what can I do to stop the stems from getting so thick?
Thick Stems on My Flowering Plants Towards the End of the Summer
Certified GKH Gardening Expert
It depends what type of plants they are. Some flowering plants naturally have thick stems and need them to support themselves. You might be able to prune the plants back and get thinner stems to re-grow (at least at first), but check the pruning instructions for the varieties you have.