suddenly (well not that suddenly) appeared in front garden - new to me
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For the first split "not rosebay" you need to look are the leaves spirally arranged or are the leaves in pairs opposite eachother -.
Flowers zygomorphic (in Chamaerion) versus actinomorphic (in Epilobium) also works.
The leaves of Epilobium are opposite basally, but tend to become alternate upwards.
Ok I'm happy - I had parviflorum as another possible but couldn't see inside the flowers, and it didn't seem quite hairy enough.
Jamie from Briantspuddle
Epilobium parviflorum is about the most variable of the Willowherbs eg in how downy it is and how split the stigma which sometimes has the 4 lobes stuck together and looking almost clavate. The downiness is the best indicator that it is not one of the others (except for E. hirsutum which has much bigger flowers).
Lat/Lng: 51.3755, 1.3252
OS grid ref: TR315692