This section is from the book "Nature's Garden", by Neltje Blanchan. Also available from Amazon: Nature's Garden; An Aid To Knowledge Of Our Wild Flowers And Their Insect Visitors.
Flowers - Very small, bluish purple, clustered in axils of upper leaves. Calyx tubular, unequally 5-cleft; teeth of upper lip triangular, hairy in throat. Corolla 2-lipped, upper lip erect, notched; lower one 3-cleft, spreading; 2 anther-bearing stamens under upper lip; 2 sterile but apparent; 1 pistil with 2-cleft style. Stem: Low, erect, branched, square, hairy, 6 to 18 in. high. Leaves: Small, opposite, ovate to oblong, scantily toothed, strongly aromatic, pungent.
Preferred Habitat - Dry fields, open woodland.
Flowering Season - July - September.
Distribution - Cape Breton Island westward to Nebraska, south to Florida.
However insignificant its flower, this common little plant unmistakably proclaims its presence throughout the neighborhood. So powerful is the pungent aroma of its leaves that dog doctors sprinkle them about freely in the kennels to kill fleas, a pest by no means exterminated in Southern Europe, however, where the true pennyroyal of commerce (Mentha Pulegium) is native. Herb gatherers who collect our pennyroyal, that is so similar to the European species it is similarly employed in medicine, say they can scent it from a greater distance than any other plant.
Bastard Pennyroyal, which, like the Self-heal, is sometimes called Blue Curls (Trichostema dichofomum), chooses dry fields, but preferably sandy ones, where we find its abundant, tiny blue flowers, that later change to purple, from July to October. Its balsam-like odor is not agreeable, neither has the plant beauty to recommend it; yet where it grows, from Maine to Florida, and west to Texas, it is likely to be so common we cannot well pass it unnoticed. The low, stiff, slender, much-branched, and rather clammy stem bears opposite, oblong, smooth-edged leaves narrowed into petioles. One, two, or three flowers, borne at the tips of the branches, soon fall off, leaving the 5-cleft calyx to cradle four exposed nutlets.
From the five-lobed tubular corolla protrude four very long, curling, blue or violet stamens - hair stamens the Greek generic title signifies - and the pretty popular name of blue curls also has reference to these conspicuous filaments that are spirally coiled in the bud.
In general habit like the two preceding plants, the False Pennyroyal (Isanthus brachiatus) nevertheless prefers that its sandy home should be near streams. From Quebec to Georgia, westward to Minnesota and Texas, it blooms in midsummer, lifting its small, tubular, pale-blue flowers from the axils of pointed, opposite leaves. An unusual characteristic in one of the mint tribe is that e five sharp lobes of its bell-shaped calyx, and the five rounded, preading lobes of the corolla, are of equal length, hence its Greek lame signifying an equal flower.