Terraria – the easiest most efficient way to farm Shadow Scales

Personally i'm using a grid of tunnels. The vertical tunnels are using every 5 block a wooden platform to jump up or down.

  |L|  M  |L|
 _| |_____| |__
 _ L _____ L __
  | |   M | |
  |L|     |L|
  | |     | |

• L = Wooden Platform (you can jump through up or down)
• M = Minerals

From the Terraria wiki page on Trees:

Growing conditions

Planting an acorn in grass will create a Sapling that will eventually grow into a tree of a random size and shape. Saplings can only grow under the following conditions:

1. A planted acorn can only be planted on a dirt block covered with grass, corrupt grass, crimson grass, hallow grass, a mud block covered with jungle grass, or on snow.
2. The acorn requires one space of open ground to either side.
3. There must be at least two open tiles between saplings or only one will grow into a tree. This matches the previous rule, i.e. each acorn needs both adjacent tiles free, which means the closest two trees can be is with two tiles between them.
4. The acorn requires at least 32 feet (i.e. 16 tiles) of vertical clearance to grow into a tree.
5. An acorn will not grow indoors - all background walls or overhanging ledges must be removed, and if you have a ceiling it must be at least 32 feet above the block where the acorn was planted as measured by a Depth Meter (e.g, if you are planting an acorn at 300 feet above, there may only be blocks at 332 feet or higher directly above it in order for it to grow into a tree).
6. Acorns will not grow in the presence of torches, wood platforms or sunflowers within its 2-block radius.
7. Saplings must be off-screen in order to become trees. They will not grow when currently in view, unlike most other plants which can spawn while you watch.

Saplings can be destroyed with a Pickaxe.

So, you can't grow a tree at the edge of a step or cliff, or next to a step or cliff, or at the edge of a patch of grass, even though you can plant acorns there. While trees can, very rarely, grow a distance of three tiles apart (two empty columns between them), they grow much faster at a distance of four tiles apart (with three empty spaces between them). For these reasons, it is recommended to plant acorns every four tiles on a long flat row of surface grass. Tiles more than 14 units above the planted grass do not have any effect on tree growth, so an optimal tree farm can have multiple 15-tile high levels."

You seem to have your acorns spaced correctly, there are no walls, and I assume you have enough head room. Although it's not explicitly mentioned in the wiki, your torches may be keeping the trees from growing. Try removing them. At least some of the acorns should grow to full trees in a matter of minutes.