No crash, nothing I can see in reported error logs.

Version : 5.8.2.387

With a Managed Arboretum, Forestry manages to plant all the saplings, fertilize them fine, and grow at least one tree. After that all trees fail to grow. If I destroy a bunch of saplings sometimes trees will then grow, but not often. I've waited about 18 days in game and still no trees have grown. I've verified the farm is receiving all required items and it's in a wide open field so there's nothing above it.

I'm not sure what it causing this for you. It could be worth testing this out in creative and using /gamerule randomTickSpeed 100 (or some other very high value) to check that only one tree will grow and not others.

I really don't know what is happening. The trees grow fine if I crank the ticks to like 2000, but I left a chunkloader in the area yesterday and when I came back absolutely 0 trees had grown. I've left it sitting here for about an hour now and still no trees have grown (on default tick), but as soon as I change the ticks they grow fine. I'm currently trying to test whether creative mode or the default tick is the reason. At least a single tree should grow during an entire day correct? I'm getting no tree growth at all, over multiple days.

running it at a high speed seems to indicate to me that the trees aren't growing because they are directly next to other saplings. When some of the humus runs out of fertilizer and then turns to sand, if there are like two blocks next to a single sapling then that sapling succeeds at growing. Otherwise the trees do not grow at all.

Interesting. What species of tree are you trying to grow?

Spruce, but I tried with Oak as well. I can actually destroy a large amount of the humus and trees will suddenly start growing, but as soon as the entire field is full of humus again trees stop growing.

So, trees will suddenly grow if only a few patches of humus are there, and the rest is sand. This definitely seems related to adjacent saplings. Should I try with a different type of sapling? Where in the code does growth handling occur?

Most of the growth stuff is handled in TileSapling I believe.

I tried to reproduce in a test world but things did grow for vanilla oak and forestry apple oak. Test world is at b477151.

So I guess we need to isolate the cause further. What does your setup look like?

I can upload my whole instance if that would help.

Yep that would be great

https://www.dropbox.com/s/6nzzdo69qa4pw68/ABitOfEverything.7z?dl=0

Hope that link works.

Which mod causes vanilla trees to grow to different shapes?

That is what's causing the problem AFAICT because it's trying to grow giant trees but there are saplings/multifarm blocks in the way. If you try with apple oak things seem to run fine.

I am not sure. I did not create the modpack and this is my first time using it. Awesome find though.

The mod that causes the big trees is Recurrent Complex.
You can disable the big trees by setting saplingTriggerChance to 0 in reccomplex.cfg.
I haven't tested a forestry multifarm yet but I assume with a normal sapling growth the multifarm will work fine.