Horticultural Questions

How does green roof media differ from soil?

Charlie says: Unlike organic soil you might use in your garden, conventional green roof media is mostly light-weight mineral material paired with a minimum of organic material. It should be designed to meet established FLL guidelines for both water retention and drainage, as green roof systems with poor water retention will require irrigation to compensate.

What kinds of plants should be used on a green roof?

Charlie says: In frost zones 4 through 8, at least half of the plants installed on an extensive green roof should be varieties of Sedum, a type of succulent. In colder climates grass-dominated covers are recommended. Reliable tropical plant lists are not currently available. However, in cooperation with the Florida Department of Environmental Protection, we have installed three green roof prototypes to evaluate candidate plants for tropical extensive systems.

Many other plants can be incorporated to provide habitat value or visual interest. Appropriate auxiliary plants will vary by region, but generally, the deeper the green roof, the more complex the plant community that can be sustained. They range from Sedum and herbs on the thinnest systems to meadow grasses and perennials on mid-weight roofs, to turf and small trees on deep intensive roofs.

We have used hundreds of different species in American projects. The experience gained from these projects allows us to confidently recommend plant communities for green roofs. Their selections will depend on many factors: depth of growing media, seasonal temperature ranges, average rainfall, irrigation strategies, and your aesthetic preferences.

For optimal plant growth and health, the plants should be selected, installed and maintained by experienced horticulturists or landscape contractors who understand the local environment and climate, in consultation with the design engineer. Fortunately, our national Roofmeadow® Network stands ready to provide the regional prowess needed for green roof success.

Are any green roof plants invasive or spread by wind?

Charlie says: The only Sedum that might be considered invasive is Sedum sarmentosum, also known as Star Sedum, Gold Moss, Stringy Stonecrop or Graveyard Moss. It has green spearhead-shaped leaves, lemon yellow flowers and long tendrils. We do not recommend using this species. Several native and non-native grasses and perennials typically used on green roofs will re-seed and spread by wind ,which may be desirable, depending on the design asthetic.

How are green roof plants started?

Charlie says: There are several methods for starting green roof plantings, and all methods require temporary irrigation unless planting occurs in early spring or mid to late fall.

Sedums: Sedums may be established from plug or container size plants, cuttings, or pre-grown Sedum mats. Sedums cuttings may be started in May through June or late September through the end of October without the use of irrigation. Broadcast on the growing media, these cuttings will quickly form roots and establish themselves. When using Sedum cuttings to establish a green roof, a permanent, bio or photodegradable wind netting or hydromulch must be used to prevent the cuttings from blowing off the roof before they have established. Sedum plants also may be installed as pre-grown Sedum mats that are delivered to the job site like rolls of turf sod and ideally installed in the spring and fall. Pre-grown Sedum mats serve to stabilize the entire green roof system against wind scour, precluding the need for a bio- or photodegradable wind netting or hydromulch. Plug or container-sized Sedums may be installed anytime during the growing season and may also require a wind stabilization blanket depending on exposure of the green roof.

Perennials and Grasses: Perennials and grasses may be installed anytime during the growing season as plug or container-sized plants.When establishing green roofs with perennials or grasses, a permanent irrigation system may be required, depending on the plant species and the media depth. All perennials and grasses will need at least some temporary irrigation to establish on the green roof. Perennials and grasses also may be seeded into the green roof anytime except during the summer months.

How long until the roof will be fully covered?

Charlie says: Generally, a green roof that is planted correctly will reach maturity in two growing seasons, or less. Occasionally, reduced sun or rain may slow the process. During maturation, it is especially important that the roof be properly maintained with weeding and fertilization based on yearly media testing, and possibly temporary irrigation.

Is there a danger of dry plants catching and spreading fire?

Charlie says: Not if the green roof is designed correctly. In Germany, green roofs have a better fire rating than conventional roofs because the mineral media layers cannot burn. The extensive use of Sedums, gravel borders and breaks also protect against fire. In an arid, fire-prone region, require your designer to engineer the system with fire-retardant features, paying special attention to mineral content, gravel layers and plant selection.

What effects will the winter freeze and thaw cycles have on the roof?

Charlie says: While not all generic green roofs are resistant to freeze and thaw cycles, freezing and thawing has no effects on our Roofmeadow® green roof systems. We use mineral-based media that stand up well against multiple freeze/thaw cycles, as well as engineered infrastructure elements to accommodate thermal contraction and expansion.

Is there erosion in the winter when the vegetation dies back?

Charlie says: The design of a green roof system should include a significant percentage of semi-evergreen or evergreen plants, so that the mature cover is never bare of foliage; minimizing the potential for erosion and damage. The risk for erosion is greatest just after the initial planting. During this early phase, the media should be anchored with erosion control netting.

Can I plant a flower or vegetable garden on my roof?

Charlie says: Generally, green roof systems are not deep enough for growing vegetables, and the tools of traditional gardening would damage the green roof and the underlying waterproofing. However, incorporating container gardens or modular elements can accommodate an owner’s horticultural or agricultural desires.

Will I be able to walk on my green roof? Can it be used for recreation?

Charlie says: If the roof is accessible from the building and the roof will support the weight, decks, patios and walkway pavers can be integrated into the design of the green roof. Among the available plant covers, only turf grass will stand up to regular foot traffic. Because of their intensive needs, turf green roofs will require a minimum of 6-9 inches of media depth and permanent irrigation is mandatory. Alternatively, stepping stones or walkways can be used to provide access through more fragile extensive green roofs.

What kind of maintenance is required?

Charlie says: Generally, after the plants have become established, most extensive green roofs only need weeding and occasional infill transplanting twice a year. The green roof should be fertilized only as needed. These maintenance visits also are a good time to inspect the drains, edging, and any other visible roof elements. Maintenance visits should be timed to intercept any weeds before they go to seed. This level of maintenance is sufficient to maintain the health of the plants and protect the underlying roofing materials. Some owners enjoy greater involvement and choose to ‘garden’ in green roof more frequently. Intensive green roofs will required the same care and maintenance as a similar garden situation on grade.