Frozen vegetables are washed in water that often contains anti-foaming agents, which are not appropriate for Passover use. Moreover, some companies also pack pasta products on shared packing machinery. Therefore, frozen vegetables require Passover certification.
However, these issues are not a concern at facilities that process frozen fruit, which does not require Passover certification. One must be careful that no additives e.g. corn syrup are used.
Frozen raspberries and blackberries are only accepted if pureed.