Plants With Built-In Sunblock

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Several local plants are uncommonly adept at blocking sunlight and reducing water loss during summer months.

Summer is a great time for outdoor activities. As humans we know we need to protect our skin with sunscreen, but did you know plants also use sunblock?

No, they don’t apply lotion to one another. But many have adapted to naturally block sunlight and reduce water-loss during summer months. In fact, several local plants have adapted to the powerful sun by covering themselves in fine tiny hairs, more specifically trichomes.

Velvet mallow (Allowissadula holosericea) is one such plant. The leaves have a similar shape to Turk’s cap, but with a different texture and color. Fine hairs give the leaves a beautiful silvery cast and when you run your hand over them you’ll find them soft as velvet. The flowers that appear in summer are a bright orange-yellow. Velvet mallow grows in shade or sun and is often seen in the rocky soils of north Bexar County, but there are some outliers growing at Medina River Natural Area on the city’s South Side.

Senna's yellow flowers stand out on Texas roadsides late in summer.Velvet-leaf senna (Senna lindheimeriana) also has velvety, hair-covered leaves. Growing in the same sunny, rocky conditions, this member of the pea family has evolved a similar strategy to beat the heat. They bloom about the same time as velvet mallow and even have a similar color. The little beans remain on the plant and are an important winter food source for birds.

White prairie artemisia (Artemisia ludoviciana) gets its white/silvery color from its hairs. These hairs don’t produce a velvety texture because they don’t stand on end. Instead they lie flat along the leaf surface, what botanists call appressed. But the hairs are just as good at scattering light and that’s why our eyes perceive the leaves as white. This plant can be rather aggressive, but it’s a good filler for sunny areas that don’t get much traffic.

It might not seem like a lot, but the fine covering of hairs on these plants reflect some of the sun’s energy and keeps the temperature inside the plant’s leaves lower. A lower leaf temperature is important for plants because they rely on the passive exchange of gasses for photosynthesis. The hotter the leaf the more water lost from transpiration while they are breathing in CO2. Plants of the same species will be more or less hairy depending how much sun they receive.

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