Petunia

Seed is sown indoors in January to March at a germination temperature of 72 to 75°F, though many gardeners instead buy young plugs or plants; pinching these back before transplanting encourages a bushier habit. Petunias want full sun to partial shade (heavy shade weakens flowering) in light, well-drained soil of medium fertility, ideally with a pH near 6.0 to 6.5 and good porosity to support root growth and discourage rot; the plants tolerate poor soil provided drainage is adequate and show moderate drought tolerance once established.

Watering should be deep rather than frequent, since light, shallow watering encourages shallow rooting; roughly 1 to 2 inches every 7 to 10 days is enough for an established planting. A balanced fertilizer worked into the bed before planting, followed by a midsummer topdressing of nitrogen and monthly or controlled-release feeding thereafter, keeps growth and bloom going through the season.

Pinching the growing tips after planting encourages branching, and deadheading spent flowers promotes continued bud production; plants that become leggy respond well to being cut back partway in midsummer to spur fresh growth and renewed flowering. Where winters are too cold for the plant to persist outdoors, it can be kept indoors over winter in a bright spot away from heated air.

Aphids, flea beetles, slugs, snails, broad mites, cyclamen mites, foliage-feeding caterpillars, spider mites, thrips, and leafminers are the insect and mite problems most often reported on petunia. Root, stem, and crown rots, gray mold (botrytis blight), powdery mildew, verticillium wilt, aster yellows, tobacco mosaic virus and other viruses are the principal diseases; petal blight can appear in rainy, humid weather and is best avoided by keeping foliage and flowers dry when watering.

Nutrient disorders and either excess or deficient watering are recognized environmental problems, and fasciation has also been noted as an occasional physiological disorder.

Petunia is grown almost entirely as an ornamental, valued for masses of color in beds and border fronts, in containers, hanging baskets and window boxes, as a groundcover, and even as winter color in interior sunrooms depending on the type. Its flowers draw butterflies and hummingbirds, making it a common choice for pollinator and butterfly gardens, and it is seldom troubled by deer.

Today's garden petunia is a hybrid derived from wild South American species, principally Petunia axillaris and Petunia integrifolia (also called P. violacea), with some accounts including a third species, P. inflata, in its ancestry. These wild petunias grew in Argentina, where exploration had long been restricted under Spanish rule; after Argentina opened to outside settlers in the 1820s, the Scottish botanic-garden gardener James Tweedie traveled the region and introduced the wild species to Europe. From there the plants reached the United States, where breeders such as Mrs. Thomas Gould of California developed the first large-flowered strains in the late 1800s, work that eventually gave rise to the modern hybrid petunia grown in gardens today.