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I thought Sweet peas were annuals, but I've one from last year still growing and flowering? It's growing inside in a poly-tunnel, it was a stray plant that sprung up and I didn't have the heart to pull it up. I guess as we haven't yet had a hard frost it's just kept on going, but It's March now. I picked a small bunch of sweet peas for Christmas day and I'm still picking flowers now which is lovely, though snowdrops and sweet peas are an unusual combination!

Any ideas?

Surely sweet peas are a perennial that are  normally grown as annual. They are perfectly hardy as long as their roots are in the ground not pots, that can freeze from the sides.