Square stems and opposite leaves are defining characteristics of plants in the mint family, Lamiaceae or Labiatae, which includes all mints, almost all culinary herbs, many medicinal herbs, fragrant plants including lavender and lemon balm, ornamental salvias and types of weeds. Many plants in the mint family have high levels of aromatic oils, responsible for their flavour, aroma or other active properties. Flowers are often rich nectar sources that attract hummingbirds, butterflies and bees. Various mint family members naturally produce purple flowers, and others have been bred as purple-flowering ornamentals.
This aromatic herb was long considered medicinal, used until the 20th century as a cure for countless disorders and as a strewing herb to deodorise and disinfect dwellings. Once modern medicine devised new cures, lavender was valued for its scent alone -- the basis of a worldwide perfume industry. There are white- and pink-flowered lavenders, but most have bloom colours that range from deep purple to -- of course -- lavender. Colourful cultivars among the fragrant English lavenders (Lavandula angustifolia) include dark purple Hidcote, light purple Munstead and deep lavender-violet Goodwin Creek. Spanish lavenders (Lavandula stoechas) include dark purple Wings of Night, purple Madrid and Fairy Wings. Lavandins (Lavandula intermedia) include purple Richard Grey and dark purple Grosso and Dutch Mill.
Summer-blooming bee balm (Monarda spp.) attracts bees and other pollinators nonstop. Like true mints, it prefers moist soil and will tolerate shade, though bee balm doesn't bloom well in shade. Most cultivars are 2 to 4 feet tall, excellent toward the back of a mixed bed. Striking purple cultivars include Purple Rooster and Peter's Purple. Grow disease-resistant varieties such as violet-blue Violet Queen and purple-red Colrain Red to deter powdery mildew.
Everyday culinary garden sage (Salvia officinalis) is as lovely in flower as any ornamental, and its abundant purple-blue flower stalks last for a month or more. For limited space or containers consider dwarf garden sage (S. officinalis minum). Other variations show off their colour in vegetation but don't bloom, including golden garden sage, with green-gold variegated leaves; purple garden sage, with dark purple new leaves that "green up" with age; and tricolour sage, with variegated green, cream and pink leaves.
The ornamental salvias -- more than 900 species and countless cultivars -- are sturdy perennials and tender perennials grown as annuals in colder climates. Most commonly grown is bedding sage (Salvia splendens). May Night (Salvia x superba) is a low-growing hybrid salvia with dark violet-blue flower spikes, good for the front of flower borders, as is deep purple East Friesian. Also notable among less common varieties is the shrubby, fast-growing Mexican bush sage (Salvia leucantha), especially the cultivars All Purple, Midnight and Purple Velvet; Peruvian sage (Salvia discolour), with pale vegetation and purple-black flowers; giant flowered purple sage (Salvia pachyphylla); and Salvia verticillata "Purple Rain." Tall purple-flowering salvias included 4- to 5-foot-tall Indigo Spires and robust, 5- to 6-foot Purple Majesty, which tolerates partial shade.