The trees I have dumped sank well with one block. I used that twisted nylon rope to tie my trees to the blocks and several of them came loose soon after sinking them. I tied a square knot in the rope but they came undone somehow. I'm going to go to a poultry farm supply store and buy a whole roll of 1/8 cable and aluminum swedges to use this year. I have a pair of the swedge crimpers. I'm going to drill a hole through the trees and fasten them to the blocks.

Did you get those trees at a Lowe's or Marvins type store?