Green Roofs Suffering From Drought Stress

I never thought that I would be writing this blogpost so early in the UK in April.  British weather has always been reliable for providing plenty of free water during the first four months of the year.  In fact we’re known for our rain sodden springs in which plants thrive. 

This year however is different.  Even though it’s raining outside at the moment, it’s only for about the third time this year. I’ve been watering vegetable plants for a month now and going out and about, I can see that the drought has really affected some of the sedum roofs in the neighbourhood.

What should a sedum roof look like in spring?

During the winter, most sedum roofs change their appearance.  It’s a metabolic thing.  The sedum plants have less light to give them energy, the ambient temperature drops and there are no insects around to pollinate their flowers so they go into a semi-dormant state. 

The leaves become smaller and look like tiny hard beads.  More often than not the foliage will change colour.  It will no longer be green or grey-green, instead it takes on a red-green hue – sometimes turning to a cherry red, sometimes almost brown. 

green roof showing red colour

Typical colour of a green roof under environmental stress - this colour foliage is normal in winter time but during warmer weather it indicates that the plants are coping with stress.  Look for signs of dehydration (wrinkled leaves, dry growing medium) especially on steeply sloping roofs.

healthy sedum green roof in spring with well hydrated plants

Sedum blanket in mid spring.  Plants are growing well, growing medium cannot be seen and foliage has nearly all turned from red to green.

Come spring, warm rains, sunshine and the promise of a visiting bumble bee or two will encourage the plants to wake up.  Gradually, the red colour subsides and greenness returns.  Leaves swell up.  The plants increase in height and breadth and you will no longer be able to see the growing medium beneath them. Flower buds start to appear in late spring. The whole roof radiates health and vitality and is a celebration of spring.

What are the signs of drought in a sedum roof?

If the plants are going to be affected by drought, it will normally strike in the middle of summer.   Sedum plants are incredibly drought tolerant due to a special metabolism called CAM.  However, no living thing is drought proof.  These amazing little plants will perish if they go without water for more that 3-4 weeks.  In the UK, it’s unusual to have a long period of drought and so sedum roofs rarely die from lack of water.  

Signs of drought in sedum roofs include

  • Unseasonally red coloured foliage (between march and November, sedum roofs should be mainly green in colour)
  • Small plants – well hydrated foliage appears verdant, lush and springy.  Dehydrated plants are small, tight and solid.
  • Wrinkled leaves.  Sedum plants sometimes look like deflated balloons when they’re suffering from drought.  This tends to be more noticeable in summer when the plants are full-sized and suddenly lose a lot of water.
  • Lack of flower buds – depending on the sedum species present, you should see signs of flowering by mid-may onwards.  If the plants are still hugging the roof and refusing to push up flowering spikes, they are probably lacking in either food or water.
  • Visible growing medium – it’s not unusual to be able to see small areas of growing medium between the plants in winter time.  As spring time progresses, the substrate should be hidden beneath the foliage
  • Large bare patches – If you have several large bare patches of roof – bigger than an A4 piece of paper your roof could be suffering from severe drought but is likely to have other underlying problems such as lack of nutrients.

dry, hungry sedum roof

This sedum roof is unseasonally red in colour, lacks flowers (the picture was taken in June) and has a lot of bare patches.  It's dehydrated and undernourished.

What should you do if your sedum roof is suffering?

If you think your green roof is suffering from drought stress and there are no decent rain showers forecast, then water it.

Turn the hosepipe on it, or switch on the irrigation system and keep watering until you see water running off into the drainage systems.

This water will be used up quite quickly if the plants are dehydrated.  Water every other day for a week (every day if your roof is on a steep pitch) until you see the plants start to perk up. 

After a week of irrigation, the growing medium on your green roof should be nice and moist – if that’s the case you’ll notice that water runs off very soon after the hosepipe is turned on.  Now is the time to feed your green roof if you haven’t already done so.

Feed your green roof whilst the growing medium is moist

As sedum plants start to recover from drought they take on board a lot of nutrition.  Just like people – when we’ve been under the weather, we need to be extra vigilant about eating nutritious food to help our bodies recover.

If you haven’t already fed your sedum roof, now is the time to do so.  (Do not feed a sedum roof between October and March – the plants may become too lush to withstand winter weather)

Simply apply a specially formulated sedum food such as Enviromat Natural Green Roof Feed exactly as per the manufacturers’ instructions.

If it’s a granular feed, it must be watered in.

Irrigating your green roof during dry weather

There’s a fine line between too little water and too much.  For a sedum green roof, it’s much better to be slightly dry than sodden.  Sedums hate having wet feet and creating perpetually moist growing conditions will encourage weeds to grow.

Once you can see that your sedum roof is no longer suffering from drought stress, it’s OK to stop irrigating for a while.  Hopefully, Mother Nature will provide some rain to support your plants.  If it doesn’t rain for two weeks – then give your sedum roof another good soaking. (on a steeply pitched roof, you may need to water weekly).  Just once mind you.  Don’t water unless the plants are really struggling.

 

Related articles

 

What makes sedum plants drought tolerant?

Find out about the unique metabolism of these special little plants More>

 

Common green roof problems and how to solve them

Sparse foliage, bare patches or unwanted plants? Read more in this article from June 2016

 

Order green roof feed online

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