Green Roof Thrive Rate
Thrive rate is a measure of the relative health of plants. Our stewardship team scores each living roof’s thrive rate on each visit. This page illustrates and describes thrive rates to clarify terms and set expectations.
Green Roof Thrive Rate: Healthy
Healthy green roof plants are always our goal. Indicators of health are vigorous roots, new growth in the appropriate season, absence of pests, tissue turgidity, appropriate color, and attractive foliage. Thrive rate is a different metric than coverage, as high-coverage green roofs are not necessarily healthy, and low-coverage green roofs are not necessarily unhealthy.
Healthy Green Roof Vegetation: A very healthy crop of Sedum reflexum ‘Blue Spruce’ and Sedum spurium, with Sedum sexangulare and Phedimus kamtschiaticum in the background.
Healthy Green Roof Vegetation: A very healthy green roof with approximately 85% coverage, but thriving plants.
Healthy Green Roof Vegetation: A very healthy green roof with small, young plants emerging from cuttings and seed. These plants are small but thriving.
Healthy Green Roof Vegetation: A very healthy mature green roof with large mounds of Phedimus kamtschiaticum.
Healthy Green Roof Vegetation: A very healthy green roof in mid- to late summer. During this time, growth slows, fewer new buds are visible, and colors might not be as vibrant; however, this is normal within a healthy green roof plant’s life cycle.
Healthy Green Roof Vegetation: A very healthy and very young green roof. Plants are young, but growing vigorously.
Green Roof Thrive Rate: Stressed
Rooftops are often stressful environments, and plants chosen for extensive green roofs should be able to tolerate that stress. Sedums are commonly used on extensive green roofs as they are adapted to growing in stressful environments (heat, drough, wind, reflected light, low organic matter, low nutrient). Sedums usually handle this stress with ease, though they might show their stress by wilting, self-pruning, or losing some color. This stress should not cause alarm, as it is a normal part of the plant’s life cycle; in fact, attempts to reduce stress (such as adding irrigation or fertilizer) may destabilize the green roof by encouraging weeds. We expect most green roofs to exhibit stress during late summer drought, and then bounce back quickly with some relief from heat and drought.
Stressed Green Roof Vegetation: This photo illustrates the difference between stressed green roof plants (foreground) and healthy green roof plants (background). There are a few healthy plants in the foreground, but most are undersized and of inappropriate color for the season. Stress such as this can be caused by differences in environmental factors (heat, water, light, drainage) or by human-induced trauma (window washing chemicals, staging materials on the roof, etc.).
Stressed Green Roof Vegetation: The Sedums in this photo are exhibiting some normal, temporary stress perhaps due to a dry spring, or lack of available nutrient. Notice that the Alliums are not showing much or any sign of stress.
Stressed Green Roof Vegetation: The green roof in this photo exhibits late-summer, drought-induced stress, combined with low coverage due to recent removal of a large area of weeds. This green roof can turn a corner by keeping weed pressure low while the Sedums bounce back during fall.
Green Roof Thrive Rate: Declining
Vegetative decline is not common in properly designed green roofs, and we do not have many examples of declining green roofs. Decline differs from stress in that plants normally bounce back from stress, and decline usually warrants replacement. Decline may be caused by pests such as grubs, or environmental factors such as excessive reflected light. Our goal is to identify and prevent potential decline before it happens, and if it does happen, propose more resilient solutions.
Declining Green Roof Vegetation: In this example, only the left portion of the photo exhibits decline, as plants have died. In this case, decline is due to excessive reflected light and head from the adjacent mechanical area. Correction will likely require a change of species and potentially some light mitigation.
Declining Green Roof Vegetation: In this example, grubs have eaten the fleshy roots of the Phedimus kamtschiaticum, but the grubs have not been attracted to the narrowleaf Sedums with finer roots. The grubs require treatment, and the kamtschiaticum will need to be replaced, preferably with multiple species for greater resilience.
Declining Green Roof Vegetation: Closeup of Phedimus kamtschiaticum, mortally wounded by root-eating grubs.
Green Roof Thrive Rate: Dead
A dead green roof is highly uncommon, but we include this as it is a score within our rating system. In fact, we only have one example. In the example below, the green roof died due to a combination of inappropriate media and plant selection. We hope never to see another dead green roof again.
Dead Green Roof Vegetation: This green roof was installed using an inappropriate extensive media, and using almost all native plants that preferred a higher organic media. Unfortunately, few plants survived.
Dead Green Roof Vegetation: Another view of the same rooftop before removal and replacement.