Horseweed in Winter

by Carl Strang

Though the friary site at Mayslake Forest Preserve has begun to develop some prairie vegetation, there still were plenty of weedy annuals in last year’s growing season. One of the more abundant was horseweed, a tiny-flowered relative of the fleabanes (though some botanists remove it to another genus, Conyza). There are two forms of this plant up on that plateau, so different that they appear to be different species at first. One shows the open tops characteristic of horseweed.

Most of the plants at the site have this form.

Most of the plants at the site have this form.

Here is a horseweed in bloom, for comparison.

As you can see, the petals are very short, the entire flower perhaps an eighth inch across.

As you can see, the petals are very short, the entire flower perhaps an eighth inch across.

Zooming in on the individual winter fruiting structures, we see that the seeds are gone.

The dotted white platforms of the central disks are all that remain.

The dotted white platforms of the central disks are all that remain.

A few scattered plants at first glance look much different.

They are shorter (a foot and a half tall rather than 3 feet), denser and darker, almost club-like in appearance.

They are shorter (a foot and a half tall rather than 3 feet), denser and darker, almost club-like in appearance.

The stems are thicker, but otherwise identical to those of the taller, more open plants.

The leaves on the lower stem also have the same linear, stemless shape, though they are more crowded.

The leaves on the lower stem also have the same linear, stemless shape, though they are more crowded.

The branching tops are compressed together.

The individual branches are much the same as on the typical plants, however.

The individual branches are much the same as on the typical plants, however.

Zooming in on these, the individual fruiting structures show the biggest difference from the normal plants.

With a few exceptions, most of the flowers appear never to have opened, or if they did, they closed and trapped the seeds.

With a few exceptions, the flowers appear never to have opened, or if they did, they closed and trapped the seeds.

I wonder if a fungal or viral disease might have hit these darker, club-like plants. The flowers appear abortive, and everything else seems to say that they are indeed horseweeds. Here is another item to tuck away in memory and watch for during the coming growing season.

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