What story was this in?

In some post-apocalyptic novel where people are struggling with TEOTWAWKI, it turns out that some old woman’s flower garden include opium poppies, which allows the survivors to produce a crude laudanum. I thought this was Alas, Babylon, but I just checked and that’s not it. Anyone?

Definitely not Alas, Babylon. It’s also not On the Beach.

I enjoy the post-apocalyptic genre myself, so I’ll be interested in the answer to this.