Not a glitch, nirnroot glows.
Sure it does. It isn't supposed to glow so much that it looks like it's trying to match the sun ...
http://www.moneycontrol.com/news-topic/skyrim-patch-1.02-ps3/video-glowing-nirnroot-tes-v-skyrim_d5Q68N8gQn0.html (Warning: Neckbeard.)
Also http://forums.techarena.in/video-games/1441213.htm
And http://www.gamesas.com/index.php?/topic/1283418-uhhh-the-nirnroots/page__view__findpost__p__19787171__hl__nirnroot__fromsearch__1