usage weed v.s. weeds

[Lost_Soul]

I need to find some kind of chemical that will keep down the weeds in the garden.

Why do we say "weeds" instead of "weed" ? I always thought that weed is an uncountable noun...

tia

[Conchita]

Weed is a countable noun in your context, i.e. plants that you don't want in a garden.

However, it is uncountable when used in the sense of 'tobacco' or 'cannabis/marihuana'