There are 4 groups of plants.
Seedium -- Plant which can be unlocked by seeds.
Gemium -- Plant which needs to be purchased with gems
Power Mints -- Plant which needs to be purchased with 100 mints
Cash premium -- Plant which needs to be purchased with $$$
Only seedium plants can be unlocked with seeds, if the plant you are trying to unlock is not belonging to this group, you will need to actually buy the plant itself first either with cash, gems or mints when it is available from the store.
Hope this helps.
Note to EA/PopCap. Please bring back the Golden Pad lock and Gem indicator etc to indicate the difference of plant groups.
