The genus Ranunculus contains hundreds of species of flowering perennials, many of them commonly called buttercups. Some of these species, such as the creeping buttercup (Ranunculus repens), which is hardy in U.S. Department of Agriculture plant hardiness zones 4 to 9, and lesser celandine (Ranunculus ficaria), which is hardy in USDA zones 4 to 8, are weedy and invasive in some areas; although they're sometimes included in flower beds or woodland gardens, they're not ideal choices for most gardeners. One species, the Persian buttercup (Ranunculus asiaticus), is well-behaved and among the most popular garden ornamentals, however.
Persian buttercup, which is often commonly called simply ranunculus, is a common garden ornamental that grows from clumping, finger-like tubers. The tubers produce a mound-shaped plant that grows to between 1 and 2 feet in height. The plant flowers between spring and mid-summer, producing colorful blooms that can last up to six weeks. The plants go dormant after flowering, and the foliage will yellow and die back by the end of the season.
Winter Hardiness and Heat Tolerance
Persian buttercup is a perennial, but it is more vulnerable than many other Ranunculus species to damage from cold temperatures. It will survive winters in USDA zones 8 to 11, but in colder zones it must be grown in the garden as an annual.
The plant grows best when temperatures are relatively cool, and in warm climates, the plant's flowering will decrease when daytime temperatures climb above 90 degrees Fahrenheit.
In zones where Persian buttercup is reliably perennial, the tubers may be planted outside in the fall, after summer temperatures have peaked and the weather begins to cool. Tubers planted in the fall will begin to flower early in the spring, usually in March.
In colder zones, tubers must be planted in the spring, a week or two before the last frost of the season. Spring-planted tubers will flower later in the season, typically in June or July. Another option is to start the plants in pots indoors in mid-winter and then transplant them into the garden after frost danger has passed; this head start will help the plants to bloom earlier in the summer, before high temperatures become a problem.
Site Selection and Planting
Persian buttercup grows best in sites with loose, well-drained soil and full sun exposure. Plant tubers with their fingers, which typically look like a small cluster of bananas, pointing downward, about 1 to 2 inches deep. In heavy soils, plant the tubers more shallowly. Space jumbo tubers 8 to 12 inches apart; small tubers may be planted as little as 4 inches apart. If you're planting in pots, plant only one or two jumbo tubers in each 10-inch pot.
Persian buttercup tubers are sold dried, and although they usually appear shriveled at this stage, they will absorb moisture and expand after planting. They will rot if they're exposed to too much moisture, however, and they must be planted in places where the soil drains quickly and thoroughly.
Water the tubers thoroughly immediately after planting, but don't water again until sprouts emerge. Water the plants through the flowering season so that the soil remains moist but not soggy, and stop watering once flowering stops.
Fertilize at planting time with an all-purpose flower fertilizer; as a general rule, scatter about 1/10 pound of 10-10-10 fertilizer per 10 square feet of planting area and water thoroughly. After the plants emerge, fertilize every two weeks with liquid fertilizer diluted to half strength until the plants stop blooming.