In New Jersey there are four main species: American (L. americanus), sweet (L. viringicus), European (L. europaeus), and northern (L. unifllorus).
American and European have leaves with lobes, rather than just teeth, especially the lower leaves of the plants. They also have sepals (calyx lobes) with long points, that extend well beyond the fruit.
American is generally mostly or entirely smooth on the stems and upper surfaces of the leaves. European always has hairy stems and generally hairy on the upper surface of leaves.
American flowers have petals twice as long as the green sepals. European has petals only a little longer than the sepals.
European will grow in waste places away from water, American (and sweet and northern) needs to be near water.
Sweet and northern have leaves with teeth and no lobes. Often the leaves are purple.Their sepals are triangular with no long points. They do not extend much beyond the top of the fruit.
Sweet has leaves that narrow abruptly to a long, winged stem. Northern leaves taper gradually.
Sweet teeth are smaller and there are more of them on a leaf than in northern.
Sweet fruit (and American and European) are indented at the top. Northern are rounded on top.
Sweet is hairy on the stems and somewhat hairy on leaves. Northern is mostly smooth.
There is one more Lycopus sp., only in southern NJ (barrens and Cape May): clasping water-horehound, L. amplectans. It has leaves that are very sessile, somewhat clasping the stem, and has few teeth along the edges of the leaves. Otherwise they are similar to sweet and northern.
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