Not All Plants are Seed Unlockable

At the request of @eieio_31310, I'm posting here some information that I posted on the r/PlantsVSZombies Reddit sub.

While most of the 190 plants currently in the game can be unlocked with seeds, there are a few plants that can't be. These are:

Gemium Plants (13) - plants that are unlocked by purchasing them for (usually) 100 Gems

Squash, Jalapeño, Hypno-shroom, Pea-nut, Homing Thistle, Ghost Pepper, Sweet Potato, Sap-fling, Hurrikale, Fire Peashooter, Lava Guava, Shrinking Violet, Electric Currant.

Cashium Plants (21) - plants that are unlocked by purchasing them with real money.

Snow Pea, Power Lily, Imitater, Chomper, Toadstool, Strawburst, Cactus, Electric Blueberry, Jack O'Lantern, Grapeshoot, Cold Snapdragon, Escape Root, Gold Bloom, Wasabi Whip, Apple Mortar, Witch Hazel, Parsnip, Missile Toe, Caulipower, Electric Peashooter, Holly Barrier

Power Mints (13) - plants that are unlocked by purchasing with 100 of the "mint" currency obtained in Arena. Typically only one Power-mint is available each month (the mint that corresponds to the Arena Seasonz plant for that month).

Fila-mint, Pepper-mint, Winter-mint, Reinforce-mint, Bombard-mint, Ail-mint, Enchant-mint, Contain-mint, Enforce-mint, Arma-mint, Conceal-mint, Spear-mint, Appease-mint.

Also, it's important to note that, while the World Plants CAN be unlocked using seeds, you may not want to. Using seeds to unlock these plants means that those seeds are lost for their primary purpose of levelling up the plants. You can also unlock these plants just by working your way through the Adventure mode Worlds. They are awarded to you as you complete the levels in each world.