Unleashed Monster

We have an unusual history with Yellow Flag, I. pseudacorus.  Many years ago while Ed was still teaching, a lad from the special class brought a clump of this plant to school for his teacher.  He had pulled it from the river flat on his way to school.   A lesson in never taking plants from the wild was delivered and a piece of this treasure came home with Ed.  We planted it in an inside corner of the stone square where its leaves provided a background for other plants.  The sword like bright green foliage looked great all summer.  The plant spread and seeded with great abandon. We soon tired of hacking it back.

Still ignorant of the  plants history and invasive nature, we moved a few small pieces back to our pond.  Some were planted in the water and others were planted on land at pond's edge.  All survived and it appeared that we had returned a native plant to its natural surroundings.
    

Some of the land between our home and the road is to steeply sloped to safely mow.  Underlying gravel keeps it very dry and I thought that we would try yellow flag here as a no care ground cover.  The lack of moisture has slowed its spread here but it is alive and well.

Time for a reality check.  Despite the fact that this plant was found in the wild it is not native to North America.  It was moved here from Eurasia as a garden plant and it has escaped cultivated settings.  This huge vigorous pest has naturalized elsewhere and is displacing our native Northern Blue Flag.  We also grow Blue Flag here and find it hard to believe that anything can crowd it out.  It too is a vigorous spreader that seeds freely.

I have no idea how to offset the damage that we have done.  Our pond will soon be full of Yellow Flag as will the wet areas around it.  This is not the first instance of an invasion by a plant that was moved for amusement and profit around gardens.  We have made this mistake several times but never on so grand a scale.