Using Sap Color to Identify Southeastern Milkweed Species: 1 is Cloudy, 2 are Clear and 18 are White

 

Asclepias viridula - Apalachicola, Florida

Currently, field and taxonomy books only account for one milkweed having clear sap. To clear this up (pun intended), the deep south's three species with non-white sap are discussed here.

Reviewing the sap color of a wild milkweed can help to identify (or rule-out) 3 of the 21 species native to the deep south. One species has semi-transparent, "creamy" sap that strongly exudes when a leaf is cut, two species present clear sap that does not strongly exude on a cut leaf, and 18 species strongly exude white sap when cut.

Note* Before reviewing the different species, it's important to remember that ALL milkweed species will produce little-to-no sap if it is late in their growing season and/or the plants are senescing (yellowing and dropping leaves) due to extreme weather, such as drought or an unseasonal frost.

The "Cloudy" Species: 

Asclepias viridula - southern milkweed

Asclepias viridula - flower profile showing distinctive 2-dimensional horns

Southern milkweed is genuinely rare and conservation-dependent, and is typically found in fire-maintained wet prairies and (on occasion) mesic microhabitats within upland natural communities. 

Vegetative plants are sometimes confused with Asclepias cinerea and/or Asclepias verticillata. Although a dichotomous key and field experience with the different species makes it easy to delineate them, and quick field indicator for southern milkweed is to lance a leaf, and observe if the sap is cloudy or solid white. If the sap is cloudy, it's southern milkweed!

Asclepias viridula - Cloudy Sap Exuding from Leaf

Asclepias viridula - Cloudy Sap Exuding from Leaf @ Node

Asclepias cinerea and Asclepias verticillata exude classic, white, latex-ish sap. Additionally, A. cinerea typically grows upwind of A. viridula in mesic-xeric transitions (in mesic or scrubby flatwoods), and Asclepias verticillata typically grows upwind of A. cinerea in xeric uplands or high pine of sandhill or clayhill. 

The "Clear" Species:

Asclepias michauxii - Michaux's Milkweed

Asclepias michauxii - upright ecotype of mesic clayhills and high pine - Brewton, Alabama

Asclepias michauxii - prostrate ecotype of the Apalachicola lowlands

Asclepias michauxii - Flower Form in a Classic Asclepiad Umbelliform

Like the southern milkweed, Michaux's milkweed is another rare Asclepiad of the coastal plain. Most populations are remnants along roadsides, with a few strongholds remaining in their range on protected, managed lands. Michaux's milkweed and southern milkweed can be found nearby to each other, but do not grow together in the same microhabitat. Asclepias michauxii will grow at the fringes of wetlands, but does not grow within the hydric soils of wetlands like Asclepias viridula is inclined to do. Until one has sufficient field experience with the species, a good field indicator is to look lance a leaf on a plant, whereby non-exuding, clear sap will be observed. 

Asclepias michauxii - Non-Exuding, Clear Sap, with Leaf Bruising

Asclepias michauxii - Non-Exuding, Clear Sap, with Leaf Bruising

Aside from field investigation by observing color of sap and other morphological features, we are often successful in discovering new populations of rare and declining Michaux's milkweed by looking for a very distinct type of habitat -> uplands native plant communities dominated by Helianthus radula  - the rayless sunflower. Typically, if we find non-wetland populations of rayless sunflower, we frequently discover new populations of Asclepias michauxii. 

Upland Community of Helianthus radula - Kinard, Florida

Finding the rayless sunflower leads to the discovery of a new population of Michaux's milkweed! - Kinard, Florida

Asclepias michauxii is a close relative of Asclepias tuberosa, and so it is unsurprising that both species present clear, non-exhuding sap. As for Asclepias viridula, it appears to be a sap anomaly of the southeastern United States milkweeds, probably as an evolutionary emergent trait of this very specialized milkweed species.

Asclepias tuberosa - Butterfly Milkweed

Wild Asclepias tuberosa var. tuberosa in Raleigh, North Carolina

This is the species (and its varieties) that many people are familiar with, and which many sources of literature will cite as the only species of milkweed to not have white sap. Regardless of where you observe butterfly milkweed, all of the varieties of this species have clear, non-exuding sap. 

Asclepias tuberosa rolfsii - Non-Exuding, Nearly Non-Rxistent Clear Sap, with no Leaf Bruising

As folks come to become masters of milkweed identification and surveying, we hope that being able to distinguish (at least three) species can be aided by an investigation into their sap color!