Plants have evolved to time their seed germination, flowering and fruit/seed formation within particular temperature ranges (often regulated by day length). Their distribution geographically is also limited by high and low temperatures. Extreme conditions affect plant performance, survival and reproduction. In 2012, in the Denver-Boulder area, we had record-setting high temperatures: We tied the all-time high of 105 degrees Fahrenheit, and June was the hottest on record. We endured 73 days over 90 degrees and 13 days of 100 degrees or more (in the last 140 years, there have been only 83 days of 100 degree temperatures in the Denver area). And because drought accompanied the heat, 62 of Colorado’s 64 counties were declared crop disaster areas by the US Secretary of Agriculture.
In our own gardens and yards, some plants performed poorly even though we could water them. Tomatoes, for example, will drop blossoms and not set fruit if temperatures are over 90 degrees. In Arizona where temperatures soared above 120 degrees, some plants died even with irrigation. Whether global climate change is a long-term or a short-term cycle, we gardeners are going to have to learn some new tricks.
High heat affects plants in many ways. Seeds may not be able to germinate, fruits or seeds may not form or will drop before maturing. Growth can be stunted, plants can die or be weakened and attract pests and diseases. These observable reactions occur because of the adverse affect high heat has on internal and microscopic activities within the plants. Problems arise in photosynthesis, the formation of hormones, enzymes and defense chemicals; in the intake of water and carbon dioxide, in weakening of the cell membranes etc.
Plants do have natural systems that respond to heat problems. Transpiration is a mechanism in plants to cool themselves by “pumping” water out through the leaves for a kind of swamp-cooler effect. (See Colorado Gardener issue XX for an article on transpiration) But high heat can cause a plant to exhaust water supplies in this process; or the plant closes its leaf pores (stomata) to prevent water loss, but in so doing reduces the potential for taking in carbon dioxide which is needed for photosynthesis. Plants can also make “heat-shock proteins” which reduce problems with over-heating, but these strategies do take resources away from other plant needs like growth, flowering and fruiting. Some native plant communities have already started shifting northward where temperatures are cooler. And natural selection is undoubtedly giving preference to plants with increased heat and drought tolerance, like ones with deep or wide-spread roots.
But what can we gardeners do to respond to hotter temperatures? And what if these changes last for years and/or become more extreme? A discussion of solutions is frustrated by the fact that the world is investing far more in the problem than in solutions, so at the present, we know little about what to do. Here are some ideas.
We can choose, select and breed plants that show a tolerance for high heat. We can trade success stories with our friends and neighbors and we can learn from communities further south that are already dealing with very high temperatures. Because high heat often goes along with higher water needs and less precipitation, we need to choose, select and breed plants that tolerate drier conditions too. These have to be genuine xeriscape plants for dry, low-humidity Colorado, not xeriscape for Virginia or Iowa.
We will have to develop our skills at soil building to provide nutrition and organic matter that will retain moisture and let rain penetrate to lower depths. Improving soil health also means cultivating the microorganisms that bring water and nutrients to plant roots and that build organic matter and improve soil structure. Supporting the health and vitality of plants is just as important in responding to heat stress as supplying water. Chemical fertilizers, with their strong salts, have more potential for burning plants in hot, dry conditions, so organic fertilizers are preferable.
We can provide conditions that help to keep the soil cooler. Mulching with local wood chips or partially composted bark, or even fine gravel will insulate the soil, keeping it cooler and reducing evaporation. Planting in partial shade can benefit even some sun-loving plants. In 2012 several gardeners reported better success with tomatoes grown in partial shade than in full sun, and many reported failures in growing tomatoes in containers in full sun where the soil gets especially hot. Watering the soil also helps to cool it.
We can also build shading structures to protect plants from heat, especially from the west, which can be particularly hot in the late afternoon. These structures, whether made from lath, shade cloth or the light fabric row covers, need to be high enough to allow good movement of air or they can trap heat.
Other techniques, like foliar feeding plants early in the morning during hot weather, the use of anti-transpirants (like Wilt-Pruf), inoculating with mycorrhizae, early planting in protecting structures, protection from wind, planting next to a rock, and even preconditioning plants under different environmental stresses; all may be somewhat helpful. Whereas it is important for us to seek local solutions in order to be able to have beautiful landscapes and be able to produce good food locally, we cannot ignore the environmental and global effects of practices that waste water, like “fracking” and inefficient irrigation of crops, and that produce greenhouse gases, like the burning of petroleum and the production of chemical fertilizers.
Here is a small list of plants that are known to be more tolerant of heat:
Most of our native plains plants and shrubs including:
Desert Four O’Clock
Sulfur Flower (Eriogonums)
SW Natives including
New Mexican Privet
Well-Adapted Plants like:
Lilacs Sedums Oregano
Blue Velvet Honeysuckle
Rose of Sharon
Boxwoods in shade
Many shrub and heirloom Roses
(not hybrid teas)
Smoke Tree (Cotinus)
Blue Mist Spirea
Silver Lace Vine
Veggies-little is known
Cherokee Purple Tomato
Arkansas Traveler Tomato
Valley Girl Tomato
Purple Calabash Tomato
New Zealand Spinach
Purple Fountain Grass
Cape Aloe (houseplant)