CG What are the advantages of designing with water irises?
CD Their tall, striking foliage is attractive throughout the growing season and acts as a terrific backdrop once flowering finishes. Also, bloom time varies amongst species, so it’s possible to have irises flowering in your water garden from May until July.
Irises provide a striking accent as well when planted with cardinal flower (Lobelia cardinalis), water lilies and small-leafed floating plants.
CG What kinds of structures and materials work well with these plants?
CD The green or variegated foliage of irises softens the edges of a pond and provides a marvellous juxtaposition when placed beside formal pools, whether they’re framed in square-cut stone or flagstone. In informal ponds, just as rocks and boulders mask the liner to give ponds a natural look, irises placed in the shallows do the same.
CG Should they be planted in groups?
CD Grouping together different species will provide blooms over a period of several weeks; the palette can include primary colours (red, blue and yellow) plus endless variations in between. Every three or four years, irises should be thinned out to keep them vigorous and blooming well.
CG What are your personal thoughts? Caveats?
CD Many of the most interesting [iris] colours are available through mail-order companies, while blue- and yellow-flowering ones are more widely available. (Yellow flag is so vigorous it has been used in sewage treatment plants and to remove metals from wastewater.)
CG How do you get season-long interest?
CD Because many of our common ornamental plants are round or mounded in form, irises can provide a much-appreciated foil or contrast with their strong, vertical lines and long-lasting foliage. In mild regions—Zones 6 and warmer—the foliage may remain erect and green even in winter. I grow irises in open-weave free-standing, plastic containers year-round, which I slip into an above-ground water feature every spring. Over the winter, I place the containers on a shelf of the pond, or plant them into the mud at its margins.
Other bog plants make ideal companions for water-loving irises:
Double marsh marigold (Caltha palustris ‘Flore Pleno’) Zone 2
Pickerel weed (Pontederia cordata) Zone 3
Double Japanese arrowhead (Sagittaria sagittifolia ‘Flore Pleno’) Zone 6