Tom's Blog

Friday, September 8, 2017

Old-field thistle in an old field

Old-field thistle (Cirsium discolor) is a favorite plant of pollinators and birds. Goldfinches love them, and many wait until the thistle plant is mature (late August or early September) to nest.

At Pleasant Valley Conservancy, old-field thistle has been in our seed mix since restoration work began. However, since this thistle is a biennial, it moves around. Fortunately, seed viability is good, and colonization is generally successful. However, one hardly ever sees first year plants, which generally grow as low-lying rosettes.

The purpose of this post is to show the origin of this thistle's common name.

The east boundary of PVC is adjacent to a neighbor's field that has been used variously to raise cover crops for a near-by organic farming operation. Three years ago, cropping was abandoned, and this field turned almost immediately into an "old field." About three weeks ago, this field was almost a monoculture of Queen Anne's lace. Yesterday when I looked at it, a large number of old-field thistle plants had developed, more thistles than I had ever seen in one place.

This field is within yards of the PVC East Basin, a five acre restored prairie that has scattered old-field thistle plants.Although thistle seeds get around very well, I suspect that most of the "inoculum" for this field came from our East Basin next door.

According to Allen Stokes, who did a detailed study of goldfinch nesting in Lake Wingra marsh in 1944-1946, late-nesting goldfinches work very fast to complete their nest building and raise their young. This makes sense when one considers that in our area the weather could briefly "crash" at any time.

Incidentally, visitors to PVC generally miss completely the East Basin and the adjacent Ridge Prairie, because the main trail leads in the opposite direction. Now would be a good time for a detour in this direction.

The old field full of old-field thistle

Typical old-field thistle plant


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