Ranunculus is a very large genus in the Ranunculaceae family. Not all of them are tuberous and could be considered "bulbs".
Ranunculus asiaticus, commonly known as 'Persian Buttercup' is native to the eastern Mediterranean region in southwestern Asia, southeastern Europe (Crete, Karpathos and Rhodes), and northeastern Africa. Photograph 1 of commercially supplied tubers on a 10 mm grid by David Pilling. The free ends of the tubers point downwards, and the ends that are joined go uppermost in the ground. Before planting tubers are soaked for 24 hours and plump up, as shown in photos 2 and 3. Photo 4 shows the resulting shoots a couple of weeks later.
Photos showing the resulting flowers in Spring. Illustration from the Gottorfer Codex, a collection paintings on vellum depicting flowers of the garden of Schloss Gottorf, created between 1649 and 1659.
Seed related photos.
Ranunculus calandrinioides, a summer dormant buttercup native to the Atlas mountains in Morocco, has a thick rootstock with fleshy roots. It is easily grown under cold glass. Grow it as hard as possible (ie don't keep it frost free under glass) and give it as much light as possible. I have also found that it keeps much more compact and much more beautiful if kept bone dry until mid winter. In this way growth does not commence above ground until early spring and is much less lax. (Watered in early autumn, after a summer rest it comes into growth well before Christmas.) Photos by Tony Goode.
Ranunculus ficaria has been moved to a separate genus, see Ficaria verna.
Ranunculus kotchii is not an accepted species name but plants are grown under this name. Tuber photo by Peter Taggart.
Ranunculus 'Rococo Series' John Fielding writes "Well over 20 years ago I attempted some crosses with Ranunculus asiaticus and other species. The resulting hybrids proved to be remarkably hardy winter growing perennials and have experienced -15ºC without harm in UK conditions. Given the Mediterranean origin of the parent Ranunculus asiaticus they are tuberous rooted and go through a summer dormant period. During this time they are more than happy to be totally dry but can remain in the ground if not watered or can be lifted and stored in a cool, dry place. The roots need to be planted just below soil level in autumn in a sunny position. They respond dramatically to feeding and put on masses of growth producing more flowers per stem than typical Ranunculus asiaticus. The thicker stems are also more robust meaning staking is not necessary and have proved themselves as excellent weather proof cut flowers. They are also excellent bedding plants associating well with late flowering tulips and other spring bedding flowering in early May. The many side buds continue to flower as the first flowers fall. The flowers develop a two tone effect as they age which is particularly attractive.
We are releasing four cultivars this year (2019), Peach, Pink, Orange and Yellow at the RHS Malvern flower show, 9th to 12th May and also the peach and pink online. I also say a huge thank you to Lyndon Mason who ran the trial for us at the National Cut Flower Centre at Holbeach St Johns".