Why sedum plants are so drought tolerant

One of the reasons that sedums are used for green roofing is that they are incredibly drought tolerant.   A sedum roof will need the bare minimum of irrigation and even then, only in extremely dry conditions.

Sedum plants have evolved to live in exposed conditions where soil is very well drained and sun and wind dry everything out very quickly and so, over the centuries, they have developed some inbuilt coping mechanisms.

sedum leaves

  1.  They store water in their leaves and stems.  One of the characteristics of sedum plants is that they have really fleshy leaves – as do their close relations the aloe vera plant and the pineapple. The plant cells are like little tiny water bombs, stowing H2O to use when rainfall and ground water are in short supply.
  2. They have a waxy covering on the leaves – a bit like a sealant to try to stop water evaporating from the surface of the leaf and to protect it from wind damage.
  3. They are CAM plants.  CAM is a super special mechanism unique to this plant family that helps prevent water loss and it’s really rather clever:

 CAM and Photosynthesis

All green plants use a process called photosynthesis to harness energy from the sun and turn it into carbohydrates that they can use for life processes.   Photosynthesis involves the plant absorbing carbon dioxide through little holes in its leaves called stomata.  Sunlight then powers a chemical reaction to combine carbon dioxide with water to make sugar.  The waste product is oxygen – which we can breathe in.

Photosynthesis can only happen during the day because it needs sunlight.  But, if a plant opens its stomata during the day it risks losing water through evaporation.  That’s why so many non-CAM plants wilt on hot days.

Sedum plants have managed to overcome this problem by keeping their stomata tightly closed during the day and only opening them at night time when there is a reduced risk of losing water.  The carbon dioxide they need to photosynthesise the next day is drawn in and then, because it’s not easy to store a gas, the CO2 is converted into a liquid called malic acid.  Next day, malic acid is converted back into CO2 and used to produce sugar.

Sedums, and other plants from the Crassulaceae family are the only plants to use CAM or Crassulacean Acidic Metabolism and that’s why they have superior drought tolerance.   Incidentally, it also makes the leaves taste AWFUL.  The Roman name for Sedum Acre is “wall pepper” because it will happily grow on walls where there is no soil and it tastes very bitter indeed.

sedum green roof


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drought tolerant sedum plants are ideal for a green roof