Gardens of the Cross Timbers: Is the climate tanking?

Becky Emerson Carlberg
Contributing writer
Hibiscus loving the heat

HOT. Goodbye July. Now we must endure August. Please rain.

The latest program in the Shackelford Webinar Series was “CLIMATE CHANGE IN THE BUILT ENVIRONMENT” by Dr. Michael Arnold, Professor of Landscape Horticulture and Director of The Gardens at Texas A&M, College Station. Are you ready? Not a pretty picture.

Regardless of the cause of climate change, the impact on built environments is likely to be strong. Look at the megafires in the western U.S. How to predict, adapt and the horticultural implications are key elements. Climate change is a long-term deviation from typical events. A number of billion-dollar extreme weather events have taken place since 2000. The number of drought years has risen. Records going back to 1901 show eight of the ten warmest years, including night temperatures, have occurred (in the 48 contiguous states) since 1998.

Hardiness Zone maps from 1990 to 2015 show a considerable shifting of warmer zones into colder zones. One third to one half of the country has warmed with only a few stable cold zones in the mountains of the southwest. Dr. Arnold continued:

URBAN HEAT ISLAND (UHI). UHIs are cities with a multitude of impervious surfaces and buildings that absorb heat. Canyon-like streets inhibit air flow. Refrigerator units and air conditioners generate heat. Less vegetation means less evapotranspirational cooling. Increased ozone pollution. Some downtown areas may be 27 degrees warmer compared to rural areas. In 60 of the largest U.S. cities, the mean daytime summer temps were 2.4 degrees higher and 4 degrees higher at night. In Madison, Wisconsin, UHI effect increased the plant growing season by five days. Plants are tricked to leaf out earlier.

Got too cold for the American Holly in February

RISING TEMPERATURE AND INSECT PREDATION. For every 1.8 degrees F the temperature rises, 10-25% grain yield decreases. Insect metabolism rises with the temperature. Predation increases. Warmer winter and drier conditions favor borer damage. Higher temps divert plant resources to cooling responses as they try to conserve water and implement defensive mechanisms. Photosynthesis drops. Plants decline.

TRENDS IN US PRECIPITATION. The southwest U.S. will continue to become drier. Tree rings and crop archaeology of pre-European peoples show the southwestern U.S. had experienced an unusually wet climate the past 100 years. Worrisome. Is this area going back to a drier time or responding to change?

PROJECTED CHANGES IN WATER WITHDRAWALS. In OK, with no climate change, water remains rather unchanged except some variability in the extreme eastern part. With climate change, water availability will decrease 25-50% in most the state and 10-25% in SE OK. Projected soil moisture drops by mid-century and continues downhill with higher carbon emissions. Easy to forget when getting rain, but nine years ago OK was in a severe drought. Ground water depletion and reduced recharge of underground water sources such as the large Ogallala Aquifer will be affected. Water levels in Kansas, Oklahoma, Texas and Nebraska are predicted to drop 20-40 feet or more.

WINTER CHILLING? Plants require a set number of chill hours during winter to leaf out, flower and set fruit, but winter is losing its cool. This affects plant dormancy. The number of chilling hours dropped 50% in California between 1950-2000. The San Fernando Valley is a major U.S. producer of fruits and wines. Prediction is by 2090 there will be no chill hours. Recent example: Thornless locust trees from northern climates were grown in Dallas. After one warm winter all had poor bud break due to inadequate chilling. They were alive but never leafed out. Chilling.

RISING OCEANS. Increase in severity of storms and flooding, as well as frequency and intensity of tropical storms. The number of main storms and those of hurricane level in the Atlantic Basin have increased the last 20-30 years. The sea level is predicted to rise 3 feet by 2100, submerging parts of the Gulf and South Atlantic coast. During the next 600 years, water will rise 18 feet; 90% of Miami, New Orleans and Virginia Beach will be well below sea level.

SEVERE STORMS. Critical coastal areas, dunes, rivers and creek riparian areas which periodically flood must be preserved or restored. A recent storm in Houston dumped 60 inches of rain, their entire year’s average. 40 inches fell in College Station (yearly average 36-38 inches). Enormous structural, economic and landscape impacts. Tornadoes are frequent escorts. Select tress with good scaffold branches. Prune to minimize canopy sail in strong winds and reduce bark inclusions (two branches grow too close together and form a V).

DYNAMICS OF PLANT GEOGRAPHY. Natural movement and migration unlikely to keep up with changing climate, which may also aid movement of invasives. Native ecosystems are weakened and prone to invasion by well-adapted exotic pests. Urban areas may serve as refuges and centers of origin for invasive plants and pests.

Got too hot for beets in July

IF TEMPERATURES ARE RISING, WHAT HAPPENED THIS WINTER? Warmer air holds more moisture but when cooled can produce more frozen precipitation with an increase in severe winter storms. The weakened Polar Vortex allowed Jet Stream arctic air to dip into the mid-latitudes. The Vortex is usually trapped up north in the colder stratosphere, but when the Jet Stream weakens during warming episodes, the heated stratosphere allows break-outs of cold air.

In TX and OK, last winter was one of the coldest winter events on record. Post oaks died in College Station Texas. Why did some winter annuals survive and sub-tropicals return from the roots? Snow cover and Provenance.

Provenance refers to original origin of genetic material of plants. With the right provenance, the plants can better weather the changes. Examples: Possum haw ‘Ilex decidua’ and bald cypress ‘Taxodium distichum’ from central and western Texas provenances were able to withstand a major drought, but the same species from eastern Texas provenances died.

Live oak ‘Quercus virginiana’ from the Texas Hill Country suffered less cold damage, but the coastal forms were hit hard. ‘Washingtonia robusta,’ the fast-growing Mexican Fan Palm, was severely damaged. Most died in the College Station area. Hardier ‘Washingtonia’ hybrids from California growing as far north as Tyler TX showed less damage and recovered.

WHAT TO DO. Embrace diversity in the residential landscape by planting up to 15 or more different plant species.

Discover the provenance of your plants. They could have originated in California. Important when considering hardiness and adaptability in Oklahoma.

Incorporate Low Impact Design features in landscapes: green roofs, living walls, swales, rain gardens.

Recognize and exploit microclimates using warm climate plants.

Accept the dynamic nature of gardens. They change and evolve.

How true.

Becky Emerson Carlberg

Becky Emerson Carlberg, graduate of Oklahoma State (Plant Pathology) is a teacher, artist, writer as well as certified Oklahoma Master Gardener and Master Naturalist. Contact her at