2015 survey of purple milkweed growth and pod formation at Pleasant Valley Conservancy

Purple milkweed is one of our signature plants at Pleasant Valley Conservancy, and Kathie and I have been monitoring it since 1999. Originally, only a single purple milkweed patch arose in Unit 12B, which was the first oak savanna area restored. The photo here was taken at that first site.

As savanna restoration proceeded, other purple milkweed patches appeared spontaneously. Indeed, even today new patches appear spontaneously, years after the major clearing work involved in savanna restoration was completed. (A nice new population appeared in the bur oak savanna of Unit 11A just this year.)

By our present count, we have 20 patches that have arisen spontaneously and each patch has been permanently marked. Some of these patches have continued to appear every year but some have arisen for a few years and then have disappeared. In some cases, patches that have disappeared have reappeared again in subsequent years, only to have disappeared again.

In addition to patches that have arisen spontaneously, we have also been able to raise plants from seed and have transplanted them to the Conservancy. These transplants have also been permanently marked. Fifteen of these transplants have flowered and survived for many years, although some have flourished for a few years and then disappeared.

The reasons for this wide variability in growth from year to year are unclear, but may partly explain the rarity of this species and why it is listed as Endangered.

In 2015 we had 18 separate locations where purple milkweed plants flowered. At many of these sites, there were multiple flowers, often three umbels per stem. One patch had 15 stems, and a total of 18 umbels.

About half of these 2015 patches were spontaneous, the rest were from transplants.

As a group, pod formation in milkweeds is highly variable. Pod formation apparently only occurs if cross pollination occurs. Thus, pollination will not be successful if a pollinator (e.g. bumble bee) moves from one umbel to another on the same plant. Given the widely spaced distribution of patches at PVC, it is not surprising that pod formation is low. This year only two plants (both spontaneous) formed mature pods. One plant formed 4 pods and the other 2. 

Details of the first 10 years of our study, with photos, can be found in the following paper.