What is this plant at Exbury Gardens?

What a wonderful day! Our first visit to Exbury Gardens - and we'll certainly be going again. Has anyone else been recently - and can you identify a plant for me? I took a photo of it, but unfortunately it didn't come out. The plant is in the herbaceous/grass section and is planted with some fennel. It is tall, has purple stems, bluish-grey leaves and looks as though it will have small (white?) flowers at the top. I wish I'd asked now - but wanted to identify it myself from the photo.....