How To Keep Your Iris Safe From Freezing Winter Temperatures

How To Keep Your Iris Safe From Freezing Winter Temperatures

The iris plant, which is named after the Greek goddess who rides the rainbow, is a common ingredient in most perfumes. It has an iridescent inflorescence that makes it a popular choice for ornamental gardens. This plant is quite hardy and can survive in cold climates, making it fairly easy to care for. However, depending on the cultivar, age, and climate conditions, you may need to take steps to properly overwinter the plant.

In locations with frosty conditions, irises must be protected with straws or structures made from dead leaves, which is not necessary in milder winters. It is also important to deeply water younger plants before winter to promote better anchorage, otherwise, they may suffer from cold damage due to the freeze-thaw cycles that occur in cold locations. Cold, hardy varieties like Siberian iris and Japanese iris can benefit from a little mulching during the winter. Still, you may need to dig out the rhizomes of other varieties and replant them in the spring.

Overwintering iris plants

Pruning yellow iris


For irises, winter care begins with fall pruning. This is done to lend them protection against acute fungal diseases and pests like leaf spot and borers, whose spores and eggs remain dormant in the dead foliage through the cold and activate in the growing season. To do so, tug out all the wilted flowers and foliage and deadhead the remainder using sharp clippers to nearly 6 inches above the ground. While you’re at it, ensure you have thoroughly removed all spotted, diseased parts (if any) and shape the remaining leaves into an upright triangular shape.

Once the ground freezes, cover the soil with a 1-to-2-inch-wide sand or mulch layer and top it up with evergreen branches. This is particularly crucial for newly-rooted iris plants, as the frequent freezing and thawing cycles make them susceptible to heaving, meaning they’re easily displaced from the ground under the cyclic impact. Moreover, it insulates them against frost damage. However, ensure the mulch isn’t water-retentive, as irises despise wet feet. Not to mention, wet soils can cause root rot and promote fungal growth. Oat or wheat straws and hay are viable options, but avoid grass clippings at all costs. That being said, if ensuring proper drainage is a tall task, then lift out the rhizomes and store them in the basement or a garage for the winter.

Other winter care tips

Indoor iris plant

Peter Turner Photography/Shutterstock

Like garden-planted irises, their containerized counterparts also need winter protection. Irrigate them occasionally if the soil is dry to the touch and the temperatures remain over 45 degrees Fahrenheit. In areas experiencing winter rain, add gravel or sand to the potting material to ensure the rhizomes don’t remain standing in water. Also, cover the containers with mesh or burlap to keep hungry critters like squirrels from munching on your hardy perennials and provide wind insulation. Alternatively, bring the pots indoors if the weather is extremely harsh and place them back when spring sets in.

As for the ground-planted irises, stagger the removal of dead leaves and the mulch once the snow melts and the temperatures rise to 50 degrees Fahrenheit or more. Start by taking off the top layer to expose the ground underneath to sunlight and drying it completely. Follow it up with the complete removal of the mulch layer on a hot day while making sure plant foliage and branches suffer no breakage. Also, check for any signs of dry rot and deal with it by cutting out the damaged parts or treating them with a fungicide.

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Dr Heidi Parkes

By Dr Heidi Parkes

Senior Information Extension Officer QLD Dept of Agriculture & Fisheries.