How can I create an efficient tree farm?

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.

Terraria wiki guide on Gardening

By placing a dirt wall inside the layer of grass blocks (not behind the tree, the block below the tree), one can prevent tall grass from spawning, maximizing efficiency. The dirt wall should peek out from behind the grass blocks and be visible behind the tree, but not actually being in the block behind the tree. The dirt wall should be behind every block of grass, not just the one under the tree.

So place one dirt wall behind the grass that the trees are on, so it just barely sticks out, and there will be no tall grass and you'll never have to weed again.

This is my method of handling a tree farm, I space it at every fourth block for planting for aesthetic reasons, and use the torches to help me mark it out. The spacing from top of ground to bottom platform (used to eliminate vines dropping down) is 18 blocks. I usually build up 3 or 4 tiers of 81 blocks wide for worlds where I want a wooden housing projects or to use platforms for skyways and "hell-evator up" mechanisms. My biggest farm was a 3 tier, 401 block wide farm, producing 100 trees per tier, average was about 2400 wood per harvest cycle.

I have made similar farms for Mahogany, but that is a jungle grass planted trough with a half block deep of water, spaced 18 blocks of height per tier. You don't plant acorns in this method, but it will produce mahogany trees very regularly. Just be aware that that will create a jungle biome, with all associated mob spawns (hard mode blood moons make this a bad thing), so ideally don't put it too close to your housing.

(EDIT from 1.3.1:) I just discovered that mahogany can be planted now with acorns on jungle covered mud. Be aware that you do not gain ANY acorns from mahogany farming, but if you have been doing regular wood, boreal wood, or corrupt/crimson wood at all, you should have a few stacks of acorns you can utilize. For my mahogany farm, I usually combine it with a jungle shrubbery farm for getting nice early fishing bait. You can also sink the tree platform under a tile of water to prevent the shrubs from spawning. (End Edit)

The only mechanism I don't have in this world's tree farm, is my auto-mowing mechanism. I use a pressure plate activated dart trap set to fly along the ground, and that "mows the verge" along the top of the ground. Makes mushroom picking fast and grasshopper gathering easy. WARNING: Birds and Bunnies will be in danger using this method.