One of the new features in Rune Factory 4 is the ability to grow giant-sized crops, used as crafting materials and to tame certain boss monsters. To do this, you will need to plant four of the same seed types within a single 2x2 square of soil. Next you will need to buy or otherwise acquire Giantizer to raise the secondary Sz. parameter of the soil. Since it affects the soil and not the crops, Giantizer can be used at any point in these steps. Once the four plants reach maturity, they should be somewhat larger than normal; if they aren't, you'll need to add more Giantizer to the soil and wait. Do NOT harvest them yet! Continue watering the four mature crops for a few more days until they merge into a single giant crop taking up all four spaces. If your plant does not become giant after a few days, double-check that they are planted within a single 2x2 square and that they are larger than normal. If you don't plant the four crops within a single square or they aren't larger than normal, they will never merge into a giant crop no matter how long you wait.