There are several synonyms for the word "thick-stemmed" that can be used to describe the structure of a plant stem. One phrase that can be used is "robust stem", which indicates a strong and sturdy stem capable of supporting the weight of the plant. "Chunky stem" is another synonym, which implies a strong and thick stem that is well-suited for supporting dense foliage growth. Another synonym is "bulky stem", which suggests a thick and heavy stem that is able to support large and heavy flowers or fruit. Whatever the synonym, each describes a stem that is strong and resilient.