What is the singular form of fennel?

Fennel, like rice or celery, is a mass noun and normally occurs only in the singular.

Mass nouns aren't instantiated; they're abstract, or liquid, or granular, and don't occur in individualized pieces, like beans or dogs or roses, which are count nouns and have normal plurals.

So the plural of a mass noun often means different kinds of something, instead of different instances of it. Thus, fennels, rices, or celeries would mean different varieties of fennel, rice, or celery, if you needed to refer to them. Otherwise, mass nouns are used in the singular only.

You should be able to look this up in a good dictionary (not an American one). Look to see whether bean is marked as count and rice is marked as mass, so that you can tell that

• a bowl of rice
• a bowl of beans

are both correct, while

• *a bowl of rices
• *a bowl of bean

are both incorrect.