Milkweed Species in Ontario

Adapted from: Native Milkweed Species in Ontario: A violation of the Weed Control Act or an Ecological Garden? - Presented by Donald Kirk (Ontario Ministry of Natural Resources) at the 2004 Weed Inspector's Conference.

Table of Contents

Milkweed and the Weed Control Act

Milkweeds are an important component of the native and naturalized vegetation communities of Ontario as they are an important nectaring plant for many species of Lepidoptera, such as the Monarch butterfly.

With the exception of common milkweed (Asclepias syriaca), most milkweeds are not invasive and generally do not pose a threat to agricultural or horticulture production. Although most milkweed species are poisonous to livestock only common milkweed (Asclepias syriaca) would normally be found in waste places and pastures where livestock graze.

Five species of milkweed are considered rare in the province (i.e. S Rank between S1 and S3) however as of yet none have been assigned a status designation by federal or provincial governments. Of all the milkweed species in Ontario, only the control of common milkweed (Asclepias syriaca) is enforced under the Weed Control Act. Enforcement under the Act will only occur if a common milkweed population is negatively affecting commercial agriculture or horticulture production.

A List of Milkweed Species Currently in Ontario


Common Milkweed (Asclepias syriaca)

S Rank: S5, (Common to widespread).

Characteristics:

  • Dense spherical umbels (flowers) at tip of stem and axils of upper leaves
  • Flowers greenish to purple or white
  • Pods, green, fleshy, covered with soft warty protuberences

Habitat: Pastures, meadows, roadsides, waste places

Range in Ontario: Very common throughout southern and northern Ontario.

To view a picture of Common Milkweed go to the Ontario Weeds Gallery.


Swamp Milkweed (Asclepias incamata)

S Rank: S5, (Common to widespread).

Characteristics:

  • Narrow-lanced shaped leaves, smooth
  • Flowers dull pink to rose-purple (more striking than Common milkweed)
  • Slender pods
  • Smoothed stem

Habitat: swamps and wet meadows, ditches, edges of streams, shorelines

Range in Ontario: Throughout Great Lakes region of Ontario

Photo of swamp milkweed

Figure 1. Photo of swamp milkweed displaying narrow-lanced shaped leaves, smooth, flowers dull pink to rose-purple, slender pods and smoothed stem. (Photo taken by Donald Kirk, Ministry of Natural Resources)


Butterfly-Weed (Ascleipias tuberose)

S Rank: S4, (Common to apparently secure).

Characteristics:

  • Distinct showy orange flower (sometimes yellow)
  • Alternate leaves
  • Leaves lanced shaped
  • Juice not milky
  • Narrow pods

Habitat: Sandy fields especially tall grass prairie and savanna communities.

Range in Ontario: Mostly Carolinian regions especially in sandy meadows, tall grass prairies and railway rights of way. Butterfly-weed is localized to prairie remnants in rest of province.

Photo of distinct showy orange flower (sometimes yellow), alternate leaves; leaves lanced shaped with narrow pods.

Figure 2. Photo of distinct showy orange flower (sometimes yellow), alternate leaves; leaves lanced shaped with narrow pods. (Photo taken by Donald Kirk, Ministry of Natural Resources)


Poke Milkweed (Asclepias exaltata)

S Rank: S4, (Common to apparently secure)

Characteristics:

  • Creamy white flowers tinged with lavender or green
  • Flowers in loose drooping umbels arising in axils of leaves

Habitat: A species of woodlands and woodland edges.

Range in Ontario: Throughout most of Southern Ontario, however most common in the Carolinian regions.

Photo of Poke Milkweed with creamy white flowers tinged with lavender or green.  The flowers are loose dropping umbels arising in axils of leaves.

Figure 3. Photo of Poke Milkweed with creamy white flowers tinged with lavender or green. The flowers are loose dropping umbels arising in axils of leaves. (Photo taken by Donald Kirk, Ministry of Natural Resources)


Purple Milkweed (Asclepias purpurascens)

S Rank: S2, (Very rare in Ontario, between 5 to 20 occurrences).

Characteristics:

  • Leaf veins form acute angle with midrib
  • Flowers deep magenta-red
  • Pods downy but not warty

Habitat: Tallgrass prairie and alvar sites. Dry to moist meadows and thickets.

Range in Ontario: Lambton, Kent and Essex counties.

Photo of Purple Milkweed with leaf veins from acute angle with midrib.  Flowers are deep magenta-red in colour.  The pods are downy but not warty.

Figure 4. Photo of Purple Milkweed with leaf veins from acute angle with midrib. Flowers are deep magenta-red in colour. The pods are downy but not warty. (Photo taken by Donald Kirk, Ministry of Natural Resources)


Sullivant's Milkweed (Asclepias sullivantii)

S Rank: S2 (Very rare in Ontario, between 5 to 20 occurrences)

Characteristics:

  • Leaves nearly stalkless and smooth
  • Pods slender and smooth
  • Flowers similar in colour to Common Milkweed

Habitat: Moist soils in tall grass prairie communities.

Range in Ontario: Lambton, Kent and Essex counties.


Photo of Sullivant's Milkweed with the leaves nearly stalkless and smooth.

Figure 5. Photo of Sullivant's Milkweed with the leaves nearly stalkless and smooth. The pods are slender and smooth and the flowers are similar in colour to common milkweed. (Photo taken by Donald Kirk, Ministry of Natural Resources)


Whorled Milkweed (Asclepias verticillata)

S Rank: S2 (Very rare in Ontario, between 5 to 20 occurrences).

Characteristics:

  • Narrow linear leaves in whorls of 3 to 6
  • Flowers are greenish-white in leaf axils

Habitat: Dry fields, tallgrass prairies, sandy meadows and barrens, roadsides, alvars. Whorled milkweed is sometimes adventive.

Range in Ontario: Norfolk, Essex and Kent counties.

Photo of Whorled Milkweed with narrow linear leaves in whorls of 3 to 6.  The flowers are greenish-white in leaf axils.

Figure 6. Photo of Whorled Milkweed with narrow linear leaves in whorls of 3 to 6. The flowers are greenish-white in leaf axils. (Photo taken by Donald Kirk, Ministry of Natural Resources)


Green Milkweed (Asclepias viridiflora)

S Rank: S2 (Very rare in Ontario, between 5 to 20 occurrences).

Characteristics:

  • Green flowers in dense nearly stalkless umbel
  • Leaves are rough, thick, lance-shaped and in pairs
  • Green milkweed is often reclining and never tall

Habitat: Dry sandy meadows, bluff prairies, sand dunes, alvars

Range in Ontario: Brant and Norfolk Counties, Pinery Provincial Park, Rondeau, Point Pelee, and Manitoulin Island.


Photo of Green Milkweed with green flowers in dense nearly stalkless umble.  The leaves are rough, thick, lance-shaped and in pairs.

Figure 7. Photo of Green Milkweed with green flowers in dense nearly stalkless umble. The leaves are rough, thick, lance-shaped and in pairs. (Photo taken by Donald Kirk, Ministry of Natural Resources)


Prairie Milkweed (Asclepias hirtella)

" No information on this species at this time.


Black Swallow-Wort (Cynanchum nigrum) - Invasive

Characteristics:

  • Dark purple flowers in leaf axils
  • Pointed and paired leaves - opposite
  • Slender pods similar to other milkweeds
  • A trailing vine that forms dense mats and thickets

Habitat: A very aggressive invasive species. Roadsides, railways, ravines, fence lines, thickets, woodlands.


Range in Ontario: Very abundant in urban settings however spreading into rural areas and natural habitats.

Photo of Black Swallow-Wort.

Figure 8. Photo of Black Swallow-Wort. The flowers are dark purple in leaf axils. They are pointed and paired leaves. The pods are slender and similar to other milkweeds. (Photo taken by Donald Kirk, Ministry of Natural Resources)

Related Links


For more information:
Toll Free: 1-877-424-1300
E-mail: ag.info.omafra@ontario.ca
Author: Mike Cowbrough, Weed Management Field Crops Program Lead/OMAF
Creation Date: 22 March 2005
Last Reviewed: 22 March 2005