The joke is talking about a garden club finding themselves in a hosta environment. A hosta is a type of eastern Asian plant. A garden is a piece of ground used to grow plants; hostas can be grown in gardens. A "hosta environment" sounds like "hostile environment". Hostile means unfriendly or threatening. "Hosta environment" can be taken literally, as in being surrounded by hosta plants. Since "hosta environment" sounds like "hostile environment" the joke could be understood as the environment is unfriendly. This joke is centered around the word "hosta" and how it sounds like "hostile" which is generating different understandings.
The garden club visited the nursery and found themselves in a hosta environment .