I have several of these plants growing in my yard. They tend to be 5-6' tall, composed of several widely-spaced stems which emerge from a single base, each almost barren except for a few opposite-leaves, most of which brown and fall off while the plant is in bloom.

The blooms are tiny (3-4mm), violet, have 5 petals, and seem to close during the day, although I'm not sure on that point. It has a square stem which leads me to think it's in the mint family, and the leaf has a FAINT hard-to-place scent reminiscent of mint.

To get a decent image of the upper portion of the plant, I broke a piece off and scanned it with my scanner.

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