I bought some 7 foot trees from Wayfair & when one blew over, I decided I needed to put them in big planters.  The planters are fine, but I decided I needed to weigh them down.  Bought some concrete bricks from lumber store, 4 bricks, 2 (in ea planter).  The bricks are very heavy and cost like $1 each.  So am quite pleased with these planter boxes.  I also bought some moss to fill in.