Ottawa Citizen, July 30, 1970 p. 46

Secret of Reblooming Irises
A. R. Buckley
Plant Research Institute, Ottawa, Ontario

One major drawback of some prominent perennials is their relatively short period of flowering. Some gardeners who enjoy a plant such as the iris, which has such magnificent blooms, avoid planting it because its flowering interest is so short. A possible remedy for this state, in the iris, at least, is to plant some of the so-called reblooming or remontant cultivars.

As the group name of these bearded irises would suggest, they give more than one period of flowers in the same season. The special irises with this quality do not require a dormant period to flower again. As early as mid-July some have a second bloom. In our Plant Research Test Garden some have even bloomed three times in one season.

The reason why these reblooming irises will do what the others will not is a matter of conjecture. Answers to many questions about reblooming of plants are being eagerly sought, and the American Iris Society now has a special group entirely devoted to the remontant types.

What is known for certain is that some cultivars will flower again and this is the most significant thing for the home gardener who favors the iris and likes to see it again out of season.

Since the second flush of bloom is dependent upon increase, the plant must be vigorous enough to produce new rhizomes of adequate size for flower stalk production in the same year. To do this the proper culture procedure is all-important. Extra fertilization is recommended and it seems to be particularly effective when done right after the spring bloom.

Irrigation during the dry period of the summer is valuable, but great care must be taken to prevent conditions that lead to root rot. Mulching is another way to maintain adequate soil moisture. Rot may result when the mulch is too close to the base of the plant so keep it off the rhizome as much as possible.

Vigorous plants should be spaced wider than regular tall bearded irises even though most of them are tall bearded kinds. They should be divided before they grow too thickly. Division is best if done shortly after flowering in the spring so the plants may become well established before fall. The year of division you will most likely lose the second bloom.

As more and more reblooming irises are being developed certain problems are becoming apparent. Most important is the climatic classification. Irises which may have rebloomed well at the Lloyd Austin nurseries in Texas, where many of them originated, may not bloom again at all in northern U.S. or Canada. If the growing season is long enough iris could be flowered again and again. A single clump of iris was flowered six times in one year in a greenhouse experiment.

Our trials have indicated that nearly all the reblooming types received from Texas ten years ago have proved to be quite hardy and most have bloomed again in August or September with Gibson Girl giving a third bloom in early October.

In this particular case it was a rather peculiar sight to see an iris that had been cut back (so that the rhizome could be ripened well by sun to avert attacks of soft rot) with one strong. stalked bloom arising from a crown of sheared leaves.

Reblooming irises are becoming more readily available in the trade. Some of those that can be recommended for our area and which may be seen in our test gardens are as follows: Gibson Girl, the old standard cultivar with mulberry and cream flowers; Fall Primrose, a clear yellow; Autumn Snowdrift, white self; Autumn Twilight, light mauve; Harvest Blue, deep blue; July Beauty, light blue; July Sunshine, expensive as yet, but a good yellow;  Final Fling, pale mauve; and Late Results, a handsome blue.

The flowers of these reblooming irises do not always meet the superior standards set by the more modern iris with very large blooms, frills and exquisite coloring. They have been bred with reblooming foremost in mind, but they are good irises and well worth growing.

A Guide to Bearded Irises: Cultivating the Rainbow for Beginners and Enthusiasts (2012)
Kelly D. Norris

Cyclic rebloomers (daylength dependent). These varieties complete two distinct cycles of growth — flowering and increasing — each growing season. The second cycle of growth does not require vernalization to produce flower stalks. Cyclic rebloomers typically rebloom in the fall, when shorter daylengths mirror those of spring.

Repeaters. These varieties produce additional stalks on old growth immediately following the first spring flowering cycle, typically within a few weeks to a month.

Continuous rebloomers (soil temperature dependent). Rebloom in these irises occurs whenever new rhizomes mature, throughout the growing season, often without pattern. Their remontancy correlates well with soil temperatures, not daylength.

CybeRose note: Millington (1878) reported that she could induce repeat blooming in callas.

Reblooming Iris Bibliography