Glaucous Acaena

Another plant I'd not heard of Siris you have some treasures in your garden :-)
I thought the leaf reminded me of a Sanguisorba and after googling see its in the same family, very nice love the leaf colour.

Bought this as a prostrate plant years ago, from a local mini nursery which no longer exists, along with Acaena microphylla, which I have lost. Shame - their burrs were prettier - red.
Also have a Sanguisorbia, probably officinalis.

That is a shame hate loosing plants especially ones I really like.
We used to have a fabulous nursery near us too, huge garden you could look round and see all the plants growing, lots of unusual perennials, trouble was it was attached to the owners house so when they decided to retire and couldn't find anyone suitable to run it they closed, still open the garden for charity days which reminds me that was yesterday!!!, damn missed it. Don't know if they still do it but you were allowed to collect seed from the garden during the autumn open day.
I have a couple of Sanguisorba too :-)Menziesii which looks very like yours but not as tall and a white very tall one Tenuifolia.

My Sanguisorbia is alpina, leaves glacous, reddish flowers like officinalis. From seed RHS 1999. I like the sound of your white tenuifolia.
Pity about these little nurseries, they often stocked unusual plants.