Tom's Blog

Saturday, June 25, 2011

Further on purple milkweed

This post is a follow-up on an earlier one this spring.

Due to the late spring, flowering of our purple milkweed (Asclepias purpurascens) stands was delayed, but they are now at peak performance and easy to see in the field. Therefore, yesterday I made my yearly survey of all the stands at Pleasant Valley Conservancy.

This is a great year for purple milkweeds, and with lush flowering it has been possible to find quite a few new locations. Several are what I call "spontaneous", since they are not from greenhouse transplants (although many of those are doing well also).

A new spontaneous population in Unit 8 is a completely new site, quite remote from all the other sites.

These new spontaneous populations have probably come from seeds that we threw out some earlier year. Purple milkweeds that are not flowering are extremely hard to find. If they arise from planted seed, it may take them several (or many) years to establish a root system substantial enough to support a flowering population.

The photo here is from one of my spontaneous populations that first appeared in 2008 (thus, now in its 4th growing year). The photo shows only part of this outstanding population. You can also see in the background part of a large charred black oak that had died of oak wilt. This was part of a small grove of black oaks that died about the time this new population of milkweeds arose. Possibly, this milkweed patch had been struggling along in the shade of these big oaks, and the increased sunlight after they died gave the milkweeds a boost.

The count of purple milkweeds that I now have permanently marked is 32, although there are probably another dozen or so others from transplants that are doing well but have not been marked. (All spontaneous populations have been permanently marked.)

Another interesting observation is that one of my permanently marked stands (AP-6) had plants this year for the first time since 2006. I have no idea why these plants come and go like that. Obviously, there is a perennial root system which is able to remain alive for years without sending shoots above ground.

With all the great flowering umbels this year, one might expect good seed production, but this will not necessarily be the case. Last year we had over 50 great flower heads and ended up with only a single seed pod. Seed production requires successful pollination, and self pollination probably does not occur. Therefore, a lot depends upon the presence and activity of insect pollinators, mainly solitary bees. If there is a spell of rainy weather at the time when pollination is critical, we may miss it all together.


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