Forget-me-nots, spring-blooming plants of the Borage family, are annuals, perennials or biennials depending on the United States Department of Agriculture plant hardiness zone in which they grow. The hardiest garden forget-me-nots grow as perennials or biennials in zone 5, where winter temperatures plunge to minus 20 degrees Fahrenheit. Other varieties grow as annuals where temperatures drop lower than minus 10. Biennial forget-me-nots produce foliage in their first year before blooming, setting seed and dying in their second.
Alpine forget-me-nots, scientific name Myosotis alpestris, grow wild in the mountains of the Pacific Northwest, Europe and Asia. Standing 4 to 8 inches high and up to 8 inches wide, these mid-spring flowering plants are perennial in plant hardiness zones 4 and higher and annual in colder regions. Their white- or yellow-centred, bright blue five-petaled flowers appear in dainty clusters above clumps of downy, light green oblong foliage. The Rosylva alpine forget-me-not cultivar has rose-pink, white-centre flowers. The Compindi cultivar's blue or blue-violet flowers emit a mild fragrance at night. Miro's medium-blue flowers open in early spring. This cultivar's short life may make it more suitable as a biennial.
Water forget-me-not, Mysotis scarpoides, has spread from its native habitat along Europe and Asia's stream banks and meadows to naturalise across much of North America. Usually standing from 6 to 10 inches tall, with upright to reclining stems, this plant illuminates pond edges and water gardens with yellow-eyed, sky-blue spring and summer flowers. Vivid green, glossy lance-like leaves add to its appeal. Dwarf water forget-me-not, Myosotis scorpioides var. semperflorens, reaches 6 or 8 inches high. Its flowers appear continually from May until August, and in smaller numbers until frost. Both zone 5-hardy perennials perform best in sun or partial shade and fertile, moist or wet soils. They tolerate as much as 3 inches of standing water.
Woodland forget-me-not, scientific name Mysotis sylvatica, tolerates winter temperatures in zones 4 through 8. The most widely grown garden forget-me-not, it stands between 5 inches and 1 foot high, with oblong to tapering, downy green foliage. Its dense, curling spikes of white-or yellow-centred, five-lobed flowers unfurl in early to mid-spring. Woodland forget-me-not's numerous cultivars include the more compact Victoria Blue. Its 6-to 8-inch foliage clump has darker blue, yellow-eyed flowers. Mon Amie Blue blooms from seed in its first year, with delicate light-blue flowers above grey-green foliage. Like alpine forget-me-nots, these short-lived perennials may perform better as biennials. They thrive in a spot with rich, average-draining soil and midday shade.
Less cold-tolerant -- and much larger -- than many Mysotis forget-me-not species, biennial Chinese forget-me-not, Cynoglossum amabile, usually grows as a cool-weather annual in zone 6 and higher. Its 1- to 2-foot stems rise from basal mounds of greyish-green, downy-textured foliage. The lower leaves measure up to 8 inches long. Stem leaves are smaller, seldom exceeding 3 inches. Where summers are cool, its one-sided sprays of light blue flowers appear from spring until frost. In hot summer climates, however, the plants may stop blooming before fall. Chinese forget-me-not grows in poor to average, well-drained soil and full sun.