MilkweedUSA

Milkweed glossary

The words this guide uses, defined plainly. Where a photograph explains a term better than a sentence can, there is one.

Common milkweed growing in a summer meadow among grasses and other wildflowers
Asclepias
The genus of true milkweeds, named after Asclepius, the Greek god of medicine. Roughly 73 species are native to North America. All 72 species →
Alternate
A leaf arrangement with one leaf per node, staggered up the stem. Unusual in milkweeds; butterfly weed and antelope horns are the best-known examples. How to read leaf arrangement →
Axillary
Borne in the angle between a leaf and the stem. Several milkweeds carry their flower clusters in the leaf axils rather than at the stem tip.
A banded yellow, black and white monarch caterpillar feeding on a milkweed leaf
Cardenolide
A cardiac glycoside, the class of compound that makes milkweed toxic by interfering with the sodium-potassium pump in animal cells. Monarchs are resistant and store it for their own defense. What the toxins do →
Cordate
Heart-shaped, with a notched base. Heartleaf milkweed is the classic western example. Heartleaf milkweed →
Corpusculum
The small dark clip that joins two pollinia and attaches the whole assembly to an insect leg. How milkweed pollination works →
Close view of common milkweed flowers, showing the five reflexed petals and the crown of five hoods above them
Corolla
The ring of five petals. In milkweeds they are reflexed, bent sharply backward away from the flower center. Reading a milkweed flower →
A monarch butterfly feeding at a cluster of pink common milkweed flowers in a summer meadow
Cultivar
A named selection propagated for particular traits, such as the white swamp milkweed sold as Ice Ballet. Cultivars of native species are sometimes called nativars. Milkweed cultivars →
Hundreds of monarch butterflies clustered together on conifer branches at an overwintering roost
Diapause
A state of suspended reproduction. Fall monarchs enter reproductive diapause before migrating, and evergreen tropical milkweed can prevent them from doing so. The migratory generation →
A restored Illinois prairie in flower, with milkweed and other native wildflowers among the grasses
Ecoregion
A region defined by shared climate, geology and vegetation. Buying seed from your own ecoregion gives plants adapted to your conditions. What grows where you live →
Empty milkweed pods on dry stems after the seed has blown away
Follicle
A dry fruit that splits along a single seam. The botanical term for a milkweed pod. Collecting seed →
Close view of flat brown milkweed seeds packed in overlapping rows inside an open pod
Floss
The silky parachute attached to each milkweed seed, technically the coma. Aquatic milkweed is the only North American species whose seeds lack it. Separating seed from floss →
Glaucous
Covered with a fine blue-gray waxy bloom, as on the leaves of clasping and heartleaf milkweed. Clasping milkweed →
Close view of common milkweed flowers, showing the five reflexed petals and the crown of five hoods above them
Hood
One of the five cup-like structures forming the crown of a milkweed flower. Hood shape is a key identification character. Reading a milkweed flower →
Horn
A curved projection arising inside a hood and arching over the flower center. Present in most but not all species. Reading a milkweed flower →
A single pale monarch egg and a newly hatched caterpillar on the underside of a milkweed leaf
Host plant
The plant a butterfly or moth lays eggs on and its larvae eat. Milkweed is the monarch’s only host plant genus. Why milkweed and nothing else →
Lanceolate
Lance-shaped: several times longer than wide, widest below the middle and tapering to a point. Swamp milkweed →
A common milkweed plant in leaf, the species whose broken stems bleed white latex
Latex
The white, sticky sap that gives milkweed its name. It defends the plant by gluing the mouthparts of small herbivores. Handling milkweed safely →
Nativar
A cultivated selection of a native species. Some perform as well as the straight species for wildlife; others do not, and the evidence varies by plant. Are nativars good for monarchs? →
Young seedlings growing on in pots, ready to plant out
Neonicotinoid
A class of systemic insecticide that moves throughout a plant’s tissues and persists for months. Neonicotinoid-treated milkweed kills the caterpillars it was bought to feed. Buying untreated plants →
Node
The point on a stem where leaves attach. Counting leaves per node tells you whether the arrangement is alternate, opposite or whorled.
OE
Ophryocystis elektroscirrha, a protozoan parasite of monarchs. Spores on milkweed leaves are eaten by caterpillars; infected adults emerge weakened and often deformed. How OE builds up →
Obovate
Egg-shaped but widest above the middle, like a paddle. Pineland milkweed is an example.
Opposite
A leaf arrangement with two leaves per node, directly facing each other. The commonest arrangement in the genus. How to read leaf arrangement →
Pedicel
The stalk of an individual flower within a cluster.
Peduncle
The stalk supporting a whole flower cluster. Clasping milkweed carries its single umbel on a conspicuously long peduncle.
Phyllotaxy
The technical term for leaf arrangement on a stem. Identifying by leaf arrangement →
Pollinarium
The complete pollen-transfer unit of a milkweed: two pollinia, two connecting arms and a corpusculum. How milkweed pollination works →
A bumblebee working across a rounded umbel of milkweed flowers
Pollinium
A waxy sac containing all the pollen from one anther, transferred whole rather than as loose grains. Only orchids and milkweeds do this. How milkweed pollination works →
Rhizome
An underground stem that spreads horizontally and sends up new shoots. Common milkweed’s rhizomes are why it colonizes so aggressively. A rhizomatous milkweed →
Scarification
Physically nicking or abrading a seed coat to speed water uptake. Rarely necessary for milkweed. Sowing milkweed seed →
Hands sowing seed into prepared soil
Stratification
A period of cold, moist conditions that breaks seed dormancy. Most milkweeds need about 30 days at 4°C before they will germinate reliably. How to stratify seed →
Stigmatic chamber
The slot in a milkweed flower into which a pollinium must be inserted for pollination to succeed. How milkweed pollination works →
Taproot
A single deep primary root. Taprooted milkweeds such as butterfly weed are drought-proof but almost impossible to transplant once established. A taprooted milkweed →
An adult monarch butterfly, wings open, feeding at a cluster of pink swamp milkweed flowers
Umbel
A flower cluster in which every stalk radiates from a single point, like the ribs of an umbrella. All milkweeds have umbels. Reading a milkweed flower →
Whorled
A leaf arrangement with three or more leaves ringing the stem at one node. Whorled milkweed →
Sonoran Desert landscape with saguaro cactus, home to the leafless desert milkweeds
Xeric
Adapted to dry conditions and low rainfall. Most western and desert milkweeds are xeric and rot in irrigated ground. Desert milkweeds →