Peter Dowdall: Green roofs can boost biodiversity and help flood alleviation

Nature isn’t something that exists outside, beyond the confines of our offices or homes. We are part of nature — some might say we are the main feature of the tapestry, though I’m not so sure. What I do know is that the tapestry weaves all around us and of all the benefits and solutions that plants and gardens offer us, uppermost on the list at the moment must be their ability to mitigate the effects of excessive rainfall.

The towering office blocks and apartment complexes which have popped up and are continuing to be built all around our towns and cities don’t exist in isolation to nature. Urban zones and “natural areas” don’t exist independently of each other and in fact, many of these concrete, steel and glass structures can offer solutions when we allow them to work with plants and not do what we are doing now, which seems to be totally isolating them from plants and greenery.

Green walls, also known as living walls or vertical gardens, consist of plants growing on vertical surfaces, using specially designed systems. These structures provide numerous environmental benefits, particularly in dealing with excessive rain as they have built-in reservoirs which store rainwater which the plants can then use over time, significantly reducing the volume of water and the speed at which the rainwater enters the drainage system, thus decreasing the risk of flash flooding and easing the pressure on the drainage system.

For green walls to be effective and for them to thrive, it is important to select the right plants that will do well in sometimes harsh conditions and which can tolerate moisture fluctuations.

Fern, ivy, hosta, some types of sedum and some ornamental grasses make good choices, but as with choosing plants for any site, you first must consider environmental factors such as the aspect, light levels, exposure to wind and accessibility for maintenance.

We are seeing more and more rooftops in Ireland each year, particularly in urban areas, these too can play a critical role in improving our urban landscapes. Apart altogether from lessening the effects of excessive rainfall, a roof garden can provide green space to apartment dwellers and office workers, along with providing insulation in the winter and a cooling effect in the summer. Along with also enhancing biodiversity in, otherwise, challenged urban environments, green roofs play a significant part in urban flood alleviation.

There are many types of roof gardens but they can essentially be broken down into two types, extensive green roofs, which are lightweight, low-maintenance systems and intensive green roofs which are more substantial, garden-like environments with a deeper soil layer and a variety of plants, including shrubs and small trees.

Which one you create depends on what you want and also the engineering of the roof. Along with providing habitats for birds, bees, and insects, and enhancing urban biodiversity, green roofs and roof gardens absorb and retain rainwater, reducing the volume of runoff that enters drainage systems. Plants and soil also act as natural filters, removing pollutants from rainwater before it reaches the ground.

A study at Michigan State University found that green roofs can retain up to 75% of rainwater during rainstorms. The water is stored by the roof’s substrate and then taken up by the plants, from which it is returned to the atmosphere through transpiration and evaporation and the US Environmental Protection Agency reports that green roofs can reduce stormwater runoff by 60% to 100%, depending on the design and local climate conditions.

The best plants to use in extensive green roof systems include sedums and other alpine plants along with drought-resistant grasses. It may seem strange to be recommending drought-resistant plants to help with too much rain but that is because when it is not raining heavily, these rooftops can be very challenging environments.

The sun heats the roof and the wind can be extreme on some rooftops. If the plants are growing on an extensive green roof system, they may only have 10cm-20cm of soil or substrate in which to grow, so choosing plants that can cope with these harsh conditions is essential.

Intensive green roofs on the other hand are what we refer to as roof gardens and though the conditions in terms of exposure to sun and wind are just the same, plants do have a much greater depth of soil in which to grow. Depending on the strength and layout of the roof you can create a real garden up high, complete with hedges and walls to define areas, in much the same way as you would when gardening on the ground.

Once again, plants and gardens offer the solution and whilst in general, letting nature do her thing is the answer, in this instance, perhaps we need to help her to do her thing for our protection in our towns and cities.

• Got a gardening question for Peter Dowdall? Email gardenquestions@examiner.ie