In Creativerse, some trees can be grown from Saplings with a bit of patience.
Currently three kinds of tree Saplings exist in Creativerse:
How to obtain
To collect Saplings, just harvest ("pull") blocks of Leaves from Ashenwood, Cragwood and/or Elderwood; eventually Saplings will be added now and then. Saplings will even appear occasionally if you place blocks of Leaves on the ground and then pick them up again.
Since update R26 you can now also occasionally obtain Saplings when harvesting Wood from trees.
However, Excavators of any kind will not yield any Saplings when used on tree Leaves and/or Wood. Also, harvesting Corrupted Wood and Corrupted Leaves will not lead to finding any Saplings, even though they can be purified into Cragwood and Cragwood Leaves.
How to plant Saplings
Saplings need to be placed on suitable types of natural blocks and into suitable surface biomes - according to the developers preferably with a bit of moisture nearby to grow.
You can plant 2x2 Saplings or even more of them together. Then a tree with a larger stem (4 blocks thick) might grow. Or maybe two trees on top of each other. The maximum size of player-grown trees seems to be 4x2 blocks in width (for Elderwood and Cragwood), but maybe you'll be able to grow even larger trees?
Suitable Soil for Saplings
To make trees successfully grow, Saplings should be planted/placed on (green) Grass or Dirt blocks. You can simply place such blocks to grow trees on, they do not have to be "naturally" generated at this spot.
Saplings won't grow on most other types of soil, like Mud, Detritus, Matted Needles, Canyon Floor, Rocky Dirt, Dead Grass, Savannah Grass, Sand, Sandstone, tree Leaves or Mold - if you try to plant Saplings on these types of ground, a message window will appear, telling you: "Item cannot grow on this block type."
Suitable Biomes for Saplings
Additionally to that, the place to grow trees on has to be on the surface with enough open space upwards ("under sunlight") and - most importantly - in a Biome that supports trees, preferably Woodlands and Forest, also parts of Mountains and others.
Grassland is not always advantageous for trees to grow, but this will also depend on the "neighbourhood". While Swamplands and Savannahs are usually unsuitable Biomes for Saplings, there might still be a few areas where they will still grow into trees, which you can only find out by trying.
Simply check the Sapling after placing it on blocks of Dirt or Grass - if the Sapling is fallow (in red letters) then the place is not suitable, most likely the whole Biome. You can try to plant different Saplings there, or you can try building a hill from Dirt, sometimes Saplings will grow up there, but sometimes nothing might help if the Biome or spot is not suitable.
You can also plant Saplings at the shores of lakes or rivers, or into a lake on a few blocks of Grass or Dirt placed directly into the surface of the Water - it definitely works fine. Oceans aren't often suitable Biomes though.
You can use fertilizer (Pigsy Droppings) on Saplings to make trees grow a little bit faster - only at about 15 minutes though. You cannot fertilize "fallow" Saplings, because they won't grow at that spot no matter what.
After that you'll just have to wait.
It will take a few RL-hours (ca. 15-16 RL-hours) until trees eventually "sprout" from Saplings - unless they are "fallow" of course. Saplings that grow will turn into full-grown trees instantly which you can observe if you're close at that moment.
Since update R22 in September 2015, the growth-system for crops, saplings and queen bees now functions in real-time, as per patch-notes: "No need to be in-game for nature to nurture."
When Saplings stay fallow
Take a look at the Sapling right after planting it. If it stays "fallow" (written in red letters) even after placing water nearby, then it will not grow into a tree no matter what. Even if it's too cold and you try heating up the area, this won't help.
You can try growing another type of Sapling there, but in the most cases the place is just unsuitable for whatever reason and trees won't grow at this spot. You can just tear the Sapling out and take it with you in order to try planting it somewhere else.
Trees you grow yourself usually will be of small height with a symmetrical shape. They might often not have any Flowers nor Beeswax on them right away, however those can grow/spawn by themselves in any tree-top later on.
Player-grown Elderwood trees will not feature any Rimecones on or under them. You can produce Rimecones yourself though, by using a Plow on Elderwood Leaves. If you grow Elderwood trees in cold biomes, their leaves will transform into Snowy Elderwood Leaves all by themselves.
Plows cannot be used on Snowy Elderwood Leaves.
Double or triple trees
It is even possible to make trees grow on other trees. It can happen unintended when planting Saplings very close to each other. However even triple-trees or higher "staples" of trees can be grown deliberately too by placing blocks of green Grass and Water (or Mineral Water) on tree-tops. See explanations on the forums.
Sapling on display won't grow
If you put Saplings into Flower Pots, Wood Planters, on Wall Shelves, Hidden Temple Altars or Placemats, then the Saplings will not grow into trees there, but will merely be preserved and displayed for decoration.