Answer: It sounds to me like the white iris is more vigorous, and can outcompete the blue iris. It may be that the white iris sprouts earlier in the spring--so that in previous years when you've burned the bank, you've set the white iris back a bit, allowing the blues some breathing room to get established. This year, with no burning, the white iris took over.
It seems like a good idea to let the fallen leaves rot every few years or so--to return some nutrients to the soil. But it sounds like it might mean sacrificing your blue iris!
By the way, National Gardening Assn. is based in Burlington, Vermont. If you'd like to join, visit us at http://www.garden.org!
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