Answer: Preen is a pre-emergent herbicide that keeps weed seeds from germinating by emitting a gas. You don't want to use it in your compost heap. It won't work and that's not why it was manufactured, so I wouldn't bother to try. It would be a waste of money. If you are composting green dandilions with yellow flowers, there won't be any seeds involved. If you are allowing the dandilions to produce seedheads then the best approach is to remove the seedheads before pulling the weeds. You shouldn't compost the seedheads, but you can compost the rest of the plant.
If your compost is heating up and you are turning it every few days (cold sides to the middle, hot inside to the outside) it should decompose faster and the heat should kill any weedseeds within the pile.
Best wishes with your garden!
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