Common Milkweed vs Purple Milkweed – Identification Differences

This is a very short guide for how to identify common and purple milkweeds, and is intended to help you differentiate Common Milkweed and Purple Milkweed in the field. Sometimes we can confuse these two plants, as they do look similar! Now for most practical purposes it isn’t a big deal, but whether it’s for botanical curiosity or you are trying to save seed from one of them, this guide will help you differentiate the leaves and macro floral structure.

I’m going to go through the main features you can use to differentiate these two species. But the most important ones are the leaves, floral structure, and the seed pods.

Using Plant Identification Apps

While there are many plant ID apps available for smart phones, they are not all created equal (really, I tested them here). These can provide a solid ‘hint’ at the identification, but to be sure you should try to make some key identification features yourself. I have tested these apps, and found that many are lacking in ability to either identify, or be cautious in their identification. For example your plant identification app may confidently misidentify a plant, but you are not aware.

Native Range differences

The native range of Common and Purple Milkweed overlap substantially. Virtually all of Purple Milkweed’s native range is shared with Common Milkweed. Click on the images below to see the differences:

Leaf differences

At a glance, the leaves of Common Milkweed and Purple Milkweed appear to be the same. But the primary difference between the two will be the veins on the leaves and the base. Common Milkweed will have prominent horizontal veins while Purple Milkweed veins will have more crossing or spider-web like appearance. For more practical purposes, one could say that the webbing begins early on the Purple Milkweed and much later (near the end) of Common Milkweed.

The leaves are mostly the same size, and the shapes are very similar too. So, since the size and shapes overlap so much it is best to focus on the veins for evidence of one species or the other. Then combine this with other information, particularly the pods and floral structure/occurance.

Floral Structure

This is a key difference between the two plants. Purple Milkweed will only put flowers at the upper-most leaf junction, while Common Milkweed will place them at multiple leaf junctions that ascend the stalk. So if you see a plant and it has blooms, or seed pods all along the stalk at leaf junctions, then it can’t be purple milkweed.

Seed Pods

Another dead-giveaway – Common Milkweed seed pods have small spikes or prickles while purple milkweed seed pods are smooth.

Conclusion

It can be difficult to differentiate these common and purple milkweed, even when they are blooming! Hopefully this guide has helped you. Remember, use the differences in leaf veins to confidently differentiate the plant before blooming, and then return to monitor later when it is blooming to confirm. Also, examine where the flowerheads occur, and note that it is possible for smaller common milkweed plants to only have a single umbel of flowers! Finally, while the seed pods of purple milkweed are smooth, don’t use this as your sole piece of information! Other species of Milkweed also have smooth seed pods (see here, here, and here), so you will need to carefully examine leaves and other characteristics!

Learn more about Milkweed here!