When filling huge flowerpots, reduce the weight and the amount of soil you need by filling the bottom with recycled styrofoam peanuts or even chunks of styrofoam. I get pieces when I need them from the dumpster behind a furniture and accessories store.
To make my large pots lighter...I have turned tin cans upside down in the bottom of large pots some years. Sometimes I have used plastic green or black pots upside down in the same manner. The tin cans will slowly rust, but so what, by that time it is the end of summer and the annuals are finished anyways. Next year, start with new tin cans.
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