Does your green roof need to be irrigated this summer?  Here are three things to consider…….

1; How deep is your substrate?

Substrate is the name given to specially engineered green roof growing medium.  It’s not soil, it’s not compost, and it’s not sand.  It is unique to green roofing and is a blend of fine aggregates, organic matter and porous particles.  Substrate is designed to be light weight (compared to topsoil), well drained, but at the same time, water retentive. 

The deeper your substrate layer, the more rainwater it will be able to absorb and retain.

There’s an interesting study here on water retention, slopes and vegetation types.

If you have a shallow layer of substrate on your roof you may need to water it in summer – it all depends on what sort of plants you’re trying to grow, how your roof is structured and the weather conditions where you are.

2: How steep are your slopes?

A gentle slope will allow excess water to trickle off the roof after heavy rainfall but will retain up to 80% of the water that falls on it.  A steep slope will only soak up a limited amount of water…..particularly if it’s exposed to drying winds and high temperatures.

A steeply sloping living roof like this one will need TLC in a dry summer – water it at least once a week and feed every 6-8 weeks to keep the plants looking as good as they do in this picture

Sloping roofs will need irrigation during summer months.  If you’ve not got a built in irrigation system – get ready with the hosepipe!

3: What plants are living on your roof?

It’s important to identify the plants living on your green roof – especially if you were not involved in the designing or planting of the roof.

When you know what’s up there, you’ll know how to look after it.

Sedum plants are commonly used on a green roof.  They have fleshy leaves – some look a bit like jelly beans – with a waxy feel to them.  The plants are typically low growing and have star-shaped flowers in the summer time.  They keep their leaves in winter but you might find that the foliage turns browny-red when the plants are stressed by drought or cold.

This honey bee is loving the wide open star-shaped flowers on this sedum plant.  Notice the fleshy leaves – they have a waxy coating on them that helps to prevent water loss and wilting

Sedums are relatively drought tolerant and only need irrigation if they show signs of extreme stress.  If the leaves are small, red and bean-like the plants are OK, they’re employing their built-in moisture conserving systems.  If, however, sedum leaves are looking baggy and wrinkled, like deflated balloons, those plants need water – and fast!

Grasses are remarkably resilient too.  If you have grass on your green roof and don’t want it there, drought is your best friend.  You can weed them out whilst the foliage is brown and dead. That way the roots won’t regenerate.

Wild flowers are not normally ever so drought tolerant. But they are great for supporting biodiversity. They need to be planted into at least 100mm of substrate if they are to have the slightest chance of survival and they will need irrigating.  A leaky pipe type of irrigation system is good for these….water goes straight into the substrate and the dense covering of plants helps to avoid evaporation. 

Fruit and vegetables – need a lot of water wherever they are growing.

Green roof irrigation vs sustainability

Here’s a contentious subject.  A green roof is often used to make a building more sustainable.  More energy efficient, more biodiverse, better at cooling the environment, better at managing rainfall.

Sedum foliage turns a lovely red colour in winter time and during long periods of drought.  If the leaves look healthy and turgid, don’t worry.  If they look like deflated balloons – the plants need water.

How then does irrigation fit in with sustainability?

It’s a matter for your own conscious I’m afraid.  Me? I’m a Libra which means I’m hard-wired to find solutions that suit (almost) everyone. 

If I were designing a green roof from scratch, I’d make the building strong enough to carry the load of at least 100mm of substrate. 

I’d make the roof relatively flat – maybe have a slope of around 2 degrees so that it drains adequately but retains most of the rainwater that falls on it.

Planting – I’d vary the planting.   Sedum matting would give me a dense covering of plants to reduce evapotranspiration but I’d add biodiversity with drought tolerant alpine plants and tough wildflowers such as birdsfoot trefoil, thrift, dianthus, wild thyme and other heath-dwellers.  In time, the plants would find their own balance.

These measures should reduce the need for irrigation, but I’d have a system in place (ideally one that uses water harvested from the roof during heavy rainfall) just in case we ever got a long hot summer with no rainfall. 

Not sure what those measures would do to the budget though – and if the estimated costs were too high, the roof would never be greened – which is the least sustainable option of all in my opinion.

Guidelines for watering your green roof

If the weather is wet and dreary – do nothing.  The plants will be OK

If the weather is warm and dry for one week 

Don’t worry, if your living roof was fed in spring the plants will be strong enough and healthy enough to go for one week without water

Two weeks without rai

A sedum roof will still be A-OK.  Do nothing. 

A wildflower roof might be getting stressed.  Look for soft, wilting leaves and drooping flower heads.  If things are looking floppy up there, give it a good watering and check on it the next day.

Three weeks without rain

Wildflowers will definitely need watering.  Try to run the irrigation system at night to avoid evaporation. 

Sedums should be checked.  If the foliage looks red and the leaves have shrunk – this is normal.  Do nothing.  If the foliage looks like saggy sacks – switch on the irrigation and soak the substrate.  Check in a day or two and repeat if there’s no improvement in condition

Four weeks or more without rain 

Water everything!!  Don’t water unnecessarily.  If the plants are looking well, leave them a day or two longer, there’s no need to waste this precious resource.

More information on green roof care and maintenance

Controlling weeds on a green roof

Green roof maintenance tips

Green roof troubleshooting