1. You must declare it because you are otherwise making a false statement on the Declaration and that will create an unending nightmare for you even if the the item itself is benign.

2. You are unlikely to be permitted to import the sprigs. My suggestion is that rather than attaching them to the presents now, put them in a baggie and attach them once you are at your destination. If the items aren't permitted, you won't have to sit there while the wrapping is shredded.

3. As a better idea, unless you are truly headed for the middle of the desert with no stores within 100 miles, you can't walk 50' in the States right now without bumping into a Christmas tree/wreath stand. Ask anywhere for a few sprigs and don't put yourself through the mess.