In creative mode you can plant saplings anywhere (as long as it’s in dirt or grass) and they will grow almost instantly into trees, regardless of area.
Not so in survival. In survival trees are very picky and will only grow if the conditions are right and will take a very long time to do so (about one game day). Even if the conditions are perfect, they may still wither. Good advice: plant lots of saplings.
To help you with deciding where each tree is to be planted, I added a thermometer. I also wanted to add hygrometer, but I ran out of time for 1.13.
As some of you (especially those who meddled in the chunks.dat file) know, each place in the game has an associated humidity and temperature values. They determine what kind of plants and terrain get generated there and what is the shade of grass/water/leaves.
With a thermometer you can instantly tell what temperature it is at a given spot. The thermometer item has a scale made of 5 dots. Here’s the temperature value associated with each dot (starting from the bottom): 2, 5, 8, 11 and 14.
Here’s what temperatures trees like the most: Spruce 1.5, Birch 7.5, Oak 10.
All trees like humidity. The more the better, but because you don’t have a hygrometer yet you have to judge by color of grass (or abundance of other plants). The more juicy the green, the more humidity is in the area.
Happy planting once 1.13 arrives.
[EDIT] Thermometer looks like that: