When we place big cedar trees in the water they are in a bucket of cement and we tie a gallon plastic jug to the very top. The jug pulls it upright.
I agree also with a wide base. the 5 gallon buckets of cement--cut some holes in the bottom and insert pvc pipe as a base. I do this with bamboo or small trees or brush like willow cuttings off of bigger trees.
The 19 footer looks great--gives me an idea.
I also used an idea posted on this site---I ordered sand bags and filled them with rocks and gravel tied to the bottom.