Why do iris plant bulbs grow better contained by tree shave? ...
WHERE TO PLANT
Irises require at least a half-day of sun. Some shade is beneficial within extremely hot climates,(but ONLY hot climates) but in common irises do best in full sun. Provide your irises beside good drainage by planting on a slope or within raised bed.
Assuming you meant "shade", I enjoy to say they don't. All the variety of Bearded Iris I have grown do better the more sun they enjoy, and they thrive in full sun as long as they achieve plenty of water.
Are you dividing the rhizomes every 3-4 years? They have need of that to thrive.
Answers: The bulbous Irises, which includes Dutch Iris & some Reticulated Irises can grow in full sun to feathery shade.
http://www.tesselaar.network.au/growingguide...
They bloom early contained by the season when the area underneath the trees are actually sunny at that time...because the trees don't enjoy their leaves yet. This is similar to planting sun-loving tulips below trees.
The Bearded Iris needs full sun but aren't planted beneath trees. They bloom later within the season & wouldn't do well growing below trees because it's too shady for them. They have rhizomes that would rot surrounded by an area that's too shady. They resembling the sun beating down on them through out the summer.
Iris Lazica is different from other rhizomatous Iris & "enjoy light shade and moist soil. The best place to grow it is contained by a sheltered spot in spotted shade."
http://www.webgardenguide.com/Iris_lazic...
Different types of Iris require different growing conditions, so it would depend on the type of Iris you're planting & your climate to determine if it would or would not do better growing under a tree.
Good luck! Hope this help.