Monday, December 26, 2016

Winter Iris Gardening

by Tom Waters

This being the day after Christmas, it seemed appropriate to write something "seasonal" for this blog post. So what does winter mean for the iris grower?

A word to the wise: Iris growers live in many, many different climates. What is true in one climate may be false in another. Be suspicious of any gardening advice on such topics as winter care that is written as though it applies everywhere. In this post, I'll mention a few things that one might want to consider, but I do not offer any absolute advice. For that, you need to speak with other gardeners in your own area or just do some trial-and-error work on your own.

All irises grow in temperate climates. They are adapted to the changing seasons. Most kinds have periods of rapid growth in spring and autumn, but slow down or go dormant in summer and winter. In fact, irises need a distinct winter with cold temperatures in order to bloom; they will not do well in tropical climates.

Selecting irises for your winter conditions. All garden irises are ultimately derived from wild iris species from different parts of the northern hemisphere. The climate where those species live can tell you something about how those irises will do in various climates. Louisiana irises, for example, are native to the southeastern US, where winters are mild and summers are warm and wet.

Among the bearded irises, winter hardiness varies a great deal. The original diploid tall bearded irises come from the species Iris pallida and Iris variegata, native to central Europe, often at rather high altitudes. They have little difficulty surviving cold winters. These diploid types are today mostly found among the miniature tall bearded (MTB) irises. Modern tetraploid tall bearded irises also have species from the eastern Mediterranean in their ancestry, meaning that some of them fare poorly in colder climates. Depending on the particular mix of genes, modern TBs can be utterly hardy or quite tender, or anything in between. So how is one to know? Checking with other growers in your own area is always good advice, but one can also take a clue from the region where the iris was originally hybridized. Irises bred in Canada or in the US midwest are almost certain to be suited to cold winters, while those from the Pacific coast are not necessarily so. Border bearded irises (BBs) have the same ancestry as TBs, and so the same considerations apply.

Among the dwarf and median classes, miniature dwarfs (MDBs), standard dwarfs (SDBs), and intermediates (IBs) virtually all have Iris pumila in their ancestry. This tiny species is a native of central and eastern Europe, growing at higher elevations and in more continental climates than most of the TB species. It is very hardy, perhaps to a fault, because it has a reputation for failing to grow and bloom well in climates with mild winters. Consequently, growers in places like southern California and Arizona sometimes find that these types (the MDBs especially) do not do well for them.

Arilbreds vary in their degree of winter hardiness. The aril species grow in southwestern and central Asia. Although some of these are adapted to the very warm climate of the deserts of Israel, Jordan, and Syria, most arils are in fact mountain plants used to extreme winter cold and extreme summer heat. So why do northern growers find many arilbreds too tender for their climates? The fault is probably in their TB ancestry. The center of early arilbred breeding was Southern California, and the TBs used in arilbred breeding were those that did well in that mild-winter climate.

Having made these generalizations, I encourage iris growers to experiment with types that "conventional wisdom" might recommend against. Every garden has microclimates, and every category of irises has cultivars that are surprisingly adaptable.

Winter care: mulching. Irises are not very different in their needs from other perennials you may grow, so in climates where winter mulch is beneficial, it can be applied to iris plantings as well. The main purpose of a winter mulch is not to keep the plants warm, but to moderate the cycles of alternate freezing and thawing that can push plants out of the ground expose them to risk of winter rot. Snow makes an excellent insulator. If your climate is such that you can count on a fairly thick cover of snow all winter long, you have the ideal natural winter mulch!

I am not so fortunate here in northern New Mexico. We get temperatures below 0 degrees Fahrenheit every winter, most often without any snow cover at all. I do not apply a heavy much, but I do allow garden debris to stay in place over winter, giving the crowns of the plants some buffer against the wind and cold. I also put down a layer of cotton bur compost in the late autumn. Winter weather gradually degrades it and incorporates it into the soil, but in the meanwhile it seems to offer a little protection.

Beware that mulches can harbor overwintering insect pests and can collect water. In climates where these are issues of concern, it is best to forgo mulch.

Winter care: water. In climates where the ground freezes, watering in winter is nor desirable, and often not even possible, so winter offers relief from this particular garden duty. Many gardening books seem to assume this is true everywhere. However, if you live in a dry climate with spells of warm weather during the winter, you should pay attention and provide a little supplemental water as needed to keep the garden from becoming totally desiccated. Not much is needed: remember that the plants are dormant or semi-dormant, and that evaporation is less because of the cool temperatures.

Seeds! For those of us who like to grow irises from seeds, winter is an important time. Like most temperate perennials, irises have seeds that resist germination during the winter, to sprout when spring arrives. The cold and wet of winter are actually part of the preparation they need to germinate. The simplest way to grow irises from seeds is to plant them outdoors in the autumn, where they can overwinter and come up the following spring. Nature is unpredictable, of course, so many people prefer to use an indoor refrigeration process to replicate winter conditions. I'm not really set up for that kind of project, so I plant mine outdoors and let nature do her work.

Seeds from my own crosses get planted around the time of the first frost in autumn. Those I get from seed exchanges or other providers usually come later, in December or January. The longer they have to experience the winter wet and cold, the better. Since winter and spring weather here is erratic, germination is unpredictable. I generally leave the seeds in place for several years.

I hope I've touched on at least a few topics of interest. What are your own experiences of irises and winter?


  1. I have bearded iris blooming in dead of winter. Just transplanted in Fall from pot into ground. A gorgeous yellow and a beautiful blue is getting ready to open! With hard Frost's!

  2. Wonderful information! Thank you for posting.

  3. One thing that I have noticed here in Phoenix, AZ, is that the varieties that put on very little growth in the winter, such as 'Good Looking', are unlikely to bloom in spring, though their foliage often looks better than average through the summer.