Growing Irises from Seed

by Tom Waters

We usually propagate irises by division: digging up a large clump, breaking apart the individual rhizomes, and replanting. This method is easy, and because most irises increase rapidly, within a few years you will have plenty of them. This post is about a different way of propagating irises: planting seeds. This is a process that has some challenges, but also has some wonderful rewards. If you've never thought about growing irises from seeds, or have wondered about it but are unsure how to start, read on!

Why?

If you are hybridizing, you will necessarily be growing irises from seed. Hybridizing refers to cross-pollinating irises to produce new varieties. When you cross two different irises, the result is a pod of seeds. Each of the seedlings will grow into a new individual, not exactly like any other iris. Propagating by division only creates exact copies of the original plant, whereas progagation from seed creates only brand new plants, different from either parent. Even if you are not intending to embark on serious hybridizing program to create new varieties to sell commercially, making crosses and raising seedlings can be fun and interesting.

Woohoo! Iris seeds just arrived from Czechia

But hybridizing is not the only reason you might want to raise irises from seed. Some types of irises may just not be obtainable from commercial growers as plants, but you may be able to acquire seeds from a collector or from a seed exchange. This is especially true of iris species, the wild irises from different parts of the world. It can be very difficult to import live plants from other countries, but importing seeds is usually much easier. I've gotten seeds of iris species from collectors in the Czech Republic and from seed exchanges run by the Species Iris Group of North America (SIGNA), the British Iris Society (BIS), the Scottish Rock Garden Club (SRGC), and the North American Rock Garden Society (NARGS). Although the seedlings from a given offering of species seeds will all be different, in subtle or obvious ways, they will all still be plants of that named species, unless the person providing the seed misidentified the plant, or unless it was accidentally cross-pollinated by a different species growing nearby.

Growing irises from seeds also has some other advantages: you get a wide variety of different plants, so you can choose the ones that do best in your climate or whose appearance you prefer. Growing from seed is also a way to eliminate virus infection, should that be a problem.

Finally, growing irises (or any plant) from seeds is a very satisfying experience. It connects you with the whole process of growth, from its very beginning. There's nothing quite like seeing the first bloom of a plant you've grown from seed yourself.

How?

The process of growing irises from seeds is not (usually) very difficult, but it does require patience and attention to factors that you might not have considered if your only experience of raising plants from seeds is growing vegetables or annuals. There have been many articles written on special ways to grow iris seeds; you can find a number of them on the web. In this post, I am not going to suggest one particular method as superior, but just give you an overview of the basics, so that you can get started and learn what works for you.

There are two requirements for germinating iris seeds:

1. They must experience several months of cold temperatures, followed by a warming period.

2. They must have adequate moisture.

Seeds planted into pots sunk in the ground
 (gravel on top protects from washout
and animal interferenece)

The simplest way to meet these two requirements is to plant the seeds outdoors in the autumn or early winter, and let nature take its course--assisting with supplemental water if natural precipitation is insufficient. Seeds can be planted straight in the soil, or in pots sunk into the ground or just left on a porch or in a cold frame. A planting depth of 1 cm or 1/2 inch is suitable for most seeds. Germination usually occurs around the time of iris bloom in the spring.

Some growers prefer to give the seeds their cold treatment ("stratification") indoors, by putting the seeds in baggies with slightly moist vermiculite, perlite, or other sterile medium and refrigerating them for 60 days or more. The advantage of indoor stratification is greater control over the conditions, and the possibility of getting germination a few months earlier. The disadvantage is that you need to be prepared to grow on the seedlings indoors under grow lights for some time, until they are ready to be hardened off and planted outdoors.

Newly sprouted seedlings!

I prefer the outdoor method, as it is less bother and easier to manage with large numbers of seedlings. If I had an extra refrigerator to use for seeds, I might prefer indoor germination.

Whatever method is used to germinate the seeds, they should be transplanted to a semi-permanent seedling bed outdoors when they have at least three leaves. Some may bloom a year later. The year after that, most should bloom and you can decide which ones to keep and which to discard.

The reason the period of cold temperature is needed is that irises, like many perennials from temperate climates, cannot easily survive a winter while still small seedlings. When germination occurs in the spring, rather than in the fall, the young plants have the best possible chance of survival. In contrast, most annuals are fast-growing, opportunistic plants that can grow, flower, and produce seed whenever there is a few months of warm weather.

Dormancy

Even if the requirements above are met, not all the seeds will sprout the first year. With garden variety bearded irises, the percentage will usually be more than 50%, and can approach 100%. Most of the remainder will sprout the following year. At the other extreme, aril irises may sprout a few at a time over a period of 10 years or more. Why is this? It is nature's "insurance policy" against calamities and harsh conditions of various sorts. If all seeds sprouted at the same time, a drought, flood, fire, or other disaster could destroy the whole population. By having the seeds sprout over the course of several years, it is virtually guaranteed that some will survive.

For the gardener, however, such protracted dormancy is a frustration. Few of us want to wait a decade for the seeds we plant to sprout! In irises, dormancy has at least two causes: the hardness of the seed coat, which makes it mechanically difficult for the seedling to emerge, and chemical germination inhibitors inside the seed itself. In nature, the action of water and the cycles of freezing and thawing serve to gradually weaken the seed coat and to leach away the chemical inhibitors.

Some growers have success by planting seed fresh, before it has a chance to dry out. For some types, at least, this can bypass dormancy and result in immediate germination. Of course, one then needs a way to care for the seedlings over winter.

A variety of techniques are used to overcome dormancy artificially. One can attempt to leach out the germination inhibitors by prolonged soaking or use of running water. One can overcome the hard seedcoat by abrasion or chipping (cutting away the seedcoat to expose the embryo). This "forced germination" procedure is often recommended for difficult oncocyclus seeds. The ultimate procedure is to excise the embryo under sterile conditions, and germinate it on a nutrient agar medium. This "embryo culture" can be used to germinate seeds that will not germinate any other way, but it is very demanding work and the young seedlings are very vulnerable.

Whether you choose to use any of these techniques for overcoming dormancy will depend on whether the basic method is giving adequate germination for the types of seeds you grow. I think the best advice is to try natural germination first, and then move on to progressively more invasive and difficult techniques if you need to.

The Payoff

I encourage everyone to trying growing irises from seeds at least once, whether it's making a cross or two in your own garden or ordering a packet of seeds from a seed exchange. It's an adventure, and the first bloom of an iris you've raised from seed yourself makes it all worth the wait!

Here are three seedlings from the same cross, arilbred 'Aztec Prince' (Tasco, 2009) X Iris pumila: