Tom's Blog

Tuesday, October 1, 2013

Collecting purple milkweed pods

This has been a great year for purple milkweed growth and reproduction. I summarized the extent of growth and flowering at the end of June. Fortunately, rainfall has continued throughout the summer, so that we have had excellent pod formation. This has been the best year for pod formation since we started monitoring these milkweeds back in 1999.

Finding purple milkweed pods is a little tricky, since by the time pods are developing the milkweed plants have senesced and are surrounded by lots of tall savanna species. Even though we have permanently marked all our purple milkweed populations, the plants move around a bit from year to year so they are almost never right by the stakes. Fortunately, our PVC crew is well-skilled in finding pods.

Milkweed reproduction depends on a number of factors. First is pollination itself, which is a complex process. Also, self pollination does not occur, so several flowering plants have to be near by so that the pollinators (mostly solitary bees) can move from one plant to another. Then the weather has to cooperate throughout the time when pods are forming.

We had 23 separate purple milkweed populations this year. About half of them had arisen spontaneously, and the rest are well established greenhouse-raised transplants. Almost all of these populations were multi-stemmed and had flowers, generally 2 or 3  umbels per stem. We visited each population in early September to see how pod formation was coming along. Eight populations were forming pods (35%). Among the  umbels on these populations 17% formed pods. (Although each umbel is multiflowered, we have never seen a single umbel form more than one pod.) We collected a total of 39 pods.

Not all pods mature at the same time, and it is a little tricky to decide whether a pod is ready to collect. Those shown in the photo are typical of good mature pods. If a pod is starting to split open it should be collected immediately. If the pod feels firm and full, it is a good bet to collect. If you have some doubts, make a small crack along the fission line and check the seeds. If they are ripe they should be a rich brown color. Sometimes you have to just collect the pod and hope for the best.

The germination efficiency of purple milkweed seeds is very good. My laboratory tests have almost always found greater than 90% germination.

We are often asked how to  recognize a purple milkweed plant if it is not flowering. There are two other species found in the same kinds of habitat that might look similar. These are common milkweed (Asclepias syriaca) and poke milkweed (A. exaltata). Common milkweed is a clone former and is often found in large patches. Also, its plants are usually larger and its pods have distinct warts. Poke milkweed usually has narrower leaves, and generally smaller and thinner pods, but these are not  reliable characters.

A reliable method is to mark any nonflowering milkweed plant found in the woods in late May or early June and return to it in mid to late June and look for the flowers.

The best time to find flowering purples is in mid-late June, especially in savannas or open woodlands.

Purple milkweed (Asclepias purpurascens) is endangered in Wisconsin.


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