Green Roofs and CAM Plants - Frost or Freeze Damage

We examined the damage frost can do to the mesophyll cells of C3 plants. Mesophyll cells are those cells across the surface of the leaf. C3 plants usually possess lots of stomata and C3 photosynthesis is a quick and rapid process, allowing C3 plants to grow rapidly, filling in a green roof.

In C4 and CAM plants photosynthesis occurs in a different manner with different cells. Today we will briefly examine one aspect of CAM plants and look at damage incurred by freezing temperatures.

CAM stands for Crassulacean acid metabolism. CAM plants generally have stomata adapted to open at night and not during the day, preserving water during hot periods or drought. Many plants living in arid ecosystems are CAM plants and can be recognized by smaller leaves with a higher leaf volume ratio (fat, fleshy leaves where water is stored in vacuoles), waxy leaf surfaces, sunken stomata areas.

Examples of CAM plants include;

• Pineapple
• Jade Plant
• Euphorbias
• Sansevierias
• Aloes and others

Green Roof CAM Plant, Graptopetalum spp.

 The photo to the left is of a Mexican Ghost plant, many of us call a sedum, however it is really a not really a sedum but a Graptopetalum.

For an informative article with many reference links about CAM and Graptopetalum, see the blog post linked here.

Many succulents such as the Mexican Ghost plant and others can endure a little frost, though some of the more tropical varieties are tender to any type of frost damage.

However unlike C3 and C4 plants, many CAM plants store water in cells called vacuoles.  I call vacuoles 'smart blobs'.  Vacuoles are storage cells used for a number of plant physiological purposes.  In CAM plants vacuoles are used to store water and other photosynthesis related substances like the carbon dioxide.

Green Roof Plant, CAM Type After Hard Dreeze, Graptopetalum spp.

As mentioned, most CAM plants have thicker leaves - because of the presence of the stored water.  We have collected data over the years showing how the Mexican Ghost plant and other similar succulents survive minor frosts yet because of the expansion of water as it freezes, their cells rupture and the plant usually dies during a hard freeze.

Water stored in the vacuoles and other parts of the succulent's leaves expands as the temperature drops into the lower 20'S F (-6 to -8C).  As the stored fluid freezes and expands in size the vacuole wall and other cells rupture, injuring or killing the plants.

Though this phenomena can be observed across the green roof, it is especially prevalent in plants along the edges of the green roof.

CAM type succulents planted along green roof edges are exposed to unbroken cold, dry winds.  However within the interior of the green roof plantings we observe less freezing damage, probably due to stored heat in the planting media and the break from constant winds.

What lessons can we learn from the Mexican Ghost plant and freezing temperatures?  Many, including plant placement on a green roof and inter-relations between C3, C4 and CAM type plants.   We will discuss some of the hardier CAM plants in a future blog.

As always, feel free to email us with questions or comments & Happy Green Roofing!

Kevin