Why cannot level up plants?

Is it possible you don't have enough coins to upgrade?

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@ss3nov wrote:
Nope coins are not at all a problem. Have plenty of them but the plants are not leveling up beyond the 5th level.

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Um... I think there are only 5 levels.

Well I am now having this problem with my Lily Pad.  It shows "Extra Seed Packets: 16" and there's a lock symbol on the left side of the yellow bar. I know I haven't leveled it up to maximum yet, and I have plenty of coins: 259,225.

Now that I've discovered that you can buy Piñatas with Gems (Doh!) I have even more blocked off plants:

Kernel-pult

Endurian

A.K.E.E.

Stallia

Red Stinger

Spore-shroom

Dusk Lobber

Shadow-shroom

I'm starting to think this is a deliberate slow down tactic for us more advanced players (like I saw suggested in another thread on this subject).

The maximum level of plants is 4 or 5 for the time being. Later in development, plants will be able to get to levels 10, 15, or 20, depending on the plant.   

(edit: external links are not allowed in these forums).

Plants, depending on the maximum level of their upgrade, are divided into 3 ranks: 20, 15 and 10. At the moment in the game it is possible to upgrade them appropriately, up to 12, 9 and 6 inclusive