Potted succulents in the Charlotte Brody Discovery Garden, by George Oberlander.
All plants need water to live and grow—the exact timing and amount varies from species to species, but there’s no getting around it in the long run. In addition to its other benefits, water is an essential component of photosynthesis, the internal chemical process that plants use to make their own food from sunlight. By using solar energy to rearrange the atoms in water and carbon dioxide to produce sugars, oxygen is produced as a byproduct, which is released through stomata, which are specialized pores in the leaves that can open and close freely.
However, opening the stomata comes with a cost—water is also lost via evaporation every time. On hot days, this can quickly add up, rapidly desiccating the plant even as the released water vapor cools the surrounding atmosphere in a natural form of air-conditioning. Given a choice between death by dehydration and death by starvation, what’s a root-bound organism to do?
As a result of this dilemma, plants exposed to intense summer heat have evolved one or more strategies to mitigate their water loss. These include:
- Grow a spine
Since the amount of water loss depends on the surface area of the leaf, one easy solution is to reduce the size of the leaf as much as possible. This is one reason why plants native to warmer and wetter climates, such as elephant ears (Colocasia spp.) tend to have wide, broad leaves, while plants native to harsher, drier climates tend to have a smaller leaves. The logical endpoint of this trend are the spines on a cactus, which are actually modified leaves with a tiny surface area compared to a standard leaf—all the better to prevent water loss in a desert. While cacti spines still retain some of their original vascular tissue, the job of photosynthesis is turned over to their bright green stems. As a bonus, they also help protect the plant from being consumed.
- Invest in storage
Many plants native to desert regions have specialized structures to store water in their tissues, and are collectively known as succulents. While cacti are some of the best-known succulents, this group includes a wide range of plants around the world, from the thick, brown, tree-like trunks of desert rose (Adenium obesum) and elephant’s foot (Pachypodium spp.) in the milkweed family, to the mucilaginous leaves of aloe vera and haworthias in the asphodel family. Because what little precipitation that falls in desert regions tends to come in erratic bursts, water storage allows plants to grab what they can when it is abundant in order to survive the dry spells.
- Protective coatings and coloration
The color white reflects light, while the color black absorbs it, meaning that a concrete sidewalk will be several degrees cooler than the asphalt street beside it. As a result, plants in arid regions tend to be pale or silvery in color, usually coupled with thick hairs or waxy coatings to help prevent water loss. Examples of plants that have adopted this strategy include dusty miller, fuzzy lamb’s ear, and palmetto.
- Take the summer off
Mediterranean climates—which include parts of California, South Africa, and South America, as well as its namesake Mediterranean region of Europe and northern Africa—are characterized by wet, mild winters and long, hot summers with little or no rain. As a result, plants native to these regions have adapted a seasonal lifestyle, blooming and growing during the brief wet spells before going dormant for the duration of the dry spells. Plants that grow from bulbs, such as crocuses, daffodils, and tulips, go completely underground, dying back in late spring and returning the following year.
North American native buckeyes (Aesculus spp.) are one of the first trees to leaf out in the spring, and will drop their leaves as early as July or August if there is drought rather than tough it out until fall.
- Work at night
To avoid losing water during photosynthesis, certain plants have developed an alternate method, where they gather sunlight during the day and wait until after dark to open their stomata and finish the job. Known as crassulacean acid metabolism because it was discovered in the stonecrop family (Crassulaceae), it is also used by orchids, cacti, agave, and marsh plants like arrowhead (Sagittaria spp.) and bulrushes (Scirpus spp.).
As you explore Duke Gardens, check out the different plants you encounter and see what their strategies are! Good places to look for heat-adapted plants include the scree beds in the Terrace Gardens, the Mary Duke Biddle Rose Garden, the path between the President’s Bridge and the south entrance to the Blomquist Garden, and the potted succulents in the Charlotte Brody Discovery Garden.