December 2009 — 'Ask the Expert' Print

Q:  Green Roofs are new and becoming more popular and Sedum are touted as ideal plants for Extensive Green Roof systems.  What is the difference between Extensive and Intensive Green Roof systems?

A: Green Roofs are actually not a new phenomenon. Plants have been grown on rooftops since prehistoric times, but the modern incarnation really began to take off in Europe after in the early 20th century.  Most of the research that we rely upon was conducted and published in the German language.  Consequently when the terminology is translated it often seems counterintuitive to native English speakers.

Basically an Extensive system has less than 6” (15cm) of growing medium, while an Intensive system has at least a foot (30cm) and usually considerably more, with 4’ to 6’ (120cm-2m) not being uncommon.
 
Intensive systems typically require more intensive planting and maintenance. They are often more of a garden on a roof.

Extensive systems have a limited pallet of plants that are appropriate for the shallow medium depth and are more often used on roofs that are not necessarily accessible on a regular basis but require coverage to mitigate heat, contain storm water and protect roofing membranes.


Q:  Just who are these people who dictate these changes to the ‘proper’ name for ornamental plants?  Do they take into account the confusion to the public and the expense incurred with tags, signage and catalogues?

A: The International Code of Botanical Nomenclature (ICBN) is the set of rules that has been agreed upon by botanists and taxonomists to govern the classification and naming of plants.  For an international identification system to work there can only be one valid name for any plant that is recognized and respected worldwide.  Since Linneaus published the Species Plantarum in 1753 the ICBN has been developed on a system of priority and descriptions referenced to a representative specimen plant.  It is this code that controls our present binomial system of a Genus and specific epithet (species) – italicized and in upper and lower case as appropriate — with cultivar names enclosed in single quotes.  It is also this system that taxonomists (concerned with classification, organization and naming of organisms) use to place plants into their proper and logical place in the hierarchical structure of Kingdom, Division, Class, Order, Family, Genus, Species.  As plants are studied in greater detail, with bigger and better microscopic techniques and DNA analysis, previously overlooked relationships and differences are becoming apparent.  To maintain the integrity of the system (i.e. correct identification) sometimes changes are necessary.  But no changes can be made on an individual basis or seemingly on the whim of a single scientist.

All changes to the ICBN must be suggested and approved by the International Botanical Congress that meets every 6 years in different locations around the world and is attended by leading botanists and taxonomists from fully accredited scientific institutions and botanic gardens. Sixty percent of those attending must agree to any suggestions, change is not taken lightly.  But realistically the economic interests and implications of decisions are not high on the priority lists when determining the correct relationships of plants.  Ensuring that the name and relationship is correct are the criteria for change.  Granted clarity to the masses on the front line is often lacking. 

Change doesn’t come swiftly as it is not unusual for a decision to take 20-30 years to filter down to the trade and thence to the public (see December Plant of the Month – Sedum/Hylotelephium).  Despite all of our complaints about ‘lumpers’ and ‘splitters’ it is in the best interest of the horticulture trade as a whole for the botanical nomenclature system to be respected and upheld.  Clarity and secure identification through the ICBN are important aspects of our industry.
 

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