Black-oil sunflower seed hulls are considered allelopathic. They contain a chemical that can inhibit seed germination in other plants so try avoid using heavy concentrations of them any place where you might be direct seeding.

Mulching around established plants - in moderation - shouldn't pose a problem and no problem adding them to the compost piles (in my experience) except they are slow to break down. This is assuming a diverse mixture in the pile and that you don't add excessive amounts at one time.

More info in the link below.

Dave

Here is a link that might be useful: Purdue Extension Service - Composting Acorns & Sunflowers

Thanks! Since they inhibit seeds, I may use them in a bed where I've been having an ongoing problem with a ground cover that has self seeded and taken over. I've pulled it up and put down weed blockers, but it's pervasive!

So far as I know, the toxins in sunflower hulls severely inhibit plant growth (which is different from inhibiting seed germination). Perhaps the toxins inhibit both?

Sunflower seed hulls are reputed to have some allelopathic tendencies which means they can stop the germination of other seeds. Sunflower seed hulls have not, in my garden, had any adverse affect on roses, poppies, daylilies, asters, sedum, lilac, lilies, cleome, salvia, cornflowers, peonies, snapdragons, etc. not started from seed. The reason some plants have allelopathic properties is to limit competition for space and nutrients from other plants so I see more of that with growing sunflowers than the seed hulls.

One thing that alarmed me about the sunflower seed hulls is that a small mountain laurel near a birdfeeder suddenly died, while four others I planted at the same time are doing fine. The dead mountain laurel is the only one in contact with sunflower seed hulls. The conditions otherwise are appropriate for Kalmias, so it's very possible that the toxins are to blame.

Claire

Black sunflower hulls have killed my pachysandra in the past.

What about Nyger seed hulls? Anyone with experience? i read that the African plants they are harvested from are members of the sunflower family.