Yellow Spiky Dahlia

The Island Bed should be renamed the Dahlia Bed - at least in the heat of mid-summer. Many original plain red dahlias are flowering by this time, as well as this bright yellow spiky flowered variety.

I'm not a dahlia expert, but I enjoy their summer flowers and bright colours. I must have purchased this yellow spiky dahlia, though - such flowers do not appear by chance. But when?

My reference book talks about cactus flowering and semi-cactus flowering dahlias. My bright yellow is bound to be one of these.