By Patsy Cotterill, WF News, July 2023
In this article I’ll take a look at the three species of the dogbane family (Apocynaceae) that are all currently in bloom (as I write in the third week of June) in some of our local natural areas.
Spreading dogbane – common locally in well-drained soils
Spreading dogbane (Apocynum androsaemifolium) is a branched, perennial herb growing 20-100 cm tall. Its reddish stems bear paired, opposite, oval, smooth-edged leaves. The pink or pink-striped flowers (6-9 mm) are bell-like with a tubular corolla and five spreading corolla lobes. They are borne in pairs in nodding clusters at the ends of the branches. There are two ovaries per flower and they form twin, narrow, reddish seed pods which at maturity can be up to 12 cm long. Each pod contains numerous seeds, each equipped with a tuft of silky white hairs that aids in dispersal.
The plant exudes a sticky, milky white latex when conducting tissue from stem and leaves is broken. This contains organic compounds called cardiac glycosides toxic to most animals; in the right amounts, however, they can be used therapeutically to treat heart disease. (What doesn’t kill you makes you stronger?!)
Spreading dogbane occurs in a variety of habitats including open forests, grassland and roadsides, but definitely prefers well-drained, sandy soils. It is rhizomatous, forming open patches, especially where disturbance has occurred. It has a wide distribution across North America.
Hemp dogbane (Indian hemp) in the North Saskatchewan River Valley
Its relative, hemp dogbane or Indian hemp (Apocynum cannabinum), occurs mostly in woods near shores, and is common in certain parts of Edmonton’s North Saskatchewan River valley.
It can be distinguished easily from spreading dogbane when in flower because its flowers are white or greenish-white and smaller (2-4 mm), with erect lobes, so appear much less showy than those of its congener. Indian hemp gets its common name from the strong fibres in the stems used by Indigenous people. A hybrid of these two species also occurs naturally.
Spreading dogbane is flowering profusely in Fort Saskatchewan Prairie
Spreading dogbane is having an exceedingly good year in Fort Saskatchewan Prairie, having appeared in large numbers in its open woods and grassy areas this summer. We attribute this to the clearing of smooth brome grass and other non-native ground cover that has been done in the last two years, both through herbiciding, and AltaLink’s maintenance of their powerline right-of-way. Spreading dogbane is an early successional species so creation of disturbed, more open ground gives it an advantage. However, other factors may also be at play in its current spread and copious flower production.
The family name Apocynaceae is derived from the genus Apocynum, in turn coming from two Greek words meaning “away” and “dog” because the milky latex present in nearly all of its members was used as a dog poison. Other members of the family besides the dogbanes and milkweeds include the periwinkles (Vinca), and the common house plant, waxflower (Hoya carnosa). Visitors to warmer climes and gardeners may be familiarwith oleander (Nerium oleander) and frangipani (Plumeria). Strophanthus sarmentosus is the source of cortisone.
About three years ago I was seriously concerned that we were losing our populations of the species. Change is a constant in plant populations, but this change is striking.
Low milkweed – also in high numbers
Low milkweed (Asclepias ovalifolia), is also having an excellent year of growth and expansion in Fort Saskatchewan Prairie and, no doubt, elsewhere.
It is related to spreading dogbane and hemp dogbane and has several characteristics in common with these species. It was once in its own family, the milkweed family, Asclepiadaceae, but this has now been transferred to the Apocynaceae as a sub-family, Asclepiadoideae.
Of the three milkweed species in Alberta, low milkweed is the only one that grows in our area. It can grow to 60 cm tall as an unbranched perennial forb, and has opposite, ovate-lanceolate, dark-green, shortly hairy leaves. The flowers are in flat-topped clusters and are small and white or yellowish-white with a beautifully intricate structure based on flower parts in fives. The dual ovaries give rise to characteristic large, soft, ovoid or spindle-shaped, pointed pods in pairs, although one pod often aborts. The pods split open to release flattish brown seeds each with an attached tuft of pure-white, silky hairs.
Low milkweed (indeed all the milkweeds) exude the same sticky, white, toxic latex as the dogbanes, which serves to deter herbivores. (See the note below on the use by the Monarch butterfly of the defensive potential of cardiac glycosides.)
We attribute its prominence in Fort Saskatchewan Prairie to the same recent clearing activities, as well as other possible environmental factors. Low milkweed spreads readily by rhizomes so is favoured by disturbance and reduced competition, and thrives in coarse-grained, sandy soils.
Our hope is that its high numbers this year will attract Monarch butterflies; indeed, serendipitously, a university student is monitoring several patches in the Prairie this year for caterpillar activity.