Cut two 12 inch squares of 3/4 inch MDF, using a table saw, for the base. Cut three small 2 3/4 inch squares and a medium sized square for the top, 8 inches wide.
Glue and nail the two largest squares together face to face, with all four edges square. Use one 1 1/4 inch pin nail every 2 inches. Center one of the small squares on top of this base and glue and nail it in place. Glue and nail a second small square on top of the first.
Cut four pieces of 1-by-4 lumber 48 inches long, using a miter saw. Apply glue along one long edge of one of these pieces. Fit a second piece to the first, overlapping it by 3/4 inch, so that the edge of the second piece is flush with the outside face of the first.
Nail through this second piece into the edge of the first, every 3 inches. Fit the remaining two pieces together in the same way and glue and nail them. Fit the two halves of the post together and glue and nail them in place to form a 4 1/4 inch post, 48 inches long.
Apply glue to the four sides of the small squares you nailed to the base. Fit one open end of the post onto this mounting block and nail through it into the block, three nails in each face.
Glue and nail the remaining small square in the center of the medium sized square and position it on top of the post, so that the small block slides into the open top of the post. Nail through the post into the block as you did at the bottom.
Use a rafter square and tape measure to mark a line around the post on all four sides at 2 inches below the top. Measure to the center of each face of the post and mark it on the line. Set your plant hanging brackets so that the top is centered on this mark, against the bottom of the line.
Use the tape measure to center the bottom of each bracket in the face of the post and attach the brackets with one 1 1/4 inch wood screw through every hole in the bracket. Drive the screws with a cordless drill.