Miscanthus lutarioriparius (Triarrhena lutarioriparia)

Very tall, if not the tallest, grass you can grow in the UK

Rapid growing, well spaced, purple/pink stems reach for the sky in Spring/early Summer. Invasive, so best either pot grown or in the ground confined by a root barrier unless you have a big space to fill. Minimum recommended final pot size 30 litre. 

Next available as potted plants hopefully sometime in May.