Most of the Willowherbs hybridize freely with one another. The above is a chart of all the known hybrids of the Willowherbs with other Willowherbs. Note how Bronzy Willowherb has no known hybrids, and that Rosebay Willowherb does not appear in the chart, for it has no hybrids either and although it is a Willowherb, it is not an Epilobium. Rockery Willowherb has only one hybrid, that with Broad-Leaved Willowherb, whereas Broad-Leaved Willowherb has many hybrids. Short-Fruited Willowherb has the most known hybrids, totalling 10 (out of a possible 13). The empty dark-blue squares show that there is plenty of opportunity for much more promiscuity between willowherbs.

N.B. Due to the symmetrical nature of the chart, being folded about the black diagonal, each hybrid appears twice.

All Willowgerbs have long seed pods, which when ripe split open like a banana to reveal hundreds of seeds with long feathery plumes, ready to be borne on the wind to set seed in far-off places. a single plant can produce up to 20,000 seeds. Little wonder then that once a willowherb has a foot-hold in a garden, it is almost impossible to eradicate, for the plants grow faster than gardeners on a fortnights holiday can control. The smaller varieties have set seed before the gardener is even aware of their presence.

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