The pretty, yellow-flowered Iris pseudacorus (yellow flag or European yellow iris) is spreading in a damp area of our field. That’s not good.

This non-native perennial that grows from 1 to 3 or 4 feet tall and flowers from May to July has escaped from cultivation in many parts of Canada and the United States, spreading by seed and rhizomes to the point that it can be weedy or even invasive. It’s gone a long way since it was introduced in the mid-1800s as an ornamental from Europe.

Yellow flag spreads readily in wetlands, ditches and moist soils. It can even populate dry soils and tolerates salty conditions. Its dense network of rhizomes takes over patches of ground, increasing in spread year after year and crowding out other species. It also produces large brown seedpods full of seed that can float downstream and propagate new stands of iris elsewhere.

According to the “New Hampshire Invasive Species Factsheets” at, yellow flag “has escaped mainly into freshwater wetlands, but also can be found in floodplain forests, lakes or ponds, rivers or streams, yards or gardens — in water up to 10 inches deep. It can tolerate… pH 3.6 to pH 7.7, and the upper zones of salt marshes.” It’s happy almost anywhere! No wonder its sale is prohibited in New Hampshire — and in Connecticut, Massachusetts and Vermont, among the New England states.

Dense areas of yellow flag can collect sediment, where trees and shrubs can take root and change the character of wetlands.

The University of Connecticut, at, reports that the plant “reduces wildlife food supplies by out-competing arrow arum (Peltandra virginica), an important duck food.” This is part of the problem with invasives: They not only deter the growth of other, often native plants, but also of the native animals – insects, birds and others – that feed on or otherwise depend on those native plants.

In our field, a few once-small patches of yellow flag have increased in size noticeably this spring and are moving in on our beloved plot of milkweed, Asclepias syriaca, so important for monarch butterflies, as well as for several other insects — bees, wasps (including those that prey on insect pests), flies, other butterflies. Despite mowing the iris foliage repeatedly, the plants are thriving.

I should have dug the clumps when they were small. Now I’ll try cardboard mulch topped with grass clippings. (The New Hampshire publication cited above notes that resinous substances in the leaves and rhizomes of yellow flag can irritate skin, so take care if you try to dig the plant.)

I’ve noticed that the snails that have proliferated here in the last few years congregate on the foliage of yellow flag, so maybe controlling yellow flag will also diminish the snail populations … a bit. Obviously, the snails are not controlling the irises.

If you want to grow the beautiful and showy I. pseudacorus, keep it in a pot and deadhead the plants before they go to seed.

Yellow flag isn’t the only problem plant we have to watch for on our property. Others include Asiatic bittersweet, multiflora rose and glossy buckthorn. We’re lucky not to have purple loosestrife, autumn olive, Norway maple and several others. The University of Maine has fact sheets for identifying and controlling 22 invasive species (and does not include yellow flag). And the Delaware Department of Agriculture has an excellent publication called “Mistaken Identity? Invasive Plants and their Native Look-alikes” at