If fairly low growing, most likely Hypericum calycinum, a groundcover form of St. John's wort that is an aggressive spreader and can easily outcompete weeds and other plants.

Ah. So perhaps I don't want to relocate some into my garden. "Aggressive spreader" sounds like a big job.

I wouldn't........it can take over :-) It is a good groundcover for sloped areas in full sun that require or can be offered minimal attention but IME, too aggressive for a garden situation. Also has a tendency to develop rust and usually looks like crap by the end of the season. Best if it can be mown or whacked to the ground once a season.

If you do decide to take some for your garden, don't dig it from that clump. Chances are you'd end up with some of the Equisetum that's growing in amongst it.

My whole garden is a constant battle with Equisetum. I'd be hard pressed to find anywhere that didn't already have it. I'd be more worried about accidentally transplanting some of the invasive blackberry.

"My whole garden is a constant battle with Equisetum."

An unfortunate fact of life for many Puget Sound area gardens :-( I had to deal with it at my previous garden and it was a constant battle!! At my current one, equisetum has been replaced by English ivy and blackberries. And yellow archangel and self-seeding hollies and cotoneaster and spurge laurel.

You just can't win :-) But you can keep it manageable if you stay on top of it. That's why I cautioned against adding the hypericum......just another aggressive thug to deal with!!

Oh we have English ivy and blackberries and bindweed too! Before we purchased this property it was the next thing to abandoned for nearly 20 years - apparently the owners used it as a vacation home and were here only a couple weeks a year, none of which they spent gardening. Underneath were some lovely old plants, remnants of a well loved old garden. But the thugs had the run of the place for the better part of two decades, and they haven't forgotten.

I feel like I can make progress against ivy and blackberries if I can put the time in, and bindweed comes up fairly easily. The Equisetum on the other hand seems to persist at exactly the same level no matter how much I pull it. And I definitely don't need to be adding any more thugs to the mix.

Definitely Hypericum calycinum, the only familiar species with the same combination of characters that is visible in your picture. It used to be a nice cover for sheltered positions where the leaves would not burn as readily in the winter as they always did in the open. But now it also rusts, wherever it is planted. (Yours apparently don't happen to be infested, at least the part you photographed).

I didn't see any rust. I hesitate to call it "mine" since it lives on the empty lot down from mine, although I suspect previous owners of this house probably planted it to pretty up the road.

It likes drier ground than Horsetail is comfortable with. I have a patch of Hypericum that only gets rust where it gets some over spray from me watering some containers. The non rust Hypericum is under tall firs and in a lot of sun in poor ground.

I've yet to find anywhere on my property that horsetail isn't comfortable, including hot dry areas in full sun.