Growing stuff underground can be a challenge - even if one finds ready sources of water. The key problem seems to be lack of readily accessible sunshine in The Underdark.
Question: what spells work for growing plants underground?
Mimicking this sunlight itself is problematic. Even spells with actual, literal names such as Daylight give neither daylight nor sunshine.
Fortunately, one does not need 'real' sunlight to grow stuff. But which illusions can offer a reasonable facsimile? The author guesses that some illusions must be better than others. Major Image lasts for quite some time should one cast this at 6th lvl+. Programmed Illusion also lasts quite a while, though it would keep turning the lights on and off. Mirage Arcane would provide light for everything inside of a square mile, but needs replacing every ten days. Perhaps make it grow via eternal torchlight? No idea.
To throw a spanner into the gears, we don't even know if illusions are real or imaginary. Possibly this fake light would not even sustain a tanning bed.
Sum up: Can powerful illusions provide enough light spectrum so as to grow real-living plants?