Above is a slideshow of the anatomy of common milkweed (Asclepias syriaca)
Milkweed plants have many characteristics that are similar to one another, and many that differ. Knowing how many petals, the presence of a milky sap and a few other helpful ID tips, will help you confidently identify milkweed and subsequently report it or protect it.
Although it is best to be proficient at identifying milkweed without flowers (so you can recognize them throughout the seasons), most are easy to spot and much easier to identify when in bloom, so we'll start with the flowers.
Milkweed flowers all have five petals (corolla), five sepals and a corona that contains hoods and horns (sometimes called beaks), which are modified anthers of the flower. Although milkweed flowers are similar in many ways, they can vary widely in color, shape, size and structure. Many species have showy globe-like umbels with numerous flowers, like antelope-horns milkweed, while others are less dense and less organized and droop, such as Carolina milkweed.
Below is a close-up comparison of three milkweed flowers. Note how they differ. The horns are obvious in the butterfly weed, but are hidden in both the antelope-horns and California milkweed species. You can see that the corolla, or the petals, in the antelope horns milkweed are not reflexed backwards like the butterfly weed and California milkweed. They have the same reproductive structures, but they all look a bit different.
Milkweed leaves differ widely. One thing they all have in common is that they are simple leaves, meaning there is only one leaf per node, rather than compound leaves, which have more than one. However, that's where the similarities end.
There are many more differences than similarities; however, this also makes it easier to tell the species apart. Some leaves are dark green, some are light. Some are pubescent (hairy) some are glabrous (hairless). Some leaves have very prominent midveins, some do not. Many milkweed plants have an opposite leaf arrangement, while others have an alternate or whorled arrangement. Some leaves have straight margins (edges), while others are undulate (wavy).
The varying sizes and shapes of milkweed leaves can make many species difficult to spot, but once you know what you're looking for, the leaves help tremendously in identification even when not flowering. Knowing the leaf arrangement, as well as the size, shape and surface characteristics can help you narrow down which milkweed species you are identifying. Below are three examples of leaf types exhibited by milkweed species. This is not an exhaustive list by any means, but is meant to be an introduction to identifying milkweed leaves based on these leaf characteristics. When in doubt, you can pinch the end of a leaf. If a milky, sticky sap seeps out of the edge of the broken leaf tip, you have milkweed. The exception to this is butterfly weed.
Keep in mind that hairy leaves (and stems) may become less so over time as the plant matures. Also keep in mind that leaf shapes may change as they mature. What starts out rather narrow, may ultimately wind up being a more broad leaf.
Milkweed Leaf Arrangements
Milkweed plants can have any of the three primary leaf arrangements and sometimes more than one:
1) Opposite: Leaves are directly across from one another on one node
2) Alternate: Leaves alternate up the stem, and there is one leaf per node
3) Whorled: Three or more leaves are equally spaced around the stem on one node
Some, like butterfly weed, technically have an alternate arrangement, but appear opposite at times as well as whorled. A large percentage of milkweed plants have opposite leaf arrangements, while a few have whorled. Below are examples of all three.
Milkweed pods (also called fruit or follicles) are one very easy way to spot these plants off-season. Once the plant has flowered, the fruit is produced in the form of unique pods. The pods contain numerous seeds that turn a dark brown when ready to be released. Some plants may spread by rhizomes, but most spread by seeds that are attached to a silky tuft of hair, called floss. The floss allows the wind to lift the seed and carry it far from the parent plant. Some long-blooming species will have both flowers and pods at the same time, such as butterfly weed. The slide below shows how milkweed pods differ from species to species.