Siberian iris (Iris sibirica) and Dutch iris (Iris Xiphium hybrids, sometimes called Iris hollandica) are spring blooming plants of the Iridaceae or larger Iris family. The two types are among the 200 or more species Iris. Unlike the tall, showy, bearded iris, Siberians and Dutch iris have no fine hairs or "beards" on their front petals. Dutch iris grow from bulbs and can reach 18 to 22 inches tall. They bloom in May and June. Siberians are even taller, rising to as much as 36 inches. Both types bloom in shades of white, cream, purple, blue-purple and yellow.
Siberian Iris hardiness
Siberian iris are native to Central Europe and Russia. Depending on the variety, the Siberians are hardy from USDA Zones 3 or 4 to 8 or 9. This means that in Zone 3, the plants can survive winter temperatures of -35 or even -40 degrees Fahrenheit in Zone 3A. The fact that the Siberians are also hardy in warm weather climates (USDA Zones 8 to 9) means that they can also withstand relatively high summer temperatures, like those experienced in Florida and Texas. It is worthwhile noting that the plants are dormant during the hottest months of the year.
Dutch Iris Hardiness
The parentage of modern Dutch iris hybrids is complicated. The parent species may include Iris xiphium, Iris filifolia, Iris Fontanesii and Iris tingitana. The native range of the parent species is from southern France through northern Africa. Sources differ on hardiness, but hybrids available in the United States are generally hardy from USDA Zones 5 to 8. This means that they can stand winter temperatures as low as -15 degrees Fahrenheit in Zone 5a and can tolerate the summer heat in Florida and Texas.
Pushing the Zone
When trying to assess the hardiness of Dutch and Siberian iris, it is useful to follow the USDA zone recommendations suggested by vendors and reference sources. However, there are still many variables. If planted in a protected spot, near a heated structure, the iris may be able to survive in a locations with colder winter temperatures than the coldest recommended zone. Sometimes consistent snow cover acts as an insulator, protecting a plant from cold. Plants grown or in light shade may thrive in areas hotter than are normally recommended for the species or variety.
Best Planting Practices
To maximize hardiness, plant Dutch iris in the fall, about 6 inches deep in well-drained soil, ideally sandy loam. To protect the iris, cover with a layer of mulch after the ground has frozen. Siberian iris is also fall-planted. Bare-root specimens should be planted about four inches deep, with plenty of space between plants to accommodate the species' clumping nature. Consistently moist soil is a must.
Plant hardiness is also conditional on soil type and soil moisture content. For example, Siberian iris are hardiest when soil is moist. If they are planted in dry places, they will need consistent irrigation or they will die. Dutch iris should be planted in free-draining soil, as too much standing moisture will make the bulbs rot.