Minecraft – How do i get farmers to replant what i want to have them replant

An option that may be more or less drastic, depending on how much work you've already done: Rearrange your storage room so all the chests face west.

The front of the chest (with the latch) will face away from any adjacent solid blocks, or face west if placed independent of other blocks. The viewpoint of players has no effect on the orientation of placed chests. —Minecraft Wiki

Since these are double chests they only have two "choices" of which way to face. If that choice doesn't include west (i.e., you've got them facing north-south), then it's probable that arranging them all to face south will have the same effect. That's assuming that the chests face west when they're alone due to the south/west rule.

To grow Melons, you need Farmland to plant the Melon Seeds in, and dirt, farmland, or grass blocks adjacent to those where the Melons can grow onto. Melons grow slowly until their stem grows to the maximum size, at which point a Melon grows on a suitable block adjacent to where the stem grew out of (where you planted the seed). The Minecraft Wiki page on Melons has a nice section on farming them. They suggest using an arrangement like this:

S S S S S S S S S |
# # # # # # # # # |
# # # # # # # # # | S = Watermelon seed
S S S S S S S S S | # = Dirt/Grass/Farmland
S S S S W S S S S | W = Water block
# # # # X # # # # | X = Random block
# # # # # # # # # |
S S S S S S S S S |