Watering your sedum roof in summer

The wonderful thing about a sedum roof is that it’s rather good at coping with dry weather.  So far this year, I doubt that many (properly installed) sedum roofs are particularly stressed with the drought.  But just in case there are parts of the UK that are not as chilly and damp as Norfolk is today, here are some tips for watering your sedum roof should the weather change.

No need to irrigate unless your green roof looks stressed

Even when growing on a shallow substrate, sedums can happily go for up to four weeks without any rain.  However, if your roof is exposed to windy conditions or if the plants are already stressed (maybe they’ve not yet had their annual feed) it will dry out quicker.

The symptoms of sedum drought stress

In uber-dry conditions, sedum plants will activate their internal drought coping mechanisms.  Their metabolism shifts slightly so that they use even less water than usual.  What we see, is that the lovely green foliage changes colour.  The green will turn to red.

This sedum green roof is well hydrated and blooming it’s socks off.  It’s a delight for the eyes and clearly doesn’t need watering.

If your sedum roof gradually becomes a red-brown colour in a dry summer, that’s normal.  Check though that the leaves are still plump and fleshy. 

Leaves shrivel.  If the sedum plants on your green roof are in mortal danger, you will see it in their leaves.   Check once a week in dry weather to make sure the leaves look like beads and not like deflated balloons.   If those leaves are wrinkled and saggy the plants need water – and fast!

Spaces between plants.   How dense was the vegetative layer before drought set in.   If the plants were already spaced far apart, the chances are drought isn’t causing too much of a problem.  If however the plants were growing close together and forming a dense vegetative layer – but they’re not any more – you may have a problem.

These sedum plants, although red leaved and sparse are fairly well hydrated.  The leaves are still quite fleshy.  However, they do look as though they’d benefit from an application of sedum feed.

Plants on a sedum roof should be numerous and luscious enough to most of the substrate.  In other words you shouldn’t be able to see much of the growing medium.   If they’re not – then it’s time for some maintenance.   Food and water are essential in these circumstances.

Getting it right with water

The whole ethos of green roofing is to support the environment – not to be a drain on it.   Heavy irrigation on a green roof, particularly if water is in short supply, is not ideal.  That’s why drought tolerant sedums are such a good choice for green roofing.

Having said that, drought tolerant is not drought proof.  Sedums do need water from time to time.  But not too much.

If your sedum roof is definitely showing signs of stress, give it a good drink.  Water it to the point of runoff.  Do this every other day for a week or so.  Then leave it alone.

If you can, water in the evening.  That way water will be able to soak into the substrate before it gets evaporated by the heat of the sun.   A built in irrigation system using leaky pipes is far more water-efficient than spray from a hosepipe.  But don’t leave it switched on all night.  Keep an eye on it and turn it off as soon as the substrate is saturated.  Don’t waste water.  If the hose is your only option, use a spray attachment and hold it as close to the plants as possible – around 15cm above them is ideal.  No waving it around and sending water droplets up into the atmosphere.  100% of that water needs to go where the plants can benefit from it.

Avoid frequent watering.  Actually don’t just avoid it.  Don’t do it.  If your sedum roof receives lots and lots of water, the sedums will hate having wet feet.  Unwanted plants however will relish the damp conditions.  You may well find your roof gets invaded by grasses, broad leaved weeds and sometimes even tree seedlings.

To summarize

Your sedum roof only needs watering if it is

a)      Newly installed and not quite established

b)      Showing tangible signs of drought stress  or

c)       Has just been fed and the fertiliser needs to be watered in