Mulberries change colour from red to black when they are perfectly ripe. Once ripe they will fall to the ground, it's best to check your mulberry tree every day and hand-pick any ripe berries. If you have a very large mulberry tree, the easy way to harvest ripe mulberries in bulk is to place a large tarp or sheet under the branch and shake the branch. The mulberries that are ripe will fall. However, ripe mulberries will squash easily, so be as gentle as you can and don't allow large piles to form, the berries at the bottom will be crushed.