Glandularia

These plants, like their relatives the vervains (verbenas), usually have showy five-petalled flowers in shades of pink, purple and blue.

Glandularia species are closely related to the true vervains[1] and sometimes still included with them in Verbena. Horizontal chloroplast transfer occurred at least twice and possibly three times between these genera, which are otherwise too distinct to warrant unification. Somehow, chloroplasts from V. orcuttiana, swamp verbena (V. hastata) or a close relative of these had admixed into the G. bipinnatifida genome. Although hybridization runs rampant in the true and mock vervains – the ancestors of the well-known garden vervain are quite obscure – it does not seem to have been the cause of the cross-species gene transfer.[2]

Research has found a signal of one more transfer event. This had introduced chloroplasts from an ancestral member of the Verbena lineage nowadays found in South America into Glandularia. Although all members of the present genus can be distinguished to have a chromosome count of five, the South American species are diploid, while polyploid hybrid Glandularia are very widespread from northern Central America northwards. The second genetic introgression must have occurred before the genus spread north, as species with the Verbena-like chloroplasts are found all over the Americas. Since the new chloroplast genes replaced the old ones, it may be that the possibly hybridogenic G. bipinnatifida actually underwent horizontal chloroplast transfer twice in its evolutionary history.[2]

A large number of cultivars, in shades of white, pink, blue and purple, have been developed for garden use, and are particularly suitable for containers - window-boxes and hanging baskets. Though tender perennials, they are usually grown in temperate areas as half-hardy annuals (sown under glass), or sold as young plants for planting out after the danger of frost has passed. They are still widely referenced in the British horticulture trade as verbenas. The following have gained the Royal Horticultural Society’s Award of Garden Merit:-[3]

• ’Claret’[4]
• ’Donalena Lavender Grace’ (Donalena Series)[5]
• Quartz Series[6]
• ’Silver Anne’[7]
• ’Sissinghurst’[8]
• ’Toronto’ (G. canadensis)[9]

The following species are recognised by The Plant List:[10]