Subject: Distinguishing Peppermint and Spearmint

I am wondering how I can tell Mentha X piperita piperita (peppermint) from Mentha spicata (spearmint). What are the differences in foliage and flower in order to identify one from the other?

If you have both plants a sniff test is by far the easiest. Peppermint has a much stronger menthol smell when you crush a leaf than spearmint. Spearmint is usually sweeter smelling but can have a hint of pungent menthol (the amount varies from variety to variety, especially if the plants were seed grown) that is missing from peppermint. Spearmint has somewhat hairy leaves while peppermint is more shiny and has darker petioles (leaf stems). But Austrian mint, a different species with the spearmint flavour also has dark petioles. Since peppermint is thought to be a hybrid of spearmint and watermint it is not surprising that it resembles spearmint. Both have their flowers in bottle-brush shaped inflorescences held on top of the plant and both have purple flowers. Spearmint flowers tend to be a bit darker in colour than peppermint and the flower stems often branch in spearmint. You will never find seeds in peppermint, since it is a sterile hybrid.
[The following was added by the editor]
For botanical descriptions of spearmint and peppermint, see the respective entries in Britton and Brown's flora:
www.swsbm.com/Britton-Brown/Mentha_spicata.gif
www.swsbm.com/Britton-Brown/Mentha_X_piperita.gif
One clear cut difference between the species is the length of the petiole, or the tiny leaf stem that joins the leaf to the stem. In spearment, the petiole is either absent (so the leaf is almost directly attached to the stem) or very short. In peppermint, the petiole is longer, usually about 1/4 inch (6 mm) or longer.