Plant Selection Principles
Cause of Rooftop Greening Failure (2)
— Are Plants Being Selected Based on Appearance or Catalog Descriptions?
Should plants be viewed in terms of “what is being used,”
or in terms of “how they will behave in a given environment”?
This difference in perspective does not become apparent immediately after installation.
Rather, it emerges clearly in the results seen several years later.
In rooftop greening, plant selection influences outcomes more decisively than design or cost.
What ultimately determines success is the plant’s ability to adapt to the environment.
When Appearance and Catalog Information Take Priority
In actual practice, however, plants are often selected based on factors such as:
・Visual impression
・Descriptions in catalogs
・The number of past installations or adoption records
As a result, plant selection is frequently driven by appearance or precedent rather than by environmental suitability.
Rooftops are environments where multiple severe conditions overlap simultaneously, including:
・Strong winds
・High temperatures
・Dry conditions
・Rapid temperature fluctuations
・Limited available soil volume
In such environments, what plants truly require is physiological adaptability—
including heat tolerance, drought resistance, wind tolerance, and root growth characteristics.
Popularity and Documentation Do Not Guarantee Suitability
Reasons such as:
・“This plant is commonly used,”
・“There are many construction records,”
do not, in themselves, directly guarantee that a plant can adapt to rooftop conditions.
Similarly, the existence of well-developed CAD drawings or standard details for rooftop greening systems is undoubtedly beneficial for efficient design and construction.
However, such documentation does not ensure healthy plant growth or long-term stability.
Ease of drafting and convenience in design practice, on the one hand, and
the viability of plants in harsh rooftop environments, on the other,
are factors that should be evaluated on entirely different levels.
When Evaluation Criteria Are Confused
When these two dimensions are conflated during planning, projects may appear orderly immediately after installation.
Yet several years later, problems surface in forms such as:
・“The greenery does not spread,”
・“Ground coverage does not increase.”
In rooftop greening, success is not defined solely by plants surviving.
Even if plants remain alive, if ground coverage does not improve and base materials or fixing components remain exposed, the objectives of greening have not been achieved.
Plant Selection as a Long-Term Judgment
Plants do not function independently.
They cannot be evaluated in isolation from the system design or operational assumptions.
Plant selection is not simply an act of deciding which plant to use.
It is the act of determining whether a plant can remain viable over the long term within the rooftop environment.
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