Terraria – How to place the torches in terraria

No, they can grow at any time.

Plants have three states: shoot, growth, and blooming. All plants start as a shoot.

They transition from shoot to growth randomly, and each plant grows at a different rate. The only thing that influences how long this process takes is (pseudo) random chance. Depending on your luck, it could be anywhere from milliseconds to years.

After they've grown, they start blooming when certain conditions are met (which I've detailed in this answer); this too can happen with the player watching.

If you're farming with clay pots, it is best to plant as many shoots of the desired type as possible to mitigate bad growth luck.

I set up two mushroom gardens to test this:

One: -200 feet, torches every few steps under the one block of mud, about 4 mushroom grass planted at the same time.

Two: -800 feet two mushroom grass planted at the same time, one with a torch underneath, one with no torch.

Result: In garden one the tall mushrooms grew all over the place at about the same time. In garden two the tall mushroom grew under the torchless portion first.

Conclusion: torches don't really matter, just be at negative depth, give about 20-25 blocks of space above. Above all, wait. The tall mushrooms take a while to show up.