So you're wondering what that interesting plant at the back of your garden is. Perhaps it was left by a previous gardener, or it's a volunteer--a plant that migrated by seed from another location and happily settled in your yard. As long as it's well-behaved and doesn't push the other plants around, take a wait-and-see approach. If, however, the plant seems invasive--taking over space occupied by other plants--you're better off pulling it up before it becomes a major nuisance.
Determine if the plant is a weed, perennial or annual flower. If you notice the plant growing throughout your yard, in your lawn and outside flower gardens, it is most likely a weed and probably invasive. If the plant grows just in a designated area, it was most likely intentionally planted and is a perennial or annual. If it returns yearly, it is probably a perennial. Some annual flowers self-sow but don't grow in exactly the same location from year to year.
Study the plant's growing conditions. Does the flowering plant prefer shade? Chances are it's impatiens, begonia, lobelia or coleus if it's an annual. Shade-loving flowering perennials include astilbe, bleeding heart, hosta, lamium, delphinium, foxglove and columbine.
Smell the plant. Some flowers are recognizable by their distinct fragrance. Marigold has a pungent, spicy odor. Lavender has a clean, soapy scent. Petunias, moon flowers and jasmine are sweet-smelling.
Examine the plant's leaves and flowers. Some produce multiple blooms on long spears or stalks, including salvia, penstemon, delphinium, larkspur and catmint. Other flowers have large, round blooms or leaves, including nasturtium, geranium, moon flower and morning glory. Multiple-petaled flowers include daisies, coreopsis, blanket flower and cone flower. These plants resemble sunflowers.