Phyteuma scheuchzeri is an unusual, easy to grow rock garden plant, with a bizarre flower. It is a close relative of the bellflower but does not look anything like it. I think the flowers look more like an Allium. The flower head consists of a number of flowers all opening at the same time. Each flower is quite spiky in appearance, creating a big spiky ball of rich blue color.
I don’t normally include biennials as a favorite plant since they are just too much trouble to keep going in a garden, but Glaucium flavum is going to be an exception. Besides large yellow or orange flowers, the horned poppy has one of the most unusual leaves I have ever seen.
Are you looking for an unusual subshrub for the fall garden? Elsholtzia stauntonii is a good choice. The Chinese mint shrub is rare in northern gardens, where its late flowering is very much appreciated.
Aconitum x cammarum ‘Bicolor’ is a special summer blooming monkshood. In zone 5, it blooms mid to late August at a time when other plants are slowing down. It can grow in full sun or heavy shade making it ideal for almost any garden.
An unusual clematis that forms a low bush of very fragrant flowers. Clematis mandshurica makes a great addition to your garden.