When I had my old flock, it seemed like they would eat anything i planted next to the coop that they could reach, those chickens were in the coop all the time. My new flock free ranges as much as possible and I just finished planting some lilacs, azaleas, jap. maples, other decorative bushes and some small flowers in beds... they 'tasted' a flower from the azalea and havent touched anything since, not even the small, nummy-looking leaves on the lilac sprouts. We also have some hostas by the house but the birds have never touched them, thankfully!
I think when they're stuck in the coop they get bored and will pick at anything they can reach.