RHS Journals

The Garden
February 2003

I can't believe it's a buttercup

With impossibly delicate flowers in a mixture of fresh, glowing colours, Ranunculus asiaticus have long been popular, and are a welcome sight in spring, either as pot plants or as bedding. Philip Clayton examines their cultivation and uses within the garden

Images: Tim Sandall

The intricate flowers of Ranunculus asiaticus hybrids create a spectacular display, especially when grown en masse. Although best cultivated in cool-glasshouse conditions, plants will also succeed outside in sheltered spots

Vibrant hues are in demand in the spring garden as an antidote to the gloomy winter months. Many spring flowers provide colour, but the palette is mostly limited to blue, yellow, or muted tones of other shades.

This accusation cannot be made, however, against hybrids of Ranunculus asiaticus. Related to common buttercup, they are derived from a species native to southwest Asia and southeast Europe, where summers are dry and hot but springs are cool and moist. Flower colour is variable, including red, yellow and white, often with beautiful bicolored forms.

Today’s modern hybrids have been bred for garden use and the pot-plant market, although earlier cultivars were popular during the 17th century, when they were known as florist’s ranunculus (see Objects of desire, The Garden, May 1999, pp370–3).

These spectacular plants were taller than the compact cultivars of today (up to 1m/39in). Many had delicate, bicolored blooms with streaked and patterned petals or colour breaks caused by similar viruses to those found in tulips. At their height of popularity, there is thought to have been 10 cultivars of florist’s ranunculus for every one of tulip.

Named selections of florist’s ranunculus no longer survive. They were not suitable for growing outside, as the flowers were easily spoiled by wind and rain - conditions that are common in a typical UK spring. In addition they were propagated from tubers (‘claws’), which gradually lost vigour and eventually died out altogether.

Glasshouse cultivation

Modern cultivars of ranunculus are more durable, short-stemmed plants better suited to use as pot plants or bedding. Increasingly they are seen in garden centres, usually sold in flower, and though they are most successful in cool-glasshouse conditions, they can be tried in sheltered parts of the garden where their brilliant flowers will not be damaged.

Ranunculus seed is sown in autumn (see Ranunculus from seed or tubers), but germination is not always easy. Resulting plants flower the following spring, from early April onwards. They should be kept cool (15–18°C/60–64°F) during the day and frost-free by night. If temperatures are too high, plants become etiolated and require staking. Ranunculus must not dry out, and it is best to keep the glasshouse damped down and well-ventilated on warm days.

Feed plants regularly with a high-potash fertiliser and deadhead as appropriate. The first flower to be produced will be the largest, but successive flowering gives a fine display that should last for six to eight weeks.

Foliage will die down from mid-summer onwards. If the tubers are to be kept, stop watering then, and store them in dry sand.

For the largest, most spectacular specimens, plants can be kept in a cold frame once the foliage has withered. Then allow the plant (still in its pot) to bake in the sun, which mimics conditions encountered in the wild. Plants should be started back into growth in autumn and potted on into 15cm (6in) pots. They can be kept in this way for several years, although they will begin to deteriorate and eventually should be discarded.

Garden and container use

Ranunculus asiaticus hybrids can be planted outside as bedding plants in April and May, where they may flower (depending on the weather) into June, so are useful for bridging the gap between winter bedding and tender summer plants. However, they are not easy to please when grown outside. A wet, windy spring will damage flowers badly, while hot, dry conditions will cause foliage to yellow prematurely and restrict flowering.

Ranunculus can be tricky to mix with other plants. Modern cultivars flower at a low level, so underplanting is not easy, but Myosotis may prove to be a suitably frothy companion to the bold ranunculus flowers.

Single-colour selections such as orange R. asiaticus ‘Bloomingdale Tangerine’ are easier to deal with as part of a planting scheme, and can be combined with long-stemmed tulips of contrasting colour, and if possible, flower form. In many situations, however, it may be better to grow them on their own with their own dissected foliage providing perhaps the best foil for the impressive flowers.

Plants are well suited for short-term outdoor use when grown in containers, because watering can be monitored and they can be moved out of adverse weather.

Modern Ranunculus asiaticus hybrids may not receive the adulation their predecessors did 300 years ago, but they are fine plants that deserve the extra attention they require.

Ranunculus on trial

A trial at RHS Garden Wisley of Ranunculus asiaticus was sown in autumn 2000 and assessed in March and May 2001. There were seven entries and plants were grown in cool-glasshouse conditions. Though none received an Award of Garden Merit (agm), due to their relatively difficult cultivation requirements, the plants gave a fine display.

‘Bloomingdale Pure Yellow’, ‘Bloomingdale Tangerine’ and ‘Bloomingdale White Shades’ were three of the best cultivars. Plants are available from garden centres, and seed of ‘Bloomingdale Formula Mixed’ from Chiltern Seeds, Cumbria (tel: 01229 581137).

Ranunculus from seed or tubers

Sow seed between August and October on moist seed compost	Cover with vermiculite which keeps seeds moist but allows light and air to reach them
When large enough, prick out seedlings into modules of potting compost and keep cool and moist under glass over winter	Plant outside or into 9cm (3.5in) pots in spring

Germination may be improved if seeds are chilled to, and kept at, 4°C (39°F) for a week, then temperatures are raised to 15°C (60°F). Seeds will take about three weeks to germinate.

Tubers or ‘claws’ of plants can be saved and stored after plants have died down in summer. In autumn, soak the tubers and plant them in potting compost	Keep cool and moist and wait for the shoots to develop

Philip Clayton is Trainee Horticultural Journalist for The Garden

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