Have you noticed that in high summer, when the plants in your veg plots and herbaceous border are wilting, cacti, sedums and other succulent plants stay perkier for longer.  That’s because these types of plants have evolved some very special features that make them drought tolerant.

Sedums make fabulous drought tolerant ground cover and stay green when traditional lawns are brown and dry

Drought tolerance is an important feature for plants on a green roof.  Living at heights means the plants are susceptible to drying winds.  Plus of course, they’re in a limited depth of growing medium.  For them it’s like growing in a shallow plant pot – and we all know how quickly the patio pots need watering.  Only on an extensive green roof, there isn’t always an irrigation system to boost water levels in the “soil”.

Fleshy leaves store water

The first design feature of succulent plants is that their leaves and stems are like water balloons.  The cells are full of stored liquid that can be accessed when the roots are unable to get water from the soil.  If you cut open a sedum, a sempervivum or a cactus it will ooze sap.  That doesn’t happen when you slice into a lettuce does it?

The fat fleshy leaves on sedum plants act as water storage vessels

Waxy coverings keep water in

Now take another look at the leaves of a succulent plant.  A good one to examine is that excellent houseplant, the mother-in-law’s tongue.  It’s got big leaves so you can easily see and feel them.  Every leaf – in fact every part of the plant – is covered with a thick, waxy cuticle.  It’s as though the leaf is wearing latex gloves. 

That cuticle virtually seals the outside of the plant and stops water from escaping from the inside of the plant. 

Compare sedum leaves with the leaves of something like a carrot or a petunia.  The plants without a waxy cuticle are softer and warmer to the touch.  The leaves are more flexible and when the soil is dry, the plant becomes floppy.  That’s because water has leached out from the surface of the leaves and the plant is dehydrated.

I’m notorious for killing houseplants with neglect.  This mother-in-law’s tongue is a CAM plant.  It has fleshy leaves covered in a waxy cuticle and a special biological adaption that makes it drought tolerant.  These plants can even survive my lacksadaisical approach to watering plants. 

Some plants have other ideas about conserving water.  Stachys (lambs’ ears) has lots of little hairs on the leaf to trap water close to the plant’s surface.  Grass has thin leaves with a relatively small surface area-volume ratio.  But none work so well as that layer of waxy stuff on the outside of a sedum leaf.

Clever metabolism avoids water loss

This is the really clever thing about sedums and other plants from the crassulacae plant family.  It’s called Crassulacean Acidic Metabolism – or CAM.

Every green plant uses a process called photosynthesis to make food from carbon dioxide and sunlight.  In simple terms, it mixes the two substances together and uses energy from the sun to “cook” them.  The molecules fuse to form a simple sugar with oxygen as the by-product.

Photosynthesis cannot happen without sunlight.

On the surface of every single leaf are tiny little holes called stomata.  The plant can open and close its stomata to let carbon dioxide in and allow oxygen to escape. 

The trouble with that system is that on hot days, water vapour sneaks out of the stomata at the same time as the oxygen.  That’s OK if the plant can easily get more water through its roots.  If it can’t then it will probably wilt and maybe even die.

Sedum plants have devised a sneaky little system to avoid this happening.  They only open their stomata at night – when it’s not as hot.

“Ahhh” I hear you say “but how do they photosynthesise in the sun when they can’t draw in any CO2?”

This is the clever bit.  At night, when the stomata are open, the sedums will pull in as much CO2 as they possibly can.  Rather than fill themselves with gas (CO2 is a gas not a liquid), they turn the carbon dioxide into Malic Acid and store it in the leaves.  When the sun comes out, they turn malic acid back into CO2 as and when they need it.  Hence the term Crassulacean Acidic Metabolism. CAM

This is also why sedums taste so bitter.  The Romans used to call Sedum Acre “Rock Pepper” because it sits on rocks and tastes peppery.