In general, most Canadian produce can be brought across the U.S. border. If the fruits and vegetables in question are clearly identified as Canadian-grown, and are within their normal growing season, there is little further scrutiny. If the produce is outside its normal season, it must be clearly identified as coming from a legitimate greenhouse or wholesale storage operation. Commercial produce importers routinely maintain the correct paperwork, but it is more problematic for individual travelers.
Most exceptions are caused by specific pests or diseases, and the degree of threat they pose to American crops. Produce coming across the border is routinely inspected for visual evidence of insect pests, and may be refused entry if such evidence is found. Similarly, produce showing visible signs of disease, such as scabby potatoes, may be refused entry.
The United States Department of Agriculture maintains a database of produce items, countries of origin, and any specific exclusions. It is accessible to anyone with an internet connection, so even travelers in their hotel rooms have the opportunity to check before they reach the border. It is called the Fruit and Vegetables Import Requirements database, or FAVIR for short.
Aside from avoiding any produce items that are specifically banned, the most important thing to remember is that you must be able to prove that the produce was grown in Canada. Commercial produce normally has an identifying sticker, but this is not the case when buying from a fruit stand or farmer's market. Be sure to get and retain a receipt from the vendor that includes their contact information.
