Blue-purple flowers  Leaves opposite

Houstonia minima

Houstonia purpurea

Houstonia pusilla

Isanthus brachtiatus

Lindernia dubia

Matelea decipiens

Mazus japonicus

Mentha arvensis

Mentha piperita

Mimulus alatus

Mimulus ringens

Paulowinia tomentosa

Penstemon cobaea

Perovskia atriplicifolia

Phlox amplifolia

Phlox bifida

Phlox divaricata

Phlox pilosa

Prunella vulgaris

Ruellia humilis

Ruellia pedunculata

Ruellia strepens

Salvia azurea