There are a myriad of different plants that are suitable for a green roof, no matter how extensive your roof may be. But will they be happy with your green roofing conditions? Or will they eventually need to be replaced at great expense?
As well as offering amazing deals on green roof feed, fertiliser and turf, Turf Online has created the ultimate guide on how to pick the right plants for your sedum green roof. This article will cover every aspect to consider when deciding, including shade coverage, maintenance requirements and, most importantly, colour!
Green Roof Plants That Save You Money
If you choose the right plants for your green roof it will save you a whole heap of money. If the plants are happy with the micro-climate on your green roof and how you maintain them they should be virtually self-sustaining. Not only will you not need to keep re-seeding, but there’ll be no re-planting either.
What to Look For When Searching For the Right Green Roof Plant
Depth of Growing Medium
Plants need a strong growing medium to anchor their roots, to provide water and nutrients. Thus, green roofs must have 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. Sedums are very shallow-rooted and are content to live in 2cm on the substrate. By contrast, wildflowers and grasses need at least 10cm.
Why does this matter? Because the substrate will add extra weight to the roof. The deeper the growing medium, the heavier it will be. If your roof is not 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. Not only will it be warmer but it will also be more exposed to drying winds. So 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 therefore make a great choice for roof plants. As do most hardy alpines such as meadow saxifrage and some Mediterranean herbs such as thyme that will withstand frost. If there are overhanging deciduous trees, this is not to worry, but 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. They’ll also need to be drought-tolerant and frost hardy plants.
If you like gardening, have plenty of time and can access the roof safely and easily then maintenance should be no problem for you. On the other hand, if 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 trimming, pruning, or 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 of any 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? All of these questions should be considered when choosing your plant.
Once you have determined which species will actually survive on your roof, then you can look at the colour pallet. The fun stuff!