The showiest ranunculus, Persian buttercup (Ranunculus asiaticus), makes 3- to 5-inch whorled double or semidouble blooms atop ferny-leafed stems in spring or early summer. Grown outdoors from fall-planted tubers in U.S. Department of Agriculture plant hardiness zones 7 through 10, its hybrids also can be raised from commercial seed sown indoors. Smaller-flowered wild buttercups (Ranunculus spp.) sometimes are grown in gardens as well, their hardiness ranging from USDA zones 3 through 10.
Persian Buttercup Germination
Plan to sow full-sized Persian buttercups by early winter and dwarfs by late winter. Because they require chilly conditions, start them in a cool, sunny greenhouse or under a grow light in a minimally heated basement or garage. Temperatures should never rise above 68 degrees Fahrenheit during the day or fall below 45 F at night.
Select a seed-starting container with drainage holes, filling it with moist and sterile seed-starting mix -- or three parts peat combined with one part perlite -- to within 1/2 inch of its rim. Scatter the seeds over the surface and sprinkle a thin coating of damp peat or sand over them.
Cover the container with plastic wrap and place it beneath a grow light set to run for 12 hours per day. In temperatures between 50 and 60 F, the seeds should sprout within 10 to 30 days, though Persian buttercup germination tends to be low and sporadic.
When the seedlings have emerged, feed them by dissolving 3/8 teaspoon of 20-20-20 plant food crystals in 1 gallon of water. Ten days later, fertilize them again, raising the amount of crystals to 1/2 teaspoon per 1 gallon of water. Continue to feed them the latter amount every 10 days, and never allow their mix to dry out.
When a seedling has four true leaves, in addition to its two seed leaves, move it immediately into its own 4-inch pot filled with a compost-rich potting soil with a pH between 6.0 and 6.5. Keep the seedlings at temperatures between 60 and 68 F during the day and 45 and 50 F at night, making sure that they continue to receive only 12 hours of illumination each day. They should flower about three to four months after they were moved to individual pots.
If you wish to transplant your seedlings into the ground outdoors, do so about two weeks before your last frost date. Gradually accustom the plants to the increased light for a week or two beforehand, and then set them 6 to 8 inches apart in a well-draining bed in full sun. Keep them well-watered until they have finished blooming, after which you can allow them to die back.
Wild Buttercup Germination
Sow the wildflower seedlings in the same way you would the Persian buttercup types. Then consult a germination database, such as that at the Ontario Rock Garden & Hardy Plant Society site, to determine which temperatures give the best results for the species you wish to start.
Wild buttercup seeds must be fresh to germinate, so begin their stratification shortly after they are harvested.
Some wild buttercups need three months at 40 F followed by 70 F temperatures to induce sprouting. Others want six weeks at 70 F, six weeks at 40 F and then six weeks at 50 F.
To keep your seed-starting mix damp over such long germination times, enclose its container in a zip-type plastic bag. Place that container in your refrigerator for a 40 F period, in a chilly basement or garage for a 50 F one, and on a warm shelf for a 70 F one. For types that germinate best if kept outdoors all winter, bury the seed pot up to its rim in a cold frame, covering it with a pane of glass to keep its soil damp.
Once the wildflowers germinate, those started in a cold frame generally can be left there -- with the pane of glass removed from their pot -- as they grow. The others can be treated like other perennial seedlings, raised at temperatures in the 60s under grow lights that run 14 to 16 hours per day. Fertilize and transplant all types as you would Persian buttercups, but feed them only once every two weeks instead of every 10 days.
Although most wild buttercups like damp, fertile soil, a few prefer drier rock garden conditions. Some thrive in either sun or shade. Their spacing also varies, according to the size of the species, from 2 inches to 2 feet.