In English to refer to all vegetables you use plural, we use singular.

You probably say "I eat a lot of vegetables" We say "Mangio molta verdura."

http://www.treccani.it/vocabolario/verdura/

Verdura (2nd definition, the one we need)

Nome collettivo di tutti gli erbaggi commestibili e ortaggi: negozio di frutta e verdura; minestra di verdura.

Whenever you see "nome collettivo" it means you tend to use the singular for many items. Like you do in English for furniture, information, hair.

I hope it helped.