Asclepias viridiflora, Green Comet Milkweed or Green Milkweed, is a native perennial herbaceous plant that blooms in the summer. In NJ, it is on the list of 'Endangered Plant Species and Plant Species of Concern' 2022.
One to four big flower clusters emerge at the top of the stem from the axils of the upper leaves. The nodding dense hemispherical shaped clusters are about 2 inches across with many flowers. All the flowers in the cluster originate from a single point and the cluster is called an umbel. Only one umbel forms at an axil of a pair of leaves.
These non-showy flowers are pale green and are about 1/4 inch across and 1/2 inch deep. It exhibits the usual milkweed structure but with one omission - it does not have horns.
Milkweed flower structure & Asclepias viridiflora, green milkweed:
Actually, all the milkweeds have similar structure. The flower has both male and female parts. The flower has 5 petals and 5 sepals.
Diagram of a general milkweed flower:
In the diagram (A) (c) are the reflexed petals. (h) are the hoods. In (B) at the top are the horns. The horns are just inside of the hoods. Green milkweed does not have the horns. In (C) is the pollen apparatus with 3 parts : corpusculum (d), translator arm & pollinia (pollen sac)
Internet Archive Book Images, No restrictions, via Wikimedia Commons
The green petals are folded back tight against the flower stalk.
The sepals are not easy to see when the flowers are open. At the bud stage, the sepals can be seen and they are pale green and just a few mm's long and hairy.
Inside/above the petals is a circle of 5 hoods. The hoods hold a plentiful supply of nectar which attracts insects. These hoods encircle the gynostegium. The gynostegium (whitish part in the center) is a column of partially fused stamens and the pistil (male & female parts).
There are 5 stamens. The anthers, in the gynostegium, are split in half with a slit between each half, called the stigmatic slit. The slit is where the legs of a pollinating insect would go and pick up pollen while the insect is partaking of the nectar in the hood.
Instead of individual pollen grains, the grains are together in a sac called the pollinium (pollinia - plural). There is a pair of pollinia inside of each slit. The pollinia are connected to the corpusculum which is the dark object outside at the top of the slit. The translator arm connects the corpusculum to the pollinia. These three parts, corpusculum, translator arm & pollinia, together are called a pollinarium. See diagram above.
Now for the female parts: In the gynostegium are 5 receptive stigmatic surfaces that are inside of 5 stigmatic chambers. Access to the receptive stigmatic surfaces is through the stigmatic slits. So both the pollen grains and the stigmatic surfaces are enclosed in the stigmatic chambers, with access only through the slits! Interestingly, each flower has two ovaries. Two stigmatic chambers are connected to one ovary and 3 stigmatic chambers are connected to the other ovary.
Here are some close up of the slit and the corpusculum (dark spot).
In the last of the 3 photos below, a pollinarium was left on top of the flower showing the dark spot, the brown arms, and the pale yellow pollen sac.
Pollinating insects must be compatible with the specialized flower structure of milkweeds. The interesting pollination process is described in detail in the Asclepias syriaca, common milkweed page with references: click here.
The green milkweed seed pods (follicles) are up to about 4 inches long, wider at the base and coming to a point at the tip. They are smooth or short hairy.
Plant - Leaves - Habitat
The green comet milkweed plant is a large upright perennial growing to 3 feet tall and at the top usually one or two flower umbels form. The stem is pale green and short-pubescent. The root system of green milkweed consists mainly of a stout fleshy taproot. The leaves are short petiolate (short-stalked) and opposite or almost opposite.
The leaves are simple, opposite or nearly opposite and paired but not whorled. The margins are smooth and sometimes wavy. The leaves are frequently not flat but wavy. The leaf stalks (petioles) are short.
The leaf shapes can be somewhat variable from narrower to wider. The leaves are somewhat coarse. They are in general elliptic-oblong (wider in the middle and rectangular with rounded ends). The leaves are about 4 inches long and 2 inches wide.
The leaf has a prominent midrib. The many smaller veins extending from the midrib to almost the edge have a distinctive pattern with the veins almost parallel to each other and perpendicular to the midrib.
Leaf upper surface is hairy.
Leaf lower surface is also hairy.
Like other milkweeds, when the leaves or stems of green milkweed are broken, they produce a milky latex. This liquid contain various levels of toxic cardiac glycoside compounds which makes the plants toxic to most insects and animals but not deer.
Rarity: In New Jersey Asclepias viridiflora is S2 (Imperiled) 2022 but globally secure G5.
Text by Millie Ling and all photos by Hubert & Millie Ling. Photos: Flowers - July 11, 2022, Hunterdon County, NJ.
Additional information / references:
- Illinois Wildflowers
- Lady Bird Johnson Wildflower Center
"Leaves are variable in shape with plants from dry sites having long narrow leaves and those from moist sites having round leaves."
- Missouri Plants
- The USDA website shows Asclepias viridiflora nativity in the US down to county level and other information: https://plants.usda.gov/home/plantProfile?symbol=ASVI
- NatureServe - rarity & distribution. https://explorer.natureserve.org/Taxon/ELEMENT_GLOBAL.2.156640/Asclepias_viridiflora
- Key to NJ Milkweeds, New England has almost the same milkweeds as NJ: GoBotany. https://gobotany.nativeplanttrust.org/dkey/asclepias/#all