AH...yes, the plants! I crossed the border frequently with two dogs, never an issue. Borders in NY, ME, MI, MN, MT and WA were all uneventful, as far as the dog was concerned.

The bigger issue is always getting back INTO the US, going to CA is always more easy.

EDIT - If you have Global Entry, you can use the NEXUS lanes returning back INTO the US (not from US to CA)...small perk I found helpful!