Plant Profile: Buttercups, Part 3

By Patsy Cotterill

Ranunculi: Part 3. The Central Parkland Species

In previous articles, we described the buttercup species (members of the Ranunculus family) found in the Edmonton area in moist, shady terrestrial habitats (https://www.enps.ca/post/plant-profile-buttercups-part-1), as well as those found in wetter conditions (https://www.enps.ca/post/plant-profile-buttercups-part-2) In this final instalment, we are going to consider four species of buttercup that occur within the Central Parkland ecoregion, all of which are terrestrial and are quite clearly members of the Ranunculus genus. The flowers are radially symmetrical, with five sepals and five bright yellow petals that have glossy upper surfaces (the better to guide pollinators and regulate heat). A ring of stamens and a central cluster of single-seeded carpels (which will become the fruits or achenes) complete the flower.

All are herbaceous perennials with a predilection for moist soils, although the two native species, prairie buttercup (Ranunculus rhomboideus) and heart-leaved buttercup (Ranunculus cardiophyllus) are inhabitants of natural grasslands. The other two species, tall buttercup (Ranunculus acris) and creeping buttercup (Ranunculus repens) are Eurasian in origin and are to be found in more disturbed habitats such as fields, ditches and roadsides. 

Prairie buttercup (Ranunculus rhomboideus) and heart-leaved buttercup (Ranunculus cardiophyllus)

The two grassland species, both hairy, have a number of characteristics in common, apart from habitat. The most obvious one is that their basal and stem leaves differ markedly in shape (as opposed to the situation in most other buttercups in which the stem leaves are usually just smaller versions of the basal ones). The basal leaves of prairie buttercup are long-stalked, simple, somewhat spade-shaped and taper to a wedge-shaped base at the junction with the leaf stalk; they are also very shallowly toothed (crenate). In heart-leaved buttercup the simple leaves have a slightly rounder outline with a rounded, flat or heart-shaped base. The marginal teeth are somewhat more deeply indented. In both, the upper stem leaves are largely stalkless and divided into 3-5 (7) narrow segments. 

Prairie buttercup (Ranunculus rhomboideus) in flower at Nisku Prairie, 2011-05-18.  Photo: P. Cotterill.

The easiest way to tell the two species apart, however, is by flower and fruit. The flowers are larger in heart-leaved buttercup (2.25 cm compared to slightly less than 2 cm) with more rounded and, more importantly, contiguous petals, making for a more attractive bowl-shaped appearance to the flower; in prairie buttercup the petals are wedge-shaped with gaps between them. The heads of hairy achenes in the former species form an oblong-cylindric head at fruiting time, whereas the hairless achenes of prairie buttercup form a globose head. 

Heart-leaved buttercup (Ranunculus cardiophyllus) in flower at Nisku Prairie, 2022-06-04.  Photo: P. Cotterill.

In Ranunculus cardiophyllus the flower stalk is long, stout and flexuous, which possibly allows it to snake through the adjacent grass and bear its large, single flower at the same height as the surrounding vegetation.  This could be important because it flowers later, in mid-spring, than Ranunculus rhomboideus, which may therefore experience less competition for light and pollinators. 

Both species occur in Nisku Prairie, blooming somewhat successively.  Heart-leaved buttercup may be better able to tolerate the neighborhood of taller vegetation, and is possibly becoming the commoner species in the absence of mowing. 

The lower growth form of prairie buttercup makes it a good rock garden plant, capable of producing small patches of brilliant gold flowers in early May.

Similar but uncommon species

Travellers in Alberta may come across two other native species with similarly differentiated leaves: the basal leaves are simple, toothed or entire and on long stalks, and the stem leaves are more or less stalkless and deeply divided.  Further south in the prairie grasslands, sagebrush or shining-leaf buttercup (Ranunculus glaberrimus) may be uncommonly encountered. It closely resembles prairie buttercup but is completely hairless and has larger petals. Graceful buttercup (Ranunculus inamoenus) is another look-alike most likely to be found in the mountains: its conspicuous upper leaves overlap the flower stalks. 

Tall buttercup (Ranunculus acris) and creeping buttercup (Ranunculus repens)

The common names of these two non-native species indicate their manner of growth. Tall buttercup (Ranunculus acris) is a tall, erect plant that can form appreciable populations by spreading from short rhizomes, but as its common name suggests, creeping buttercup produces extensive patches from prostrate stems that root at the nodes in moist soil. The flower stems grow upright. 

Tall buttercup (Ranunculus acris) at Watson Creek Provincial Recreation Area, near Cadomin, 2023-07-14.  Photo: P. Cotterill.

Tall buttercup has long-stalked basal leaves that are 3-5 palmately divided into narrower, lobed and toothed segments, the whole leaf roughly circular in shape. The stem leaves are similar, but short-stalked and smaller, usually being cleft into three divisions. The flowers are about 1 cm across, with conspicuously hairy undersides to the sepals, and the smooth achenes form a rounded head. 

Ranunculus acris is not especially common in the Edmonton area, but it occurs along some trails in Elk Island National Park, where hooved animals may help to spread its seeds in their tracks, which retain moisture. It is much more common in the moister climate of the foothills and mountains, where it may be a relatively common component of hayfields and roadsides. At the mining hamlet of Cadomin, southeast of Hinton, it is present in large expanses of yellow pasture, no doubt from an earlier historical disturbance caused by traffic, and aided by abundant moisture from the mountains. 

Creeping buttercup (Ranunculus repens) found in waste ground in a park, Burnaby, BC, 2010-06-13.  Photo: P. Cotterill.

Creeping buttercup (Ranunculus repens) is usually a less hairy plant,  and has long-stalked basal leaves which are larger and looser than those of tall buttercup and are divided into three stalked leaflets. Stem leaves are smaller and relatively fewer. The flowers in both of these non-native species are large at 2-2.5 cm and the achenes are clustered into subglobose heads. 

Creeping buttercup is even less common in our area than tall, but it does form two or three extensive patches in different places close to the creek in Whitemud Park South in Edmonton. Its occurrence there is likely related to the fact that some bridge renovation work was carried out in recent years. 

Both species are typical flowers of hayfields in northern Europe. 

All buttercups contain a toxin, a bitter tasting substance that deters herbivores. (This toxin is not present in dried hay). The sap can cause skin blisters. 

References

En.wikipedia.org(wiki)/Ranunculus

Flora of North America. Volume 3. Ranunculus.

Kershaw, Linda and Lorna Allen. 2020. Vascular Flora of Alberta: An Illustrated Guide. Self-published. Kindle Direct Publishing. 

Moss, E. H. 1983. Flora of Alberta. 2d ed. Edited by J. G. Packer. Toronto, University of Toronto Press.