Trees won't grow if they're planted too close together and cedar trees will only grow in the northern part of town. Banana and coconut trees can only be planted on the beach. You don't need to water the trees to make them grow.
Make sure you leave a space between the trees - make sure you would be able to run the whole way around the trunk without bumping into anything (that's the way I do it anyway) and you should be good.
I know that the town plaza you can plant trees right up in there I'm doing that right now, but usually not close by houses, buildings, rocks, or public work projects ect, and I forgot where I read this, but if you are planting trees and bushes next to each other, they will grow if it's 12 consecutively placed, I think that's why sometimes when you want a bush or something to grow but it won't is because it exceeded the 12 amount limit, but if I'm wrong you can correct me xD
You should specify about where and the distribution you're planting the trees in. Usually, you can't plant more twelve trees and bushes in a row, and you can't plant two trees or bamboo shoots next to each other or plant them right next to a building or PWP, either.