I am sorry buy you will not like my answer any better.
Public gardens are not a place to harvest. Taking without permission is theft, plus you have robbed other park visitors of the enjoyment of viewing such a fine specimen. Had you asked, permission might have been granted. It is also possible that the park people wanted to save the seeds for next year. Also, I agree that the seed head needs to stay on the plant until the seed head dries and the seed matures.
The birds have been deprived of food and you have seed which is probably not mature enough to grow.
Please, next time ask first. Either ask permission for the park people or ask here on ATP and you will get a better answer.
