Geraniums may be annuals, perennials or neither, depending on the geraniums in question. The garden varieties with bright flowers on mounds of pungent leaves are known as garden geraniums (Pelargonium spp.), and they are tender perennials. True geraniums (Geranium spp.) produce pastel clouds of blooms and may be either perennials or self-seeding biennials. Complicating matters is the fact that home gardeners typically treat garden geraniums as annuals.
Annual, Perennial or Biennial
Every plant on Earth has one goal: survival of the species. Flowering plants such as geraniums survive and reproduce in several ways, including suckering and seed production. Also, gardeners can divide the plants' clumps and bulbs to produce more plants.
Each plant has a lifespan to accomplish its goal of species survival. An annual plant grows from a seed, flowers, sets new seed and dies within one growing season. A perennial plant lives three or more seasons, producing seeds yearly. If it dies back due to cold, its roots put up new growth the following spring. A biennial plant takes two years to grow, often not flowering until the second year.
Tender Perennial Geraniums
Zonal garden geraniums (Pelargonium x hortorum), ivy geraniums (Pelargonium peltatum) and scented-leaf species geraniums (Pelargonium spp.) are all tender perennial natives in U.S. Department of Agriculture plant hardiness zones 10 through 11. A hard freeze kills those geraniums. So in areas colder than USDA zone 10, they are typically discarded and replaced with new geraniums in spring, as if the plants were annuals. The geraniums may be overwintered indoors as houseplants or held dormant in paper sacks, however.
All of the tender perennial geraniums have rounded, more or less deeply lobed or web-shaped leaves and produce florets in reds, pinks, white, orange and even yellow. Zonal geraniums are so-named because of the dark circles on their green leaves. Another member of the tender perennial group, the regal or "Martha Washington" geranium (Pelargonium x domesticum), is a cultivar typically grown as a houseplant.
Biennial Cranesbill Geraniums
Cranesbill geraniums (Geranium maculatum) may be perennials, short-lived perennials, biennials or long-lived annuals. Also known as wild and hardy geraniums, they are natives in USDA zones 3 through 8 in North America. Bigroot geranium (Geranium macrorrhizum) has a delicate scent and also is hardy in USDA zones 3 through 8. Some hybrids may be less hardy.
Cranesbills are ground-cover or rock-garden plants, and their florets are less dense than those of their garden cousins. Their blooms are also more subtle: soft pinks, whites and blues. Biennial cranesbills self-seed prolifically, which means new plants sprout every year; so replacing them is seldom necessary.
Garden geraniums and cranesbill geraniums grow in full sun, but choose cranesbills for shady areas. Both types bloom best in moist, well-drained soil, and both kinds develop rot or edema when overwatered. Many cranesbills tolerate some dryness, but both species grow best with the weekly 1 inch of water that benefits various flowering plants.
Seed heads on garden geraniums also feature the long, pointed capsules that earned the cranesbills their common name.