Do Elite Pathfinder Challenges give Iris Petals?

They are a top-up, not a route: Elite challenges alone will not reach the Irisalis milestones. Petals add up toward milestones that unlock rewards, the legendary Irisalis encounter among them, and collecting them is slow. That holds most of all if you stick to the permanent sources.

So clear the Elite Pathfinder Challenges as you meet them and count what they pay, but plan the bulk of your petals around other sources. The Legendary Journey event screen has a built-in "How to obtain" section listing the ways you still have left to earn Iris Petals. Check it whenever your count stalls instead of grinding one activity.

The bigger contributors include the eight petals placed around the open world, first clears of each Alpha, the silver collection badge at 70% progress in a region, the Windchasers Departure journey quests and Branch Nurturing. You can still earn petals all season through normal progression, permanent objectives, weekly rotations and the shop, including after the event is finished, so a slow start is not a missed chance.

Not settled: Which source supplies most Iris Petals is not settled. It may be permanent progression rewards, or it may be the stamp shop and event shop, and that changes how small the Elite challenges' share really is.