Yes, plant-based foods can be halal, and in many cases, they are by default. Fruits, vegetables, grains, nuts, and legumes are all naturally halal as long as they are not processed or prepared with haram substances. […] If these foods are handled, flavoured, or packaged with non-halal ingredients such as alcohol, gelatine, or animal-derived enzymes, they may no longer be considered halal.

I don’t know how reputable Halalification is, but it was high on the list of search results and the entire answer seems well-reasoned.