I’ve heard conflicting advise about when to plan ranunculus bulbs in Ontario. Should I start them in pots now (March) and put them in the ground at the end of May?
Thank you for contacting the Toronto Master Gardeners with yor inquiry.
Ranunculus asiaticus (Persian buttercup) plants are cool season flowers that grow best in spring-like temperatures of about 55°F (13C). In warm climates (zones 8-10), Ranunculus can be planted in beds and borders in the fall. Unfortunately, in the GTA a zone 5-6, Ranunculus is not winter hardy so you must treat them as an annuals.
You can start your Ranunculus tubers indoors about twelve weeks before the last frost date, which here in Toronto is usually May 9. Plant one or two tubers in a pot and place them where it will be exposed to full sun for 6-8 hours. Ranunculus are happiest with about six to eight weeks of cool sprouting temperatures: 60 F (16 C.) during the day and 45 to 50 F (7-10 C.) at night to break dormancy. Transplant your buttercups outside when all danger of frost is passed and the first true leaves are evident. Make sure to acclimatize your plants to the outdoors about 3 weeks before planting them in your garden.
Alternately, you can plant the tubers outdoors when the danger of frost has passed- in late spring. Place your Ranunculus where they will receive full sun(6-8hrs). Plant the tubers 2” deep and 4”-6” apart. Set the tubers in the soil with the pointed part of the claw-like structure pointing down. After planting, water well to settle the soil around the tubers and then allow it to dry out before watering again
After blooming has finished for the season leave the foliage in place; don’t cut it off. The leaves will gather sunlight and provide nourishment for the tuber. At the end of the summer the leaves will yellow and die back as the plant slips into dormancy. Foliage many be removed at this point. Ranunculus actually prefers not to be watered while dormant.
Most gardeners, grow Ranunculus as annuals, discarding them at the end of each season, alternatively you can dig up the tubers for dry storage once the leaves have withered slightly. Gently dig up the tubers, wipe off the soil and allow them to dry in a warm location for a few days. Then store them in a paper bag in a cool, dark location over the winter.