One of the problems with molding these tree bases is that the darn flower gets in the way. The "flower" part just snaps on to the plastic stem, so theoretically you could just unsnap the flower... but on these flowers, the parts are designed not to "un snap"...
So my thought was to take a spare bit of flower stem, mold a base around it, then remove the stem when the base was dry - leaving the perfect-sized hole to glue the flower into.
As you can see, the base "buckled" quite a bit as it dried - and it didn't help that I got carried away and created a larger base than I intended. Some filing gets rid of the worst flaws, but one corner still rises significantly.
Here's the flocked-and-sealed tree base, with the plastic stem removed. It was about at this time that I realized that the darn plant stem narrowed from top to bottom, and the piece I'd used to make the hole was about four times the size of the stem I needed to fit in the hole!
I used white glue to attach the "trunk" to one side of the hole. When the glue was dry, I packed the rest of the hole full with Renaissance Ink "medium" flocking gel. When that was dry, I painted it a terracotta color, and glued more flock on top.
Again, the base seems strong enough, but the warping is even more evident with the larger size.