The results of a five year study by the Chicago Botanic Garden list nine plants rated "excellent" and 67 rated "good" for use in a green roof environment. Meanwhile, Sasha Aguilera, Design Consultant for Xeroflor Canada, an industry leader in pre-vegetated roofing, has put together some suggestions for green roof maintenance and design.

The Chicago Botanic Garden has released the results of a green roof study to determine which plants are best suited for growing on a botanical roof. Five years of research on the green roof of the Garden's Daniel F. and Ada L. Rice Plant Conservation Science Center has led to the publication of Plant Evaluation Notes, which highlight the best plants for green roofs in Zone 5. The study is the largest of its kind ever in the U.S.

A green roof consists of growing plants that help insulate the building below from temperature extremes in hot or cold weather. The plants also help control the amount of rainwater run-off that occurs from the roof of a building. The plants used are intended to be self-sustaining varieties that require little if any care.

A diverse group of 216 herbaceous and woody plants were evaluated in the extensive (growing depth of 3 to 6 inches) to semi-intensive (growing depth of 6 to 8 inches) green roof garden. Nine plants received five star "excellent" ratings for their overall performance and survivability, including Antennaria dioica, Calamintha nepeta ssp. nepeta, Juniperus chinensis var. sargentii'Viridis', Phlox sublata 'Apple Blossom', Phlox subulata 'Emerald Blue', Phlox subulata 'Snowflake',Rhus aromatica 'Gro-Low', Sporobolus heterolepis, and Sporobolus heterolepis 'Tara'.

Top-rated plants consistently displayed good vigor and robust habits, superior ornamental qualities, disease resistance, heat and drought tolerance, and winter hardiness/ survivability throughout the evaluation period. Additionally, 69 plants received four-star "good" ratings for their strong performances.

"Ultimately, the success of a green roof is due to the success of the plants growing on it," said Richard Hawke, plant evaluation manager at the Chicago Botanic Garden. "Plant trials like the one undertaken here are crucial to increasing the knowledge about the best plants for green roof culture."

The Plant Conservation Science Center is a 38,000-square-foot, LEED gold-rated research laboratory with two 8,000-square-foot gardens, one on the north and and one on the south side of the central clerestory.

The Ellis Goodman Family Foundation Green Roof Garden South features regional and national native plants, many of which are not currently used as rooftop plants, while the Josephine P. & John J. Louis Foundation Green Roof Garden North features a mix of plants known as good green roof plants, plus native and exotic plants that have potential for green roof use.

"The sky's the limit for plants we can grow on green roofs," Hawke said. "We will continue to incorporate more plants into our trials as the Garden further develops its recommended list of best plants for roof gardens."

Maintenance Is Key to Roof Integrity and Longevity

There is a constant need to maintain these roofs so they don't become overgrown and potentially damage the very building they are a part of.  Green roof maintenance doesn't have to be difficult, but a plan must be developed.

Many jurisdictions, such as Toronto, Canada, are now requiring a maintenance plan to be submitted when getting a permit for a green roof.  In Washington, DC, rooftop inspections are required twice a year.  Many green roof systems require ongoing maintenance as a condition to maintain the warranty on their products.

Green roof design and plant species selection should take into account the maintenance requirements, both of the system being used, the plants, and the jurisdiction, if any.  There are many systems available that require very little maintenance and are lightweight, something to be considered when engineering the building's structure.

In the photos below, the green roof system on the left is a technologically advanced system using synthetic retention layers with no "loose" growing medium. It is lower in maintenance and lighter in weight when compared to the photo on the right, a system with 4? of growing medium. The thicker green roof system is not only heavier, it is also overrun by harmful and aggressive trees and requires significantly more maintenance at this point.

Sasha Aguilera, Design Consultant for Xeroflor Canada, an industry leader in pre-vegetated roofing, has put together these suggestions for green roof maintenance and design:

Plant Selection

Locally grown hardy and drought tolerant plants such as Sedum and mosses are ideal. Maintenance personnel should be familiar with green roof plants and the owner's green roof aesthetic preference as some "weeds" may or may not be tolerated.

Irrigation Plan

Access to water, with adequate pressure, close by the rooftop is critical for supplemental irrigation in the establishment phase and during periods of drought.

Fertilizing Plan

An annual application of slow release fertilizer, usually in spring, helps feed plants nutrients over a 3-9 month period. Care must be taken to avoid applying nutrients on plants preparing to go into dormancy in the fall season.

Weed Control

Weeding is necessary on all green roofs. Without proper maintenance, invasive weeds can overtake a green roof and pose a threat to the original green roof design. Woody plants can potentially harm the waterproofing membrane. A thin-layered system with synthetic water retention layers tends to be less inviting for harmful weeds than systems with thick growing medium. These thin-layered systems therefore require less routine, manual weeding.

Debris Removal and Drain Inspection

Inspection of the drainage paths is very important. Blocked drains and pooling water can cause root rot, and green roof plants may drown.

Document Your Visits

Once a properly installed extensive green roof is well established, its maintenance requirements are usually minimal. However, there should still be a person or team responsible for maintaining the system, performing visits 3 or 4 times a year. If the property does not have the personnel to do this, it is recommended to hire a professional green roof maintenance contractor with horticultural knowledge and training in working at height. A report should be written at each visit and recent weather conditions should be recorded. Photos are a must for future reference and any potential warranty claims.

Source and Photo: PR Newswire, CNW Group/Xeroflor Canada Inc

First half of piece by Glenn Meyers. Second half by Dawn Killough