Planter Box is a very handy item. With more Planter box, you can grow more Plants at one time which aids you in Road Trip battles. In the beginning, 10 planter boxes are given in the first Lot for free. It is also given occasionally, in number of one or two, when you expand to new Lots. You can buy more of it by paying 125 Gems each. Plants on Planter boxes are dysfunctional, either in initial form or fully grown form, until you actually put it on the field beside or on the Path.
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