Okay, I think I found out why nobody is posting anything about Rose Lake here lately. Good thing I took my clippers with me ...
I cleared out the trail from the Woodbury lot, take the right fork, around the corner, take the left fork down the long hill, first left up and over the hill, then out to the main trail, then left back to the parking lot. This loop is now clear of thorns and can be ridden cleanly. Not only that, it's a fun loop with climbs and downhills and lots of twisty character. Especially fun when it's almost dark! And the shadows look like bears

Anyway, I think that might just be an especially bushy and thorny section, the wide main trail had a lot fewer thorns on it, and I suspect the South section (take the left trail from the Woodbury lot) has nowhere near as many. Next time I'll go check out the South section and the ridge trail and take my clippers with me.
Please post up if you clean out any sections or just to let us all know where the worst thorns are - maybe we can make this trail rideable and fun all summer long. I've read about how people stop riding here at this time of year, but it would be nice to keep it clear enough to ride, it's a fun trail.