Excerpted from The Gardener's Iris Book

Try Your Hand at Hybridizing with Irises

Irises are easy to hybridize, but careful planning helps ensure success

by William Shear

Why another book on irises? The Gardener's Iris Book is specifically keyed to American gardens and introducing American gardeners, experienced or otherwise, to one of the most diverse and complex of all plant groups used in gardens.

In this excerpt, William Shear provides detailed information on creating your own iris hybrid.

Debby Rairdon
 
'Debby Rairdon', a humble backyard hybrid, was awarded the Dykes Medal, irisdom's most coveted prize.
With their large, simple flowers, irises are very easy for even a beginner to hybridize. But before rushing out to the garden to make some crosses, consider carefully. Planned crosses almost always provide better results than ones made at random or impulsively. Choose parents that already exhibit qualities you would like to see in your seedlings, such as vigor, disease resistance, and habit typical for the kind of iris, and, of course, attractive form and color. Inferior parents will rarely produce a superior offspring. It is only through selecting outstanding parents for crosses that the rare individual will be obtained that may exceed them both in desirability.

Even with the most careful selection of parents, you must prepare yourself for disappointment. The genetics of irises are complex, and the best qualities are not always passed on to offspring. Geneticists have long been aware of a phenomenon called regression to the mean, in which the offspring of outstanding parents have a tendency to drop back in many of their characteristics to the average for the population as a whole. It is well to remember the vegetable breeders who crossed radishes and cabbages (which are actually quite closely related plants), hoping for radish-rooted plants that would bear cabbages above ground. What they got, unfortunately, was plants with cabbage roots and radish tops.

A long and detailed treatise on iris genetics is beyond the scope of this book. For an excellent introduction to the subject, readers can consult The World of Irises, edited by Bee Warburton and Melba Hamblen (Publishers Press, 1978).

Getting started
You won't have to invest in specialized equipment, but you will need a few items before you begin. A small pair of forceps or tweezers is the tool of choice for collecting the pollen-bearing anthers. You may also need some small glass or plastic vials or tubes to store the anthers in, and tie-on paper labels for tagging the pollinated blooms and for connecting the seed pod you hope will form with the notes you have taken recording the parentage (see The importance of keeping careful records) -- and for that you'll need pencils and a notebook. You will also appreciate having a box, tray, or other container to hold your gear and make it easy to carry everything into the garden. Some professionals prefer to use an apron with many commodious pockets, or a fisherman's vest.

Pollinating the plants
Once you have selected the first parents for your new hybridizing program and assembled your gear, you are ready to go into the garden and actually make the crosses. Let's assume for now that you are going to work with Tall Bearded Irises.

Making the cross
To make a cross, pollen from the anther of one plant (the pollen parent) is removed and transferred to the stigmatic lip of the other plant (the seed parent).

Once in the garden, the two parts of the iris flower that you must be able to distinguish are the pollen-bearing anthers and the stigmatic lip, which receives pollen. These structures are close together in the floral anatomy, the stigmatic lip near the tip of the style arm, and the anther attached to the base of the bloom, tucked in beneath the style arm. The variety or species that will contribute the pollen for the cross is called the pollen parent, while the one that will develop the seeds is the seed parent. For most kinds of irises, a plant can play either role, but in a few cases, some types may be either pollen sterile or seed sterile, or both. The fertility of a particular plant can be ascertained only through experience with the plants themselves.

Now examine the anthers of the pollen parent carefully. If they are mature and producing pollen, you will be able to see a yellow, white, or bluish powder adhering to them. It usually takes at least a few hours after a flower first opens for the anthers to mature, open, and show their pollen.

Fertilizing the flower
 
To fertilize an iris flower, pollen from the same or another variety is applied to the stigmatic lip of the style arm. Photo: Mike Lowe.
Making the cross could not be simpler. Using forceps or tweezers, remove one of the anthers from a bloom of the pollen parent. Select a bloom on the seed parent that has been open for one day or less. If the seed-parent plant is nearby, you can simply carry the anther there. Then, after pulling back the crests of style arms with thumb and forefinger to expose the stigmatic lip, wipe some of the pollen from the anther on to the lip. It may take some hours after the bloom first opens for the stigmatic lip to become receptive, but even if pollen is placed on a not-quite-ready stigmatic lip, it will effect fertilization when the proper time comes. Although placing pollen on just one stigmatic lip is probably sufficient to produce a full pod of seeds, it is wise to pollinate all three stigmatic lips on the flower as insurance.

Often, pollen parents and seed parents do not bloom at the same time. If the pollen parent blooms first, anthers can be collected in small vials or tubes stoppered with cotton, which can be stored in the refrigerator until the seed parent blooms. If the seed parent blooms first, an iris grower in a more southerly region might have the pollen parent already in bloom. Pollen can be shaken off into an envelope and sent through the mail without evident harm. The pollen then can be removed from the envelope and transferred to the seed parent’s stigmatic lip using a small water-color brush.

Pollen grains are too small to see with the naked eye, and unbeknownst to you, a few may adhere to the brushes or vials used to transfer or store them. So before using them for another cross, thoroughly wash such implements -- you wouldn’t want pollen from the wrong parent to effect fertilization. Some hybridizers use toothpicks or matchsticks to transfer pollen and throw them away after use.

Watching for results
Within about three days of pollination, if you used a fresh flower of the seed parent, the ovary of the pollinated bloom will enlarge slightly, indicating that perhaps the cross was successful. Be careful not to snap off any pollinated flowers; the process of fertilization may take some time after pollen transfer. The pollen grains must sprout and send a long, hollow tube down through the stigmatic tissues. In the ovary, these tubes eventually reach the ovules, or potential seeds, and fertilize them. Only fertilized ovules will become seeds. Warn garden visitors not to be “helpful” in snapping off spent blooms!

Seed pods Mature seed pods
Partially mature iris seed pods are green. The pods at left are formed on plants of Iris tectorum. Mature iris seed pods turn brown and crack open at the top (right). Collect the pods quickly at this stage, or you risk losing the seeds. Photos: Roger Foley.

After several more days have passed, if the cross was unsuccessful, the ovary will shrivel, turn yellow, and drop off. If the cross was successful, the ovary containing developing seeds will become noticeably larger. Successful seed pods will grow larger through the summer, eventually turning yellow or brown and cracking open at the top. (Rarely, a pod will be completely empty, what experts call a balloon.) If the exposed seeds are brown and glossy, they are ready for harvest. Collect the seeds in an envelope, and label it with the number you gave the cross in your notebook. Then sow the seeds and care for them.

Iris enthusiast William Shear is a biologist at Hampden-Sydney College in Virginia. His love of irises began more than 40 years ago, leading him to grow irises in diverse regions of the country where he has lived. His writing has been published in Natural History, American Scientist, Flower and Garden, and Fine Gardening.

Drawing: John H. Hartley, Jr.

From The Gardener's Iris Book, pp. 152-155





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