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Lobelia erinus, sometimes called edging lobelia or trailing lobelia, is a species of ornamental flowering perennial that is a native of southern Africa but has become commonplace in several U.S. states. Although this plant is perennial in warmer climates, it is grown like an annual in areas that are colder. It is hardy to USDA zones 10 and 11. Lobelia erinus produces flowers that are usually violet-blue with a white middle, but cultivars are available that produce red, pink, white, light blue, or purple blooms. Gardeners often use the upright species of lobelia for edging a garden bed, but the varieties known as trailing lobelia do best in containers, hanging baskets, or window boxes.
A member of the bellflower family Campanulaceae, lobelia erinus tends to grow to about 6 inches (15 cm) high and can spread up to 1 foot (30 cm). Its leaves can be dark or medium green. In some cases, the leaves could develop a burgundy or bronze tinge, depending on the amount of sunlight received. This plant produces a very small fruit containing the seeds for propagation. Some of the cultivars of lobelia erinus include Rosamond, Crystal Palace, Fountain Series, Blue Moon, Sapphire, and Cascade.
Lobelia erinus has no significant problems with disease or insects. It produces flowers during spring and summer in most locales. The blooms are said to attract butterflies. Lobelia erinus will cease to bloom when summer temperatures rise too high. If the plants are cut back at this time, they are likely to bloom one more time when temperatures drop in autumn.
Some gardeners find lobelia erinus difficult to start from seed. If buying young plants in the spring, choose stocky specimens that are not wilted and show no indication of disease. It is best to select plants that have not flowered yet, because they will be capable of greater root development. Seedlings should be planted about a week before the last spring frost in a sunny location that receives a small period of shade during the day. Lobelia erinus thrives on rich, moist soil with the aid of organic matter such as compost.
To create fuller foliage and profuse blooms, it is helpful to pinch off the lobelia's ends. Lobelia will die quickly without sufficient watering, so it is vital to keep the soil moist throughout the growing season. When used as an annual, these plants should be discarded after the first substantial autumn frost.