Iris pseudacorus – Yellow Flag Iris

09 May

It is the end of spring and the Gardens’ lake is starting to spruce up. Impressive upright leaves that adorn the lake’s bank throughout the year are raising long stems topped with big yellow flowers. These are plants of Iris pseudacorus, commonly known as yellow flag iris, whose flowers are among the first to bloom in our lake and among the last wild irises to bloom in Israel.

It is a rhizomatous herbaceous perennial. The rhizome grows and spreads in the muddy soil, creating large colonies of a single plant. The yellow flowers have a three-fold symmetry, as do all iris flowers. The petals are decorated with delicate brown stripes like markings on an airstrip for pollinators:

Photo: H. Zell

I. pseudacorus is one of two wetland irises growing wild in Israel, the other being Iris grant-duffii. Both are endangered in Israel due to habitat destruction and loss. Wetlands are typically disjunct in our dry country, and overpumping has aggravated the situation. A wild population of I. pseudacorus persists in Ein-a-Tina (Ein Nutra) in southwestern Golan, and rescue operations have successfully reintroduced it to the HaHula reserve.

Indeed endangered in Israel, yet widely distributed worldwide. Yellow flag iris grows in flooded areas, swamps, and diverse water courses and bodies in Morocco, Europe, East Asia and up to west Siberia. In addition, it has been introduced as a gardening plant in New Zealand, Australia, South Africa and the Americas. In these areas it has spread and become invasive.

Photo: Ori Fragman-Sapir

The plant reproduces sexually by seed (seeds are able to float for a month without rotting before arriving at a safe shore) and vegetatively by the rhizome, creating local colonies or distant colonies as broken rhizome sections are washed downstream in strong flows. The thick rhizome enables the plant to withstand short drought periods between floods.

The plant is resistant to herbivores and herbicides as well as being resilient in acidic and anoxic conditions.  These characters partly enable its invasiveness.

In addition to these traits, I. pseudacorus has the ability to absorb heavy metals and reduce bacteria populations such as E. coli and Salmonella, thus making it useful in the purification of urban wastewater.

Beautiful and useful, yellow flag iris is also suitable for the private garden. It is recommended for regularly irrigated soil, at the edge of a water pond, or immersed in a water body, planted in heavy soil. Light conditions should be full sun to half a day of sun. The tall leaves (80-150 cm) will provide a striking vertical element throughout the year, and the attractive flowers will be a nice addition to your garden’s spring display. And though in some countries horticulture was the source of the invasiveness problem, this problem does not exist in Israel. On the contrary, this is one of a few rare plants saved here through garden cultivation.

Photo: Ori Fragman-Sapir

May 2017