The first time I realised there was a problem with our flag irises was when Mark intervened to pull a photo from a House and Garden feature on our garden a few years ago. It was a view similar to that above. “It’s a weed,” he said. “It is embarrassing to be shown with a weed.” It has taken a while, but this week, I have dug out the last of it.
The problem with Iris pseudacorus is its resilient and invasive ways. It can survive in water, even in salt water, beside water, on water as a floating mat of rhizomes and even just on heavy soils and flood-prone areas. It is also poisonous to both humans and stock so is not controlled by animals. It is on most New Zealand weed lists and some regional councils require active management to control its spread.
Our flag irises have never set much seed, though all the descriptions say that seeding is a problem. We would have acted faster, had we seen threatening seed set. But the rhizomes, the rhizomes! They are sign enough of an issue. They form a dense mat both on and just under the surface. It reminded me of digging out wild ginger with their ability to form a solid barrier, choking out other growth. It was heavy, muddy work digging them out and prising stray rhizomes from farthest reaches with me, precariously balanced. I did not want to end up in the stream at this time of the year.
We had them planted by running water; the upper reaches of the Waiau Stream flows through our park. It was clear that floods or any disturbance of the rhizomes could cause some to wash downstream and from there, populate the countryside. It would not have been an issue had they been by a self-contained pond with no means of escape.
True, we grow Primula helodoxa and that is sometimes flagged as a weed issue by waterways but we dead head it. It is the seeding that is problematic with that plant.
Fortunately there are alternatives to the flag iris which do not seem to be a weed problem and are arguably considerably prettier. Iris sibirica, the Higos and the Louisianas are all happy in similar conditions and give us a League of Nations in our park – Northern Europe and Siberia, Japan and USA. We just happened to have plants of all hanging about waiting to be planted. So while the three patches of flag irises are now muddy and apparently bare, in a few months’ time, they may even be flowering again in hues other than yellow.
We are enjoying the water meadow effect we are achieving in that area of the garden.