The compounds that make the plants inedible or poisonous break down in the compost, and they can't be taken up by other plants anyway. The only plant I'd be careful with is poison ivy, because the woody parts take a long time to decompose.

Mostly plants use toxins to keep herbivores like us from eating them -- unless we can help with seed distribution. Once the plants are dead, they have no further need for chemical defenses, which nature cleans up as part of the decomposition process.

It's a good question!

Thanks. I'm glad to know I don't have to separate these kinds of plants out from the rest of my compost!

I wanted to grow some castor bean plants (source for ricin)for quick shade and did a lot of research on them, as they are poisonous. Many sites I found said "do not put castor bean plants in the compost pile, as it will poison your compost." That said, I don't think milkweed would cause any problems. Susan