The East London Garden Society

Watering Roof Gardens

Water is the single most important thing in a garden and getting water to plants in a roof garden presents its own challenges.

A roof garden is basically a container garden. Plants in the garden have the luxury of spreading their roots deep into the ground where the soil stays wet and the sun doesn't bake the moisture away. Plants in containers simply can’t do this so you have to take greater care to keep them well watered.

In a container there is a lower proportion of soil to roots. If you were to look at the container of a plant which has been established for many years you’d see that the majority of the container is filled with the white roots with soil packed in between. It’s the soil which holds the moisture in and so the less soil you have the less ability you have to hold moisture.

There are special composts available which retain moisture better and although they do work well in containers they are no substitute for keeping the plant regularly watered.

If you look at a dried out container plant, even after a few days of not being watered, you’ll see that the soil has become parched. Not only is the soil dry but if it’s dried out too much then it will also have lost the ability to hold water when you do water it. If you pour water onto the top you’ll see it run out the bottom pretty quickly. The obvious answer is not to let the plant get this dry in the first place but if you have done so then you need to get a container larger than the pot, fill it with water and literally soak the whole container in water for an hour or two until the soil is absolutely sodden.

But how do you stop the plant getting into that state in the first place? One way is to use an automatic irrigation system. Each plant has an individual dripper and it runs twice a day, giving the plant just enough water to keep it going but not enough that water is wasted by pouring through the container and running away. You can get smaller systems for balconies and even for houseplants but such a system is essential for a roof garden. Even with the best will in the world, you’ll find there are times when you can’t water the garden enough by hand. Bear in mind that with a watering can and hose you need to visit every container and not just soak the ground like you would in a ground‐level garden.

There are other things you can do to help. Try to avoid ground cover plants. A plant getting its leaves wet does nothing to help the roots, and the more of the soil in the container which is covered, the less the rainwater is going to hit the soil and keep it well watered. Also pick plants carefully. There are drought‐tolerant plants available so even if things do go awry you can always rescue them with some heavy watering after the event.

Casualties do happen due to lack of water despite the best efforts of the watering system to keep up but don’t let this put you off roof gardens though. The watering issues can be overcome but you need to be mindful of it. Of course, there are other things you can do with water in a roof garden such as water features which aren’t as hard as you’d think.

By Dan McNeil