2015 Kansas Wildflower of the Year
About Spider Milkweed
Spider milkweed (Asclepias viridis), also known as green antelopehorn, is the Kansas Native Plant Society (KNPS) 2015 Wildflower of the Year (WOY). Spider milkweed is 18” to 24” tall with green flowers showing May through July. It is found on dry prairies in the eastern 2/3 of the state with a substrate ranging from sand to limestone. An especially common species in prairie pastures, cattle do not find it palatable due to its production of toxic cardiac glycosides. The common names are given for the common presence of crab spiders hunting for insect prey around the flowers, and small antelope horn-like appearance of the seed pods. Previously belonging to the milkweed family (Asclepiadaceae,) the genus Asclepias is now classified in the subfamily Asclepiadoideae of the dogbane family (Apocynaceae.)
The plant resources committee chose this species primarily for its importance as a host plant for the monarch butterfly which has had a perilous population decline in recent years. According to Chip Taylor from Monarch Watch, Asclepias viridis is the next most desired host plant for monarchs after common milkweed (Asclepias syriaca). With importance to monarchs, showy flowers, exquisite seed pods, and a shorter stature and less weedy growth habit than common milkweed (perhaps making it more suitable for native landscaping), Asclepias viridis emerged as the best choice for this year’s KNPS WOY selection.
Spider Milkweed Species Account
The following species account comes from Growing Native Wildflowers by Harder and Platt, 1997. page 97.3.
Spider Milkweed or Green Antelopehorn
Derivation of Names
"Spider" - no explanation located for this term; "milkweed" - from the milky sap of the plant: "green" refers to the greenish flowers; "antelopehorn" - from the similarity of the shape of the pods to small antelope horns: Asclepias (from Greek) name of the Greek god of medicine and healing; viridis (from Latin) green.
Other Common Names
Oblong-leaved milkweed, green-flowered milkweed, greenhorn milkweed, green milkweed (this name is better applied to another species), antelopehorn.
Scientific Names (Synonyms)
Milkweed family (Asclepiadaceae). This species can be recognized as a milkweed by the similarity of its flowers to the typical milkweed flower structure (see Bare 1979) and the milkysap. Milkweeds of the genus Asclepias constitute a large group of plants native to North America, South America and Africa.
The spider milkweed is one of the earliest flowering milkweeds and has some of the largest milkweed flowers. The flowers are not strikingly colored but are interesting because of their unique floral structure. Spider milkweed in bloom accents the spring prairie with attractive clumps of pale green. The thick smooth gray-green stems may grow erect but often recline with only the ends erect. The plant is seldom more than one foot tall. Each stem is single or has few branches but there is often a clump of stems. The simple smooth leathery leaves are gray-green with pink-tinged veins and wavy margins. If any part of the plant is cut it exudes a milky white sap. Flowering occurs from early May (5/7) to July or later. The plant will often bloom in the fall when it has been cut earlier in the year. The flowers occur in loose rounded clusters, 3 to 5 inches (7 1/2 to 13 cm) across; each cluster on a stalk that arises near the tip of stem. Betz and Lamp (1992a) found 1 to 8 clusters per stem in the plants they examined. Each cluster has 1 to 23 flowers (Betz and Lamp 1992a), a relatively small number for a milkweed. The flowers are up to an inch in diameter (large for a milkweed) and the 5 green petals spread upward rather than being reflexed as in most milkweeds. In the center is a purple and white "star" composed of 5 radiating hoods and a central column (fused anthers and pistil).
Ecological Function and Importance
The spider milkweed belongs to the spring forb guild, a guild that has low coverage in the prairie but is a showy element in the spring (Kindscher 1994).
Growing Characteristics and Management
The spider milkweed is a low-growing perennial which has one to a few main stems that are usually sprawling. The leaves are mostly alternate on the stem although some may be almost opposite in arrangement. This plant has a thick taproot with horizontal lateral roots at various levels. Spider milkweed does not spread vegetatively and it does not produce large numbers of seedlings. It does not bloom until the second or third year after planting.
This plant is a source of a soft green dye (Marilyn Jones, personal communication).
Relations to Animals
Grazers do not normally eat this plant, and it is suspected of being poisonous (Stephens 1980). The nymphs of the orange and black small milkweed bug (Lygaeus kalmii) and large milkweed bug (Oneopeltus fasciatus) feed on seeds in the developing pods. Bumblebees commonly pollinate the flowers.
Seeds and Fruit
The erect pods (follicles) develop on deflexed stalks from the ends of stems. The pods are spindle-shaped, 3 to 6 inches (7 1/2- 15 mm) long. Betz and Lamp (1992a) reported an average of 1.9 (0-5) pods per stem in the plants they examined. As they mature, the pods turn from green to yellow to brown. They open along one side to release the flat dark brown elliptic seeds. Each seed is approximately 1/4 inch (7 mm) long and is attached to a tuft of hairs that allows it to be carried in the air. Betz and Lamp (1992a) found an average of 110.3 (85-154) seeds per pod.
The pods should be picked as they mature in late June or early July but before they open and release their seeds. A string tied around the middle of the pod when it is approaching maturity will prevent the release of seeds before the pod is collected A pod should be picked when a slit shows along one side and the seeds are dark brown (the pods may still be yellow). The pods can easily be opened and the seeds removed. The tufts of hairs on the seeds can be picked or burned off before planting (Snyder 1983).
We obtained no germination from seeds that had only been dry stratified. Moist stratification for 10 to 12 weeks resulted in a 56% germination rate (with 66-90 days of stratification. 154 out of 276 seeds germinated). There is evidence that seeds that are more than one year old germinate better. However seeds that are older than 4 years germinate poorly.
Unstratified seed can be planted in the field in autumn or stratified seed can be planted in spring. Seedlings can be started indoors in pots or in flats and transferred to pots when the second set of true leaves has formed. The seed should be planted 3/8 inch (10 mm) deep.
A pair of medium oval lightly veined cotyledons on a thin stalk emerge at germination. The first true leaves develop on an elongating stem between the cotyledons and are long and narrow, with smooth margins and pointed tips. Seedlings grow well in the greenhouse and can be transplanted to the field at 8 to 12 weeks.
Spider milkweed is found in the eastern 3/5 of Kansas. The species range extends from Ohio to southeastern Nebraska and central Kansas south to Florida and Texas.
Spider milkweed is found in well drained soils of tallgrass and mixed grass prairies and roadsides. It is more common on rocky, often limestone, or sandy soils.