This is a photo of a weed called mugwort. I used google image search to identify it. At first I thought maybe it was a chrysanthemum since there was an old photo of the garden with some mums growing in the same general area.
A few weeks ago, I dug up and divided a larger plant that was growing under the bleeding heart plant. Thinking it was a mum, I planted the two halves in different parts of the garden - dug holes for them (no small feat with the hard, clay ground), watered them almost daily, even trimmed the leaves twice so that they would puff out with flowers later in the year. Hahahaha.
One way to tell that it's a mugwort (versus a chrysanthemum) is to look at the underside of the leaves - they are silver/white - in contrast to the the darker green of the chrysanthemums. The shape of the leaves are also somewhat different than those of mums. Mugwort leaves are spikier. Anyhow, I dug up all the mugworts (three clumps today). Begone. Time to focus on the nicer plants, like some of the begonias, etc.