I've built an underground forest chamber in my main fort as a convenient lumber supply. It has a dirt floor and plenty of torches, but the trees I planted there haven't sprouted yet. I have heard that wheat will grow under torchlight (I believe X's Adventures in Minecraft mentioned this). Do trees also share this property?
Trees can definitely be grown underground with sufficient light. According to this thread on the Minecraft Forums, you need 7 blocks of space above the sapling. (Perhaps this condition isn't being satisfied.)
7 blocks of height is required.
A neat trick is to put stone next to the saplings, so you don't have tons of leaves that you have to deal with.
from above, this is what I do:
XXXXXXXX SXSXSXSX XSXSXSXS XXXXXXXX
S <- Sapling X <- Rock
Note, the Sapling has to be planted in dirt. Put torches on the rock pillars to light the saplings,and you are good to go!
Any light will work for trees. However, you may need more space.
My underground (automatic >:D) tree farm is 200x200, for the trees at least, and I've found trees require at least 1 block on all sides, and at least 7 blocks up. I have it at 10 right now, but to get the bigger trees you'll have to expand it to around 20-30. To get large jungle trees, you need around 60 blocks space, and yes, I have done this before.
I know this is somewhat old thread, but here goes...
Bring bone soup (just mash bones of skeletons) and throw it at the tree and it works as a super fertilizer. If it doesn't grow, your tree does not have enough space. Sometimes you need to use the fertilizer several times for the tree to grow. I wouldn't waste more than 5-6 to see if the tree "pops up".
Just my 2 cents.