When is a seed a seed? My wife and I are in agreement on not putting weeds with set seeds in the compost (and the “Easy Compost” book says just that). However, we are less sure about weed flowers (probably OK), and what about seed cases that haven’t formed seeds yet? I’m thinking in particular of foxgloves right now, as the flowers are coming to an end and leaving behind the undeveloped seed cases. I’m unsure whether to compost them or not. Just an aside: our compost pile doesn’t get superheated.
That is a very good question. I found an article in Fine Gardening magazine which discusses harvesting wildflower seeds. It is relevant because it suggests that some unripe seeds may continue to ripen even after being harvested from the plant before maturity. Whether unripe seed will eventually germinate may have to do with the permeability of the seed coat: the more permeable, the more likely the chance it will germinate.
The book Seeds by Peter Loewer (Macmillan, 1995) says that plants with tough seed coats (like legumes and morning glories) “are virtually impermeable to water and must be nicked by the gardener or soaked in warm water for twenty-four hours before they germinate. If these jackets are not broken, scratched, or eroded, water never enters and germination never begins.”
I have found several references to the immature seeds of invasive plants (Ailanthus, teasel, yellow flag iris, to name a few) being capable of germination. The Complete Compost Gardening Guide by Barbara Pleasant and Deborah Martin (Storey, 2008) “weeds that show up in your garden are fair game for compost, even if they are holding seeds. […] Weeds that have not yet begun to bloom and lack viable root buds that help them grow into new weeds can be added to any compost project, but it is important to keep weed seeds to a minimum every chance you get. […] In every climate there are plant criminals known as noxious weeds […] Unless you are confident and committed to processing the compost made from noxious weeds with a high-heat procedure, collect them in a black plastic garbage bag and subject them to various forms of torture before dumping them in an inhospitable place. Cook bags of them in the sun, add water and let them soak into slime, and keep track of what works and what doesn’t. If your superweeds survive your torture methods and you don’t have a spot in your landscape suited to use as a little landfill, discard them as garbage.”
If you want to be on the safe side, avoid putting anything seedy (even green and immature) in the compost, especially if the pile is not going to get especially hot and speed the decomposition process.