Observing the laws of Kosher can be a bit tough sometimes, but the rewards are certainly greater than the challenges.

Most frozen vegetables are acceptable for year-round use (excluding Passover), as the freezing process does not pose any Halachic concerns; though you should always be on the lookout for insects which occasionally infest vegetables.

Based on my research, it appears that due to the strong likelihood of insect infestation in the following frozen vegetables, they should only be bought with a reliable kosher certification: broccoli, cauliflower, brussel sprouts, asparagus, cut onions, spinach, potatoes, and artichoke hearts.

Also, any vegetable containing added ingredients such as pasta, oil, spices, sauces, flavoring, or coloring require Kosher certification.

According to the CRC, a noted kosher certification agency, any vegetables that are packed in Israel or China require reliable certification.

Best wishes,

Rabbi Baruch S. Davidson