What is the proper abbreviation for subspecies according to ICBN? Is it 'subsp.' or 'ssp.'? Is one preferable to the other if both are acceptable?
I believe both are acceptable according to the ICBN, but subsp. is used in all of the examples throughout the text, so that is what ought to be followed. We use subsp. on our labels at the garden and throughout the website, although our accessioning system currently uses ssp.
ssp. is preferable over subsp. by merit of practical economy of the abbreviation using fewer characters.
I was curious as well so here is an update. The official recommendation continues to be "subsp." as per the newer (2011) ICN, or International Code of Nomenclature for algae, fungi, and plants, but some online databases use ssp. (e.g. USDA, ITIS), while others use subsp. (e.g. Tropicos, GCC). http://www.iapt-taxon.org/nomen/main.php?page=art5&emph=subsp