@Stringer2048 When potato mine costs a certain amount of suns to use, it is down to the player to decide whether this strategy is well used on the limited amount of suns given. When potato mine reaches a high level and becomes 0 sun cost, it is indeed being greyed out thus preventing it from being pre-planted.
Your Blover + knock back strategy might work well but similarly so does an explosive + shrinking/transformation strategy.
Please do remember the game is designed for players to use their chosen plants with their strategy, in boosterama, it is actually better to NOT pre-plant the potato mines as dropping it onto the arena during live game play make more sense on sun economy. The game cannot be won always with high power or even maxed out mastery plants, the correct selection, correct usage and correct timing and plant placement (aka strategy) also plays a big part.