Cut and secure a piece of wood as a divider inside the box. This will divide the planting space from the pond space. We divided our box to one-third planter and two-thirds pond.
Cut four 1×2-inch pieces of scrap wood. Apply sealant to one side of each piece. When dry, nail a piece, sealant side down, to each corner.
On the inside of the box, install a plastic liner or use a water sealant that is safe for aquatic life. Read the sealant label carefully or check with an expert. Apply one coat more than recommended on both the pond and planter sides. On the planter side, drill three holes in the bottom for drainage.
Fill it! Add water, plants, fish, and ornaments to your planter and pond, then sit back and enjoy.
Source: Better Homes and Gardens
Have fun with it and enjoy it.
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