Tom's Blog

Friday, June 8, 2012

Purple milkweeds at their peak

This seems to be a good year for purple milkweeds (Asclepias purpurascens) and right now they are at their peak. This is an excellent time to search for new populations, as the flowers really stand out among all the lush greenery.

Purple milkweed is best found in savannas, open oak woodlands, or (rarely) along roadsides. It is endangered in Wisconsin and is not really common anywhere in its range (eastern North America). Fortunately, we had a few native populations at Pleasant Valley Conservancy that had survived years of neglect and they have expanded greatly as a result of our savanna management. In 1998 we started with two sites and through continued management as well as transplanting from greenhouse-raised plants we now have 28 separate populations. About half of these populations appeared spontaneously, and the rest arose from transplants.

The number of separate stems and flower umbels at a site varies from year to year. This year most of the populations are very flush, possibly due to the early spring. The photo below shows one of the better populations in a white oak/Hill's oak savanna.

Purple milkweed population in a lush savanna area

Early June is when I always make my purple milkweed survey, checking all existing sites (permanently marked) as well as looking for new sites. Our crew is encouraged to report new sightings and Amanda has already found one.

I published a paper on my purple milkweed work in Ecological Restoration in 2009 and an illustrated version of this paper can be downloaded from this link.

One of the problems that has arisen this year is the invasion of some of our purple milkweed populations by woodland sunflower (Helianthus divaricatus).  If you have been following these blogs for a few years you know that we have been battling this plant off and on throughout most of our savanna areas.The photo below is not atypical of what can happen if this plant is not controlled. The stem density of these sunflower clones is very high, and absolutely nothing is able to grow with them. The approach we are using is to kill the scattered plants that are moving in with a careful leaf spritz of Garlon 4 in oil whereas giant clones such as this one are being sprayed with glyphosate and the whole area reseeded with native vegetation.
Monoculture of woodland sunflower (Helianthus divaricatus) at the edge of a white oak savanna.  


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