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Articles : The Milkweeds of Canada (Asclepias spp.)

Status, Distribution, and Potential Impact from Noxious Weed Legislation

2.0 SPECIES ACCOUNTS
The following section presents information on the 14 species of Milkweeds (Asclepias spp.) known to occur in Canada. The species are arranged in alphabetical order and the format generally follows the style of COSEWIC Status Reports (Haber and Members of the Subcommittee for Vascular Plants, Mosses, and Lichens, 1994) except the following treatments are more brief than formal Status Reports. Plant nomenclature follows Kartesz (1994). The species descriptions are based on Gleason (1968) and herbarium specimens. Total range many are provided for all species. Provincial range maps are provided for rare species where recent maps available. To illustrate each species, there is a photocopy of a herbarium specimen from the Canadian Museum of Nature, Ottawa. The species habitats are based on relevant floras and data from herbarium specimens. The total range maps are based on Woodson (1954), and the most recent floras and rare plant publications. A literature search of relevant journal articles was performed using the library computer database AGRICOLA and BIOSIS.


Asclepias exaltata L. - Poke Milkweed

Description

This Milkweed grows to 1.5 m tall and has opposite leaves that are broadly elliptic and taper at both ends. The flower clusters are loosely few-flowered on spreading or drooping stalks. The flower color is white to pink or pale dull purple and the hoods of each flower support a crown of five incurved horns.

Illustration

An illustration can be found in Gleason (9168) vol 3, page 76.

Biological and Economic Significance

Poke Milkweed is an uncommon to rare plant of open woods whose natural significance is largely unknown. The species could be used occasionally as a food source by the larvae of Monarch Butterfly (Danaus plexippus), however, due to its limited range, scarcity, and woodland habitat, it is probably not an important source of food.

Distribution

Asclepias exaltata has a wide range in the eastern United States from New England to Minnesota and south to Georgia. In Canada it occurs only in southern Ontario and southwestern Quebec.

Figure: North American distribution of Asclepias exaltata

General Habitat

Open woods and clearing in moist to dry upland woods and thickets.

Population Biology

In southwestern Ontario, Oldham et al. (1991) report Poke Milkweed to be very localized, often with only a few plants at each locality. Wilbur (1976) reports colonies of from 10 to several hundred plants in Michigan. Shannon and Wyatt (1986) reported on four colonies in Virginia with 16 to 40 plants each.

Poke Milkweed does not reproduce vegetatively (Shannon & Wyatt, 1986). This Milkweed suffers extensively from herbivore damage but it has a low mortality due to its large root storage capacity (Wilbur, 1976).

Evidence of Threats to Survival

No imminent threats are known. Due to its woodland habitat, it is not likely to be targeted by "weed control" programs even though it bears a superficial resemblance to Common Milkweed (Asclepias syriaca).

Present Legal or Other Formal Status

Poke Milkweed has no legal protection, however, it is considered rare in Quebec in Bouchard et al., (1983). Its status in the United States is unknown.

Assessment of Status

Asclepias exaltata is rare in Quebec and uncommon in southern Ontario.


Asclepias hirtella (Pennell) - Prairie Milkweed

Description

This Milkweed grows to one metre tall and has opposite or sub-opposite leaves that are linear or lance-shaped and taper at both ends. The flower clusters are densely-flowered and occur in the upper leaf axils. The hoods of each flower lack horns but the former are separated from the drooping petals by a distinct column.

Illustration

An illustration can be found in Gleason (1968) vol 3, page 80.

Biological and Economic Significance

This Milkweed is a very rare plant whose natural significance is largely unknown. The species could be used occasionally as a food source by the larvae of Monarch Butterfly (Danaus plexippus), however, due to its very limited range and extremely rarity, it is probably not a significant food source.

Distribution

Asclepias hirtella has a moderate range in east-central United States from Michigan and Minnesota south to Texas. In Canada it occurs only in extreme southwestern Ontario.

Figure: North American distribution of Asclepias hirtella

Figure: Ontario distribution of Asclepias hirtella

General Habitat

Dry, sandy soil.

Population Biology

In southwestern Ontario, Oldham et al., (1991) report it to be very localized, often with only few plants at each locality.

Evidence of Threats to Survival

No imminent threats are known. Due to its woodland habitat, it is not likely to be targeted by "weed control" programs.

Present Legal or Other Formal Status

This Milkweed has no legal protection, however, it is considered rare in Ontario in Argus & Keddy (1984). It is considered to be rare in Canada and is given the highest priority ranking of "1" in Argus & Pryer (1990) because of its absence or rarity in bordering states and because it is considered to be "N1" nationally and "S1" in Ontario where "1" indicates "critically imperiled because of extreme rarity". Also regard as "S1" in Ontario (extremely rare" in Oldham (1994).

In the United States, it is regarded as a "G5" species (Argus & Pryer, 1990) which means that it is "abundant and demonstrably secure with many occurrences" - Mississippi; and status uncertain or unknown - Georgia and Tennessee.

Assessment of Status

Asclepias hirtella is very rare in southwestern Ontario - its only Canadian occurrence.


Asclepias incarnataL. - Swamp Milkweed

Description

This Milkweed grows to 1.5 m tall and has opposite leaves that are lance-shaped and taper at both ends. The flower clusters are on upright or spreading stalks. The flower colour is pink to red and the hoods of each flower support a crown of five incurved horns.

Illustration

An illustration can be found in Gleason (1968) vol 3, page 75.

Biological and Economic Significance

Swamp Milkweed is a common to uncommon plant of wet woods, ditches, and wetland edges whose natural significance is largely unknown. The species is used occasionally as a food source by the larvae of Monarch Butterfly (Danaus plexippus), however, due to its scattered occurrence and wetland habitat, it is probably not an important food source.

Distribution

Asclepias incarnata has a wide range in the United States from Maine to North Dakota and south to Florida and Texas. In Canada, it occurs in southern Nova Scotia, Prince Edward Island, southwest New Brunswick, southern Quebec, southern Ontario, and southeastern Manitoba.

Figure: North American distribution of Asclepias incarnata

General Habitat

Swamp Milkweed prefers the moist soil of wet woods, ditches, riverbanks, and wetland edges.

Population Biology

In eastern Ontario, it occurs singly or with only a few plants at each locality (DJW, pers obs.). Wilbur (1976) reports it to be generally of scattered occurrence in a study area in Michigan, but reports one linear colony along a wetland edge consisting of from 400 to 500 plants. Vegetative reproduction occurs infrequently, if at all (Kephart, 1981). Individual plants tend to be short-lived--five to ten years at most--because its habitat of floodplains and wetland edges is very dynamic and prone to erosion, slumping, etc., (Wilbur, 1976). Swamp Milkweed tends to suffer little damage from herbivores and may bloom in its first year after germination (Wilbur, 1976).

Evidence of Threats to Survival

No imminent threats are known. Due to its wetland habitat, it is not likely to be targeted by "weed control" programs.

Present Legal or Other Formal Status

Swamp Milkweed has no legal protection but it is considered to be rare in Nova Scotia (Maher et al., 1978). It is considered common or uncommon in the rest of its Canadian range. Its status in the United States is unknown.

Assessment of Status

Asclepias incarnata is rare in Nova Scotia, but is relatively common from New Brunswick to Manitoba.


Asclepias lanuginosa Nutt. - Hairy Milkweed
(Asclepias otariodes auct. non Fourn.:Acerates lanuginosa (Nutt.) Dcne.)

Description

This generally hairy Milkweed grows to 30 centimetres tall and has alternate or sub-opposite leaves that are oblong or lance-shaped. The single flower cluster is terminal and the hoods of each greenish flower are without horns.

Illustration

An illustration can be found in Gleason (1968) vol 3, page 80.

Biological and Economic Significance

This Milkweed is a very rare plant whose natural significance is largely unknown. The species could be used occasionally as a food source by the larvae of Monarch Butterfly (Danaus plexippus), however, due to its very limited range and extreme rarity, it is probably not a significant food source.

Distribution

Asclepias lanuginosa has a limited range in the central United States from North Dakota to Illinois and south to Kansas. In Canada it occurs only in a small portion of southwestern Manitoba. According to Morton & Venn (1990), reports of this species from Ontario--such as in Scoggan (1978-1979)--are based on misidentifications of Butterfly-weed (Asclepias tuberosa).

Figure: North American distribution of Asclepias lanuginosa

Figure: Manitoba distribution of Asclepias lanuginosa

General Habitat

Sandhills.

Population Biology

The average population size for this Milkweed is unknown.

Evidence of Threats to Survival

No imminent threats are known. Due to its rarity and sandhills habitat, it is not likely to be targeted by "weed control" programs.

Present Legal or Other Formal Status

This Milkweed has no legal protection, however, it is considered rare in Manitoba in White & Johnson (1980). It is considered rare in Canada and is given a priority ranking of "4" in Argus & Pryer (1990) despite being considered to be "N1" nationally and "S1" in Manitoba where "1" indicates "critically imperiled because of extreme rarity."

In the United States, it is regarded as a "G?" species (Argus & Pryer, 1990) which means that its status is "unknown".

The status in individual states from Argus & Pryer (1990) is as follows: "S1" (Critically imperiled) - Iowa and Illinois; "S2" (Imperiled because of rarity) - Wisconsin.

Assessment of Status

Asclepias lanuginosa is very rare in southwestern Manitoba--its only Canadian occurrence.


Asclepias ovalifolia Dcne. - Oval-leaved Milkweed

Description

This slender Milkweed grows to 50 centimetres tall and has opposite leaves that are oblong or elliptic shaped. The single flower cluster is terminal although occasionally there may be a few additional flower clusters in the upper leaf axils. The hoods of each greenish flower are without horns.

Illustration

An illustration can be found in Gleason (1968) vol 3, page 77.

Biological and Economic Significance

This Milkweed is a very rare plant whose natural significance is largely unknown. The species could be used occasionally as a food source by the larvae of Monarch Butterfly (Danaus plexippus), however, due to its somewhat limited range, it is probably not a significant food source.

Distribution

Asclepias ovalifolia has a limited range in the central United States from Illinois and Wisconsin to South Dakota. In Canada it occurs in southeastern British Columbia, southern and central Alberta, southern Saskatchewan, southern Manitoba, and extreme northwestern Ontario.

Figure: North American distribution of Asclepias ovalifolia

Figure: Ontario distribution of Asclepias ovalifolia

Figure: British Columbia distribution of Asclepias ovalifolia

General Habitat

Silty riverbanks, open woods, thickets, roadside ditches, sand dunes, and slopes of valleys.

Population Biology

The average population size for this Milkweed is unknown.

Evidence of Threats to Survival

No imminent threats are known. Due to its riverbank and thicket habitat, it is not likely to be targeted by "weed control" programs even though it bears a superficial resemblance to Common Milkweed (Asclepias syriaca).

Present Legal or Other Formal Status

This Milkweed has no legal protection, however, it is considered rare in British Columbia in Straley et al., (1985) and rare in Ontario in Argus & Keddy (1984). Since it is not rare in all of its Canadian range, it has no priority ranking in Argus & Pryer (1990). It is regarded as "SH" in Ontario (known from old records only) in Oldham (1994). Its status in the United States in unknown.

Assessment of Status

Asclepias ovalifolia is very rare in northwestern Ontario and in southeastern British Columbia. It is relatively common in Manitoba and in southern Alberta.


Asclepias purpurascensL. - Purple Milkweed

Description

This Milkweed grows to one metre tall and has opposite or sub-opposite leaves that are broad in the middle and taper at both ends. The flower clusters are many-flowered and are terminal or sometimes also in the uppermost leaf axils. The hoods of each flower hide the much-shorter, incurved horns. The flowers are a distinctive, deep magenta-red. The seed pods are downy.

Illustration

An illustration can be found in Gleason (1968) vol 3, page 77.

Biological and Economic Significance

This Milkweed is a very rare plant whose natural significance is largely unknown. The species could be used occasionally as a food source by the larvae of Monarch Butterfly (Danaus plexippus), however, due to its very limited range and extreme rarity, it is probably not a significant food source.

Distribution

Asclepias purpurasens has a moderate range in east-central United States from Minnesota to New England and south to Arkansas and Kansas. In Canada it occurs only in extreme southwestern Ontario.

Figure: North American distribution of Asclepias purpurascens

Figure: Ontario distribution of Asclepias purpurascens

General Habitat

Dry to moist thickets, open woods, and prairies.

Population Biology

In Michigan, Wilbur (1976) reports a population of 32 plants--the only one in his study area. It rarity in the Michigan study area may be due to its low seed production and high level of damage by herbivores (Wilbur, 1976).

Evidence of Threats to Survival

No imminent threats are known. Due to its rarity and thicket habitat, it is no likely to be targeted by "weed control" programs even though it bears a superficial resemblance to Common Milkweed (Asclepias syriaca).

Present Legal or Other Formal Status

This Milkweed has no legal protection, however, it is considered rare in Ontario in Argus & Keddy (1984). It is considered to be rare in Canada and is given the priority ranking of "3" in Argus & Pryer (1990) because of its absence or rarity in bordering states and because it is considered to be "N2" nationally and "S2" in Ontario where "2" indicates "imperiled because of rarity". Also regarded as "S2" in Ontario (very rare) in Oldham (1994).

In the United States, it is regarded as a "G4G5" species (Argus & Pryer, 1990) which means that it is "apparently secure to abundant and demonstrably secure with many occurrences".

The status in individual states from Argus & Pryer (1990) is as follows: "SH" (Historical-not-recorded since 1900) - Vermont; "S1" (Critically imperiled) - ?Georgia, New Hampshire, New York, and Wisconsin; "S3" (Rare or uncommon) - Maryland; "S4?" (Apparently secure with many occurrences) - Mississippi; and status uncertain or unknown - Minnesota and Tennessee.

Assessment of Status

Asclepias purpurascens is very rare in extreme southwestern Ontario--its only Candian occurrence.


Asclepias quadrifolia Jacq. - Four-leaved Milkweed

Description

Four-leaved Milkweed grow to 50 centimetres tall and has narrow to oval-shaped leaves that are broadest in the middle and taper at both ends. The leaf arrangement is quite distinctive because the leaves normally occur at three nodes--the upper and lower node each support a pair of small leaves while the middle node has a whorl of four much larger leaves. The flower clusters are many-flowered and are terminal or sometimes one or more clusters occur also in the uppermost leaf axils. The hoods of each flower hide the much-shorter, incurved horns. The flowers are white, pink, or lavender and the seed pods are very slender on erect stalks.

Illustration

An illustration can be found in Gleason (1968) vol 3, page 78.

Biological and Economic Significance

This Milkweed is a very rare plant whose natural significance is largely unknown. The species could be used occasionally as a food source by the larvae of Monarch Butterfly (Danaus plexippus), however, due to its very limited range and extreme rarity, it is probably not a significant food source.

Distribution

Asclepias quadrifolia has a moderate range in east-central United States from New England to Iowa and south to Oklahoma and Georgia. In Canada it occurs only in extreme south Ontario.

Figure: North American distribution of Asclepias quadrifolia

Figure: Ontario distribution of Asclepias quadrifolia

General Habitat

Open, dry woods.

Population Biology

The average population size for Four-leaved Milkweed is unknown. This species cannot reproduce vegetatively (Cabin et al., 1991).

Evidence of Threats to Survival

No imminent threats are known. Due to its rarity and woodland habitat, it is not likely to be targeted by "weed control" programs.

Present Legal or Other Formal Status

This Milkweed has no legal protection, however, it is considered rare in Ontario in Argus & Keddy (1984). It is considered to be rare in Canada and is given the priority ranking of "3" in Argus & Pryer (1990) because of its absence or rarity in bordering states and because it is considered to be "N1" nationally and "S1" in Ontario where "1" indicates "critically imperiled because of extreme rarity". It is regarded as "SH" in Ontario (known from old records only) in Olham (1994).

In the United States, it is regarded as a "G5" species (Argus & Pryer, 1990) which means that it is "abundant and demonstrably secure with many occurrences".

The status in individual states from Argus & Pryer (1990) is as follows: "SH" (Historical-not-recorded since 1900) - Delaware; "S1" (Critically imperiled) - Kansas, New Hampshire, and Rhode Island; "S3" (Rare or uncommon) - Vermont and Iowa.

Assessment of Status

Asclepias quadrifolia is very rare in extreme southern Ontario - its only Candian occurrence.

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