There are a myriad of different plants that are suitable for a green roof, but which ones are they and will they be happy on YOUR green roof? Or will they eventually need to be replaced at great expense?
How to choose plants for a green roof and save yourself money
Choosing the right plants for your green roof will ultimately save you a whole heap of money. If the plants are happy with the micro-climate on your green roof and with the maintenance regime you can reasonably manage, they should be virtually self-sustaining so that you won’t need to keep re-seeding or re-planting at great expense.
Factors affecting plant choice
Depth of growing medium.
Plants need growing medium to anchor their roots, to provide water and to provide nutrients (and this must be a specially engineered green roof substrate, not ordinary topsoil or compost). Depending on the root system and the plants’ nutritional requirements, some species will need a deeper substrate layer than others. For example, sedums are very shallow rooted and are content to live in 2cm on substrate. Wild flowers and grasses need at least 10cm.
Why does this matter? The substrate will add extra weight to the roof. The deeper the growing medium, the heavier it will be. If your roof is not super-sturdy you may need to limit your plant choice to sedums or sempervivums. If the building will take a great weight, you have a greater range of plants to choose from.
Sunlight or shade?
If your roof is shaded by trees or by other buildings you will need to choose shade tolerant plants such as woodland bulbs (bluebells, aconites etc), woodland grasses, red or white dead nettle and primrose. Shade tolerant plants are most likely to flower in spring and tend to offer foliage rather than colour for most of the year.
On a sunny roof, bear in mind that the temperature on the roof is likely to be at least a couple of degrees higher than the temperature on the ground; It will also be more exposed to drying winds, and therefore the plants on a sun baked green roof either need to be very drought tolerant, or they will need an irrigation system.
Sedums are adapted to living in dry conditions where the soil is thin or poor and so they make a great choice for roof plants. As do most hardy alpines such as meadow saxifrage and some of Mediterranean herbs such as thyme that will withstand frost.
NB if there are overhanging deciduous trees, be prepared to clear away fallen leaves in the autumn.
Irrigation on the roof.
If you are wanting soft-leaved plants that are prone to wilting in hot dry conditions, make sure you have irrigation available, especially if you have cut it fine on the substrate. Irrigation needn’t be a super-sophisticated system. It could just be yourself with a hosepipe….but be sure at the planning stage that you will have the time and the energy to water your roof should you need to.
The higher the roof, the more exposed it will be to the weather. If you have a tall building in a windy spot, choose plants that are low-growing (so they don’t get buffeted and scorched by the wind) and drought tolerant. They’ll also need to be frost hardy.
If you like gardening, have plenty of time and can access the roof safely and easily then maintenance should be no problem to you. If, on the other hand it’s not practical for you, or a contractor, to be on the roof more than once or twice a year, choose plants that don’t need strimming, pruning, vast quantities of feeding, weeding or general care. Remember though, that any green roof will need maintenance at least once a year. That’s a feed, weed and clear out drainage outlets.
How soon do you want your green roof to be entirely green? Can you wait for seeds or plug plants to establish or do you want the instant coverage of a sedum mat? Is your substrate layer deep enough to plant into? Do you have irrigation to keep seeds moist until they get a good start
Once you have determined which species will actually survive on your roof, then you can look at the colour pallet.