For anyone considering a green roof, choosing the right plants is the essential decision. It’s typically believed that a green roofing environment carries a resemblance to a delectable garden. The reality is quite different – tough plants, that establish themselves fast and are long-lived, are a building block of a green roof.
Another shared misconception is that native plants are best suited for green roofs thereby serving as a part of landscape restoration. While theoretically this may make ecological sense to recapture the native grassland that was destroyed by the urban development, the truth is that the rooftop microcosm presents a demanding niche which makes this impossible. Natural conditions such as wind and heat are much more harsh on a rooftop – a reality that must be taken into account.
Anyone deciding on the green roof plants, needs to consider the following list of qualities that green roof plants must possess.
- have shallow lateral root system that binds the soil together
- rooting system must persists all year long
- be able to suck out water from the soil to the atmosphere
- be able to survive longer periods when the soil is dry
- have enough foliage surface area to shade the soil and prevent the germination of weeds
- be long lived, preventing the need for expensive and wasteful replacement
- provide wildlife habitat and food for pollinators
- have ease of establishment
- have modest nutrient and maintenance requirements
- have resistance to damage from insects, diseases
- have absence of windborne seeds
- be of light weight at maturity level
Evidently, this is a demanding list of requirements. So, which super-plants posses these qualities?
These qualities are most easily found in hardy succulents such as Sedums (album, kamtschaticum, sexangulare and others). The reason for the use of Sedums is their resilience and the ability to withstand a wide range of conditions relative to other plants. Sedums have the ability to modify their metabolism during droughts and modify it again during moisture-rich conditions. This survival skill will help Sedums to survive when most other plants die.
In some cases, even the appropriateness of Sedums on a green roofing can vary dramatically across regions. It has been recorded that Sedum acre performs great in the Chicago area, but poorly in the St. Louis area.
Other suitable plants are Sempervivum (Houseleek), Jovibarba (Hen and Chicks), Talinum (Fameflower) and Delosperma (Ice plant). All these successfully conserve and store water in their leaves. Opuntia (Nopales or Paddle Cactus), also makes a good green roof plant.
Annuals and perennials
Annuals, perennials, grasses, and bulbs can be used too on a green roof. Annuals are planted as seeds to add a quick color to a green roof and fill in empty spots keeping the weed pressure down. Perennials and grasses add color and texture in protected areas on roofs that are primarily planted with succulents. Bulbs, too, may bring a hint of color during spring.
Here is a list of small bulbs that have performed well on green roofs:
- Allium neapolitanum (Naples garlic, native to southern Europe, north Africa, and the Middle East)
- Allium oreophilum (Ornamental garlic with pink and purple flowers that give punch of color early summer)
- Crocus chrysanthus (Snow crocus, native to Turkey with yellow flowers)
- Crocus tommasinianus (Woodland crocus, native to Europe with lilac flowers that appear in early spring)
- Iris humilis (Iris, with yellow flowers, native to E. Europe and Russia)
- Iris reticulata (Iris, native to Russia with early flowering blue flowers)
- Muscari comosum (Grape hyacinths, native to Eurasia with bluish-violet flowers)
- Narcissus willkommii (Daffodils, native to Europe)
- Scilla bifolia (Alpine squill, native to Europe)
- Scilla siberica (Siberian squill, native to Russia)
- Tulipa humilis (Tulip, native to Eurasia)
Variety is the key
It is important to avoid planting mono-crop planting schemes. Climate conditions and pests that are lethal to specific species target a monoculture green roof and may wipe out every plant. It has been shown that mono-crop planting schemes that thrive for many years, suddenly succumb to catastrophic elements, so it’s best to avoid them altogether. Instead it is smart to use assorted plants consisting of at least six or seven different species. When choosing plants the preference should be given to plants with varying leaf structure. These produce a variety of textures and achieve greater resilience as they metabolize water differently under varying conditions.
- The Green Roof Manual
- Planting Green Roofs and Living Walls
- Roof Gardens: History, Design, Construction
- Green Roof Construction and Maintenance