When Dry Times Come to a Green Roof

Apr 14, 2023

Dry times do come to a green roof. In spite of the lack of rain what is one to do or not to do? The first reaction, when a green roof loses it’s green colour is to think water. Out in the natural world, browning of vegetation, is quite normal. So why do react negatively to a green roof that has turned.

During long drought 2011 - Dry tiimes

Chives flourishing during long spring drought 2011

Dry Times can bring surprises

In the first place a drought can lead to fears that one’s green roof is going to die. Notwithstanding this natural fear, a drought can actually have very positive results. The picture above shows chives flourishing during the long spring drought in the UK back in 2011.

As a result of the same drought, bee orchids appear on the green roof on John’s house. For twenty years the seeds must have lain in the soil waiting for the right conditions. Three months of drought broke up the soil. Under these circumstances, we believe, the bee orchid seeds were able to germinate the next year.

And a number of commercial green roofs in London, are taking this advice. Reducing irrigation to zero has helped the roofs develop into really interesting meadows.

Installing Irrigation on a Small Green Roof

Subsurface irrigation in action on a green roof

Irrigation is an option not a necessity in dry times

In the first place we always recommend that irrigation is used as a last resort. Lack of water can have a positive effect on the green roof, in that it can remove unwanted vegetation. A good drought can kill of grasses that may well not be wanted. Furthermore a drought can have the effect of toughening the plants up. Those that return when the rain comes will be more conditioned to drought than those that don’t make it. Drought is a form of plant stamina training.

If in the event you just can’t resist using water:

  • wait as long as you can before switching it on
  • do it over night
  • and use as little water as possible

This will also be stamina training light.

Installing Irrigation on a Small Green Roof

The guide provides details of how to install irrigation within a green roof. Although we recommend NOT irrigating as standard we do describe how to install a system if you want to. In the end we just don’t know what the extremes of weather will bring. Excessive long droughts may well call for some watering.

If it is installed, then it can be used. Surface watering of a green roof is wasteful. It’s the roots that need the water.

The rule of thumb should be – resist irrigating but if you have to ensure that it is subsurface.