When to Plant Ranunculus

The two limits that set your planting date

Ranunculus timing comes down to two temperatures. On the cold side, NC State Extension reports injury below about 28°F, and Alaska Master Gardeners report that plants and corms die below about 25°F. On the warm side, Solano County Master Gardeners note that flowering ends once days stay above about 70°F.

Between those limits, the plant needs about 90 days from planting to first flowers, according to Clemson and Solano County, and then blooms for up to 6 to 7 weeks. Your job is to fit those three months of growth, plus some weeks of flowering, into the cool season where you live.

Zones 8 to 10: plant in fall

Where winter lows rarely fall below the mid-20s, plant in fall. The corms root and grow through the cool months and flower in spring, before the heat.

• Central Texas: Texas A&M AgriLife in Travis County plants in October, with blooms in April and May.
• California valleys: Solano County Master Gardeners plant in fall, mostly for convenience, and say late winter or early spring also works. Flowers there last until about late April.
• South Carolina: Clemson treats ranunculus as a fall-planted, spring-flowering bulb, and in most of the state the corms overwinter in the ground.
• Arizona low desert: University of Arizona’s Phoenix-area chart shows planting in October and November, or again from February into mid-March, with flowers from mid-February into May.

Fall corms need little water until shoots appear. Mulch the bed, and keep frost cloth ready for nights forecast to drop toward the mid-20s.

Zones 3 to 7: plant in late winter or spring

Do not plant ranunculus outdoors in fall here, because winter will kill the corms. You have three ways to time a spring planting.

1. Plant dry corms outdoors. NC State Extension says to plant several weeks before the last frost date. Clemson says to plant in late winter or early spring once the extreme hard-frost dates have passed. In practice, that means after your last nights in the low to mid-20s, a few weeks ahead of the average last frost. The dates for your zone above show that window. Soak the corms for 3 to 4 hours first.

2. Pre-sprout, then plant. Alaska Master Gardeners first pick a safe planting date, one after which plants will not see temperatures below 25°F. About two weeks before that date, they soak the corms, set them in moist potting mix in a dark spot at 40 to 50°F, and let them swell and root for 10 to 14 days before planting out. Pre-sprouted plants flower about two weeks earlier than dry corms planted the same day. Their plants bloomed 12 weeks after pre-sprouting began.

3. Start in pots well ahead. Iowa State’s Reiman Gardens starts corms in 6-inch pots about 12 weeks before the average last frost, which is early February in Iowa. The pots are grown bright and cool, no warmer than 60°F by day and 45 to 50°F at night. The plants go outside just after the last frost and bloom into midsummer. This gives the earliest flowers but needs a cool, bright indoor space.

Low tunnels and frost cloth widen every one of these windows. An Anchorage gardener who uses them pre-sprouts in mid-April and plants at the end of April, while gardeners without protection there wait until about mid-May.

Matching the plan to your summer

The shorter your cool spring, the more an indoor start pays off. Count about 90 days forward from your planting date. If that lands well into hot weather, pre-sprouting or potting up gives you back two weeks or more. In cool-summer southcentral Alaska, a Master Gardener’s spring-planted ranunculus bloomed in June.

Buying corms on time

Buy in fall, when online sellers stock corms for warm-zone gardeners. Alaska Master Gardeners note that spring orders may not ship until the freeze risk on the trucks has passed, which can mean May delivery. Dry corms store well over winter in a cool, dry place until you are ready to soak and plant them.