This is Euphorbia lathyrus - the 'Caper Spurge' occasionally reputed to
rid gardens of moles. An annual plant, it develops a solitary stem
anywhere between 60 - 90cms.high with a rather pleasingly cruciform
arrangement of blue green leaves, terminating in a cluster of lime
green flowers and bracts. It does well in sun or very light shade and
pops up regularly if you let the seed pods ripen and explode on hot
sunny days. (Gertrude Jekyll used to sit and write in her summer house
as the seeds rattled on the roof above - she found it a pleasant
experience and you'll probably do too.) Nuff of that - it's a fine
plant that looks well in the right place and although it can be a
prolific seeder, unwelcome youngsters are easily removed. Being a
Mediterranean 'invader' that does well when happy and can't be
persuaded to grow when not, it's a much welcomed visitor in many
gardens.