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This may be the watershed moment when everyone involved in choosing plants—from homeowners to maintenance companies to landscape architects—reaches for native plants first. For decades, we have eagerly adopted new imported plants and cultivars that respond well to the Texas urban heat. But in the wake of the longest, coldest freezing event in more than 30 years, we have been temporarily stunned—and left wondering how to react to this rare event. The experience brought some truths into focus.
In the urban landscape, microclimate is everything. Our buildings and infrastructure create small extremes and shelters, where landscape architects and designers have had success using native and exotic species suited to other regions. In San Antonio, for example, Trans-Pecos natives tend to perform well in parking lots, while Lower Rio Grande Valley natives tend to thrive along south-facing walls and in courtyards. So we have nurtured a lot of nearly-native and adaptable plants with little trouble for decades—and favorable microclimates may have saved plants in the recent freeze event. In general, plants showing worse freeze damage are located farther from shelter of buildings and trees. Container plants have a greater risk of being killed because their roots are not insulated by surrounding earth.
Non-native plants suffer the most in temperature extremes. Many consecutive hours of subfreezing weather are a big ask of a plant living outside its ecoregion, as it must adapt to moisture and soil chemistry different than its parents had. The stresses have been most obvious in plants imported from other continents.
Native plants may be better prepared for resilience than others. I am optimistic that native plants will survive this extreme weather, based on the weeks after the freeze. While stresses such as leaf drop and twig dieback are seen in Texas natives, even within their natural habitats, they have centuries of evolution betting on their survival. It is not surprising that in a recent AgriLife Extension webinar, nearly all the species cited as damaged in the freezes were exotics.
The source of cultivated plants could be a factor. Just as the live oaks on the Gulf coast and in the Hill Country have very different climate adaptations, other species that range across the state may show variations in adaptability to extreme cold based upon the source of the original seed or cutting. We typically only scrutinize provenance when restoring wild landscapes, but it could help explain how two specimens weathered differently in similar urban settings.
Better-established plants will be more resilient. A species that would normally survive this deep freeze may not make it if the root system is not well established. Plantings with more consistent moisture and care will probably fare better, as moist soils hold more heat.
In the weeks after the deep freeze event, I made some specific observations in the San Antonio region, where I work and garden, and where the junction of four ecoregions may make those observations relevant to readers around the state.
Most Texas species of Agave have some damage in San Antonio, where none are endemic. Perhaps the greater moisture in our soils did not allow these species to dry out adequately for the winter, leaving their flesh vulnerable to freezing. That damage is showing as bleached or pulpy, deflated-looking flesh—the latter being the result of moist cells freezing and rupturing. A. americana is showing lots of pulpy flesh trailing on the ground, but the youngest leaves at the center are still erect, even if bleached. Even the Texas mountain natives A. lophantha/univittata and the more cold-tolerant A. parryi show some bleached or wrinkled areas, especially on outer leaves. There is potential for healthy, living leaf tissue within the cone-like centers of the plants, but the survivors probably will be disfigured for some time. Several species of prickly pear (Opuntia) show similar deflated-looking damage, regardless of age, care or location. The commonly cultivated “spineless” form, however, seemed to suffer less damage than the others. As with other succulent plants, removal of the pulpy flesh is expected to allow the remaining healthy tissue to grow.
The extreme cold has burned foliage on many normally evergreen species. While you might wonder whether these plants are dead, bear in mind that brown leaves do not signify death in all cases. If the plant still has living tissue, it will shed these leaves and grow new ones. Barbados cherry (Malpighia glabra) is often evergreen in San Antonio, for example, but this year its leaves are browning. Horticulturists suggest we resist the urge to trim until the frosts have passed, allowing time for those new buds to form by midspring.
Mexican olive (Cordia boissieri) is a species that suffered many losses in the freezes of the 1980s outside their native Lower Valley range. At the present, however, a mature specimen grown in the open in San Antonio does not show any split bark. Time is expected to tell whether any branch dieback occurred.
Species showing little to no damage in the San Antonio area include four-nerve daisy (Tetraneuris scaposa), Nolina species (bear grass, devil’s shoestring or sacahuista), dwarf palmetto (Sabal minor), pink skullcap (Scutellaria suffrutescens), Texas mountain laurel, yaupon and native yuccas. Native early-season perennials, such as Engelmann daisy (Engelmannia peristenia) and heartleaf skullcap (Scutellaria ovata), show no damage either.
Damage to normally hardy late-season perennials could be hard to gauge until the normal leafing time. However, some native Salvia species, such as mealy blue sage (S. farinacea), cedar sage (S. roemeriana) and autumn sage (S. greggii) are showing green, leafy rosettes, as is typical this time of year. Native annuals that are sprouting in February seem unaffected by the deep freeze, including many of our most common wildflowers and forbs.
As a designer, I am taking the following lessons from the freeze event. Zone denial comes with risk. In an urban setting, extreme cold is not an every-year event, so you may find it worth your while to pamper and protect plants from outside your region. Take your risks with calculations. Realistically consider the microclimate to be planted, and evaluate the care and requirements of the plants you consider. Natives are tough, but endemics are tougher. Even the agaves native to high deserts were no match for the extreme low temperatures in wetter areas, for example.
It takes great effort sometimes to nurture plants in urban environments. It can be dispiriting—but times like these bring us opportunities to learn even more about our native plants, and to adopt the ones that can flourish alongside us, wherever we are.
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**ARCHIVED POST AUTHOR: mpecen
