The Q&A Archives: White Siberian Iris Superseding Blue

Question: I planted a stream bank, approximately 150 square feet with blue Siberian iris, and a few white about ten years ago. When I discovered it invaded by quack grass, about three years ago, I pulled out every iris and tried to get rid of the grass. I now watch and weed it carefully, every spring and later. This year I note that it is now about three-quarters taller white iris, and sparse shorter blue iris. This is the first year I did not burn the bank in the spring because I decided I should occasionally leave some of the fallen leaves to rot. What is happening?

Above you ask if I am a member of National Gardening Assn. I believe I am. Is that based in D.C.?

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!

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