This very large genus comprises about 250 species of widely differing habit and appearance, but the three listed here are the ones most commonly grown in gardens. Not browsed by deer.
R. asiaticus. PERSIAN BUTTERCUP. Zones LS, CS, TS; USDA 8-11 (see below). Native to Asia Minor. Tuberous-rooted plant to 12 ft. tall and wide, with fresh green, almost fernlike leaves. Blooms profusely in spring, when each flowering stalk bears one to four 3- to 5-in.-wide, semidouble to fully double blossoms that some say resemble small peony blooms. Flowers come in white, cream, and many shades of yellow, orange, red, and pink. Popular Tecolote Giant strain is available in single colors, mixed colors, and picotees. Bloomingdale strain offers the same range of colors on dwarf plants 810 in. high. All types are good in the ground or in pots.
Tuberous roots are hardy to 10F. In the Coastal and Tropical South (USDA 9-11), plant in fall for bloom in winter, early spring; treat plants as annuals there. Beyond hardiness range, plant in spring as soon as ground is workable; or start roots indoors four to six weeks before the usual last-frost date. Nurseries sell tuberous roots of various sizes; all produce equally large blossoms, but bigger roots yield a greater number of flowers.
Grow in full sun, in organically enriched, very well-drained soil (if necessary, plant in raised beds). Set roots with prongs down, 2 in. deep (1 in. deep in heavier soils) and 68 in. apart. Water thoroughly, then withhold water until leaves emerge. Birds are fond of ranunculus shoots, so protect sprouting plants with netting or wire. Or start plants in pots or flats, then set them in the garden when they're 46 in. talltoo mature to appeal to birds. (You can also start with nursery-grown seedlings.) Remove faded flowers to encourage more bloom.
When flowering tapers off and leaves start to yellow, stop watering the plants and allow the foliage to die back. Where tuberous roots are hardy in the ground, they can be left undisturbedas long as soil can be kept dry during summer. Some gardeners dig plants when foliage turns yellow; cut off the tops; let roots dry for a week or two; and store them in a cool, dry place until planting time. But because roots don't store that well, most people find it simpler to discard the plants and set out new roots when the time comes.
R. ficaria. LESSER CELANDINE. Zones US, MS, LS, CS; USDA 6-9. From Europe and Eastern Asia. Aggressive, weedy perennial spreading by bulblets on stems and underground tubers. Forms a dense mat of heart-shaped, 1- to 2-in.-wide, shiny, dark green leaves, mounding to 34 in. tall. Bright yellow, buttercup-like flowers rise above the foliage in spring. Dies back in late summer. Has become a rampant weed in some areas. Full sun or part shade. Less aggressive selections include 'Brazen Hussy', with purple-black leaves; 'Collarette', with heart-shaped leaves marked with silver; and 'Randall's White', with cream-yellow flowers and leaves marked with silver.
R. repens pleniflorus. CREEPING BUTTERCUP. Zones US, MS, LS, CS; USDA 6-9. From Eurasia; naturalized in North America. Vigorous plant with thick, fibrous roots and runners that root at the joints. Forms a lush, glossy, green mat to 1 ft. high, 6 ft. wide; leaves are roundish, deeply cut into three tooth-edged, 2-in.-long leaflets. Fully double, 1-in., button-shaped, bright yellow flowers are held above foliage on 1- to 2-ft. stems in spring. Can be invasive in constantly moist soil. Attractive deciduous ground cover for full sun to deep shade. Basic species is single flowered and just as aggressive as R. r. pleniflorus.