As gardeners we are already aware of how amazing plants are. I mean they give us life sustaining oxygen – nothing more even needs to be said. Beyond this though their ability to survive in harsh conditions without the means to get up and move mean that they have developed mechanisms of survival through adaptation that could fill numerous numbers of pages.
A lot of these traits go unseen but I am asked a lot about one particular stress adaptation on a regular basis – plants, especially succulents, turning various shades of red. Have you ever stopped to wonder why a plant is doing this? The science behind it is pretty amazing. (Calm down, I am not going to get too technical and wordy)
We all know plants photosynthesize using chlorophyll (green pigments). In addition to chlorophyll, photosynthesis requires light and water. Water is a limited resource a lot of times for succulent plants and on the other hand light is too plentiful. The solution – mask your chlorophyll or even break it down to prevent water loss from photosynthesis. The pigment which becomes dominate is anthocyanin. This is the red pigment in almost all the red fruits and vegetables you eat high in antioxidants. So why the red pigment and not say carotene (orange pigment)? Red is the hottest wave length in the light spectrum. When we see red we are in fact seeing the red light being reflected off of the plant and not absorbed, therefore the plant is staying even cooler.
During the summer this is evident on a lot of our succulents. Many cacti and succulent growers who enter plant contests encourage this because it is pretty darn beautiful having red hues as well as the green ones.
Once temperatures cool and rain returns the plants will revert back slowly. Red leaves however can happen during the winter months as well. Cold is also a stress and succulents do much better in the frost and cold with high moisture content in their leaves therefore preventing water loss is also important. This is actually why the east coast has better fall color. The sudden change from warm humid conditions to cold stresses the leaves resulting in quick breakdown of chlorophyll. This results in beautiful fall colors!!
An example I love to use in the greenhouse to show students heat adaptation is an epiphytic cactus from Brazil, Rhipsalis pachyptera. When in its ideal habitat of a humid, shady forest the cactus has wide, dark green stems. We have the same species growing in our desert room in low humidity, high temperature, and light. It is growing but not happily and the difference in appearance is phenomenal. The desert growing one is turning dark red with twisted stems trying to self-shade. The shaded parts of the stem are more green while the exposed parts are red. The shade growing one needs as much surface area and chlorophyll as it can get to capture light which is a limited resource in the tropics.
Not all succulents will do this so before you withhold water trying to force your plants into a rainbow of red make sure you know its growth habit. Some succulents when stressed will brown and look more crispy. They can look like they have literally curled up and died (but do not worry they most likely just look that way). Others will just endure summer with a bleached appearance. Examples of some that will put on a show for you are: Many Echeverias including Echeveria ‘Big Red’, E. runyonii, Aloe striata, Aloe microstigma, A. buhrii, Graptopetalum paraguayense, Kalanchoes, Haworthia comptoniana (grown in sunny window), many Sedums, and some Euphorbias such as E. tirucalli ‘Rosea’. So really why is it some plants look phenomenal and extra gorgeous when stressed but when I am stressed I have no such luck!!!