Not to be confused with the other uses of Plant Food.
Plant Food boost is an item that can be activated before a level is started when selecting the plants that lasts for the duration of that level. Any plant from that seed packet will automatically use its Plant Food effect every time upon being planted. There are only two ways to activate it. One is to have a boost already obtained from the Zen Garden, which is obtained by fully growing a plant besides Marigold and then activating the seed packet that it spawns. The other way is to pay a varying price of gems (either 10, 12, or 15) to activate the boost for that level. Planting a plant with a Plant Food boost on a Power Tile will not trigger the Power Tile, and will only affect the plant.
When equipped to Puff-shrooms, it is extremely powerful as it will trigger all other Puff-shrooms, and at the same time refresh all of them, making Puff-shrooms practically invincible. In addition, their 0 sun cost and fast recharge makes them even better, as planting them will be easy and you can quickly get a powerful army. Finally, if you have no space and must dig one up, there is no disadvantage, as it costs no sun and has a very fast recharge.