Last season was a strange one in my garden, notable not only for the unseasonably cool and wet weather — the talk of gardeners all over New England — but also for its climate of paranoia. One flower was the cause: a tall, breathtaking poppy, with silky scarlet petals and a black heart, the growing of which, I discovered rather too late, is a felony under state and federal law. Actually, it’s not quite as simple as that. My poppies were, or became, felonious; another gardener’s might or might not be. The legality of growing opium poppies (whose seeds are sold…