More info for the terms:
corpusculum,
follicle,
forb,
fresh,
fruit,
monoecious,
root crown,
seed,
warm-seasonIn the Willamette Valley of Oregon, showy milkweed grows primarily in open grasslands.
It sometimes occurs in riparian zones and savannas with small trees [27].
In plains grasslands of western Nebraska, showy milkweed is found in grama-buffalo grass (Bouteloua
spp.-Buchloe dactyloides), grama-needle-and-thread grass-western wheatgrass (Hesperostipa
comata-Pascopyrum smithii), and sandsage-bluestem (Artemisia filifolia-Andropogon/Schizachyrium
spp.) grasslands. Green needlegrass (Stipa viridula) and threadleaf sedge (Carex filifolia)
are common codominants within these grasslands [31].
BOTANICAL AND ECOLOGICAL CHARACTERISTICS
SPECIES: Asclepias speciosa
Charles Weber ©California Academy of Sciences
GENERAL BOTANICAL CHARACTERISTICS: This description provides characteristics that may be relevant to fire ecology,
and is not meant for identification. Keys for identification are available [
6,
12,
17,
22,
28,
30,
34,
38,
39,
44,
60,
61,
63].
Showy milkweed is a warm-season perennial forb [
17,
22,
29,
38,
61,
63]. Stems are
ascending to erect and 19.7 to 47.2 inches (50.0-119.9 cm) tall [
6,
17,
22,
24,
28,
38,
44,
61,
63].
Leaves are 2.4 to 7.9 inches (8-20 cm) long
and ovate-lanceolate [
6,
22,
24,
44,
61]. The inflorescence is a large, showy umbelliform cymes [
6,
22,
63]. At 0.59 to
1.10 inch (15-28 mm) wide, they are the largest of all Asclepias species
[
38,
61]. The fruit is a 2.4- to 4.7- inch (6-12 cm)
follicle [
6,
17,
22,
24,
44]. Seeds are elliptic and 0.24 to 0.35 inch (6-9 mm) long
[
6,
17,
22,
63], with hair-like tufts (see photo above).
Showy milkweed has "deep" rhizomes [
10,
22].
RAUNKIAER [49] LIFE FORM:
GeophyteHemicryptophyte
REGENERATION PROCESSES: Showy milkweed reproduces both sexually and vegetatively.
Breeding system: Showy milkweed is
monoecious.
Most Asclepias spp. are self-incompatible, requiring crosses between genetically
different individuals to produce viable seeds [
11,
63,
65].
Pollination: Showy milkweed is pollinated by wind and insects. Insect pollination
is accomplished by lepidopterans and hymenopterans. These
insects remove the pollen packet, or pollinarium, when a groove in the plant's
corpusculum entraps the hairs or appendages of the insect. The arms of the pollinarium twist, facilitating
insertion of the pollinarium into the stigmatic chamber of another flower as the
pollinating insect continues foraging [
9,
27,
63,
65]. The flowers
of Asclepias spp. produce large amounts of nectar, which serves as a
germination medium for the pollen [
65].
Seed production: Stevens [
54] determined that showy milkweed produces an average of 630 seeds per stem
with a weight of 0.208 ounce (5.890 g) per 1,000 seeds.
Seed dispersal: Seeds are
dispersed when wind catches the hair-like seed tufts. [10]. Showy milkweed's
persistence along waterways [6,12,17,28,30,34,38,42,44,53,60,61]
also suggests dispersal by water [14].
Seed banking: Seed may be stored in water or in soil, as shown in the following studies. In
a study by Bruns [
14] in the Chandler Power Canal in Prosser, Washington, showy
milkweed seeds survived in fresh water for several months. Three sets of seeds
were submerged in fresh water up to 12 months to
determine viability. Extended viability of seeds indicates the potential
of seeds trapped or carried in irrigation water as sources of potential sources
of colonization. The following table represents the percentage of seeds
that were decomposed after 3 to 12 months of submergence in fresh water, and
germination of the submerged and dry-stored seeds after the same periods [
14]:
Months after test initiated 0 3 6 9 12
Percent of submerged seeds that decomposed ---- 1 2 48 48
Percent germination of submerged seeds ---- 96 90 5 45
Percent germination of dry-stored seeds 4 14 14 26 12
Chepil's [
15] study suggests that showy milkweed has a short-lived soil seedbank.
In a study performed in Saskatchewan, showy milkweed seeds were tested for
longevity, periodicity of germination, and viability of seeds in cultivated soil.
One thousand seeds were mixed in a 2.5-inch
(6.4 cm) layer of sterilized
soil in the field. From 15 November to 31 March, the soil was turned with a
trowel to a 3-inch (7.6 cm) depth 2-3 times, then sown to spring wheat (Triticum aestivum) or
cereal barley (Hordeum vulgare) in alternate years.
Ninety-five percent or more of the showy milkweed seeds germinated during the 1st year, with
the greatest germination from 7 May to 31 May, though substantial
amounts of seeds germinated until 31 July. Overall, showy milkweed seed dormancy did not
exceed 2 years [
15].
Germination: Seeds can germinate in soil or when submerged in water, although submergence may lower the
germination rate if the seeds remain in water over 36 months.
The longevity of showy milkweed seeds in fresh water was
tested by Comes and others [
16]
in the Chandler Power Canal in Prosser, Washington. One hundred showy milkweed
seeds were collected from the field; dried; placed separately into nylon cloth
envelopes; placed in plastic screen bags; and
submerged 11.8 inches (30 cm) beneath the water surface. Comparative germination tests were
made on seeds that had been stored dry in glass bottles at room temperature
[
16]:
Months after test initiated
3 6 9 12 24 36 48 60
%
Germination of submerged seeds 13 39 38 5 7 1 0 0
Submerged seeds remaining firm 55 47 32 14 7 3 0 0
Germination of dry-stored seeds 72 70 68 69 58 65 45 71
Germination in water was at a maximum level of 55% between 3 and 12
months, and no germination occurred after 48 months. Germination of dry-stored
seeds at the 3- to 6-month period ranged from 45% to 72% [
16].
Seedling establishment/growth: During the seedling stage, showy milkweed directs most of its energy
into root development. This contributes to drought tolerance, but the
aboveground portions of showy milkweed grow "very slowly" [
2].
Asexual regeneration: Showy
milkweed spreads clonally from deep rhizomes [
10,
22], and probably from from
sprouting from the root crown.
SITE CHARACTERISTICS: Showy milkweed prefers wet areas of prairie habitats and moist,
sandy soils along waterways, ditches, streambanks, floodplains, and washbottoms.
It is common along the banks of irrigation ditches
[
1]. It can become weedy in cultivated fields, roadsides, railways, "wastelands," and
fence rows [
6,
12,
17,
28,
30,
34,
38,
42,
44,
53,
60,
61].
Elevation: Showy milkweed occurs at the following elevations:
Arizona 6,000-9,000 feet (1,800-2,700 m) [
39]
California 0-600 feet (0-1,900 m) [
28]
Colorado 3,500-7,500 feet (1,100-2,300 m) [
24]
Nevada 2,200-8,500 feet (700-2,600 m) [
38]
New Mexico 6,000-9,000 feet (1,800-2,700 m) [
44]
Utah 2,700-8,500 feet (800-2,600 m) [
61]
Soil: Showy milkweed prefers moist soil [
6,
12,
17,
28,
30,
34,
38,
42,
44,
53,
60,
61].
It grows in all soil textures [
53] and tolerates alkaline soils [
17].
SUCCESSIONAL STATUS: Little research has been done addressing the successional status of showy
milkweed; however, it is considered a "weedy" species [
1,
17] that
occurs in early and mid-succession. It has the
ability to quickly colonize disturbed sites (Riser, personal communication [
50])
in open grasslands and partially shaded habitats.
SEASONAL DEVELOPMENT: Across showy milkweed's range, flowering occurs from May to August
[
10,
17,
22,
39,
44,
45,
63]. Follicles dry during August and September,
and seeds disperse in September and October [
10].
