture. Consider shade covers on vegetable crops. And be vigilant, watching your plants for signs of heat stress such as wilting.
Water moves constantly from the soil to roots, stems and leaves. There it escapes through leaf pores. When the rate of water lost is greater than the water absorbed, plants wilt and need water.
Trees often jettison some leaves to conserve water when it’s hot. Fewer leaves need less water. Unless leaf loss is dramatic, there is no cause for concern.
Vegetable plants slow production in high heat. Blossoms drop when temps top 80 degrees. Without blossoms, plants can’t make fruit.
Tomatoes, squash, peppers, melons, cucumbers and beans are likely to drop blossoms. It’s a passing phase. Plants will make flowers and fruit again when temperatures cool.
Plants have developed miraculous adaptations to high temperatures, but sometimes need our help. So watch, water and marvel at the many ways nature finds ways to beat the heat.
Annette Cormany, horticulture educator, University of Maryland Extension – Washington County