Bulletin of the AIS no. 170 (1963)
Photoperiodism in Irises
William G. McGarvey

STUDY of the reaction of plants to light and to the various components of white light has resulted in many interesting and useful discoveries since Garner and Allard made their first report on the subject in 1920.*
     Much is known today about how certain plants differ in their need for light and in the obviously related need, their need for darkness. Poinsettias, for example, are known to require 12 hours of uninterrupted darkness in order to flower. Chrysanthemums require short days and long nights for flowering as contrasted with corn which must have long days and short nights in order to mature.
     The following material concerns the reactions of certain varieties of irises to a single light condition, exposure to light 24 hours every day. The results obtained seem interesting enough to justify their being reported. However, the results seem even more interesting in terms of the questions they raise than for the immediate information produced.


On October 20, 1962, a number of reblooming irises carrying bloomstalks with pods were potted and moved into a greenhouse in order to avoid frost damage to their late-set seed pods. These plants were in good growing condition when lifted and showed very little sign of any retardation in growth after transplanting. The plants selected for this reason included:

5 tall bearded tetraploid reblooming seedlings

Another plant was selected for a similar reason. This plant was lifted from the L. F. Randolph garden at about the same time. When dug it carried a bloomstalk in bud. It also responded favorably to being transplanted by going on to flower and to set seed following pollination. This plant is described by Randolph as an
Iris obliensis type from San Angelo, Italy, 40 chromosomes, and as a consistent rebloomer

Two potted rhizomes of an oncocyclus species were taken to the greenhouse at the same time. These plants showed no signs of growth above the soil level at this time. These plants were:

I. lortetii (obtained from Israel via California)

Still another group of 86 seedlings which germinated in October, were potted and introduced to the greenhouse at the same time. This group included:

50 tall bearded tetraploid seedlings
  5 tall bearded tetraploids X I. varbossiana
  1 tall bearded tetraploid X I. imbricata (Elburz Mts., Iran)
  2 tall bearded tetraploid X I. reichenbachii (Holomondos Mts., Greece)
  1 diploid tall bearded X I. lortetii
12 I. mellita (T 19 A) X I. imbricata (Elburz Mts.)
  I. reichenbachii (Mt. Olympus, Greece) X self
  I. imbricata Astolot X I. imbricata (collected in Russia)

79 bearded

 I. fulva self (These are from a clone that survives New York winters and also reblooms


Greenhouse Environment. Because these plants were available and since the loss of the seed pods on the mature plants would not have been too serious an event, it was decided to subject all of them to an experimental light condition of an extreme nature. These plants were given constant light from October 20, 1962, to date (March 18, 1963). During each 24 hours they received daylight (but no sunlight until March 1) in whatever amounts were available depending on the length of the day and the conditions of the sky (clear or overcast). White fluorescent lamps overhead were kept lighted 24 hours of every day. A supplemental source of light was provided by other white fluorescent lamps between the hours of 6:00 A.M. and 8:00 P.m. of every day.
     Measured at plant level, the overhead 24-hour lamps provided 30 footcandles of light on a constant basis. Measured at plant level, the 14-hour supplemental lamps provided 50 footcandles of light for 14 hours of each 24-hour period. Because of variation in the amount of daylight there were large variations in the intensity of light provided during the daylight hours. During the period after 8:00 P.M. and before 6:00 A.M. and between the dates of October 20th and March 1st all plants received 30 footcandles of constant light.
     Temperature. Throughout the entire time of this experiment the greenhouse was maintained at a constant temperature of 68 ( 3°) F., until after March 1st at which time sunlight reaches this greenhouse, and temperature variations for short periods of time were as great its 8°F.
     Humidity and Moisture. Humidity during the time of this experiment was quite variable in the greenhouse, ranging from a low of 30% to a high of 95%. However, the soil in the pots containing the plants was maintained in a moist but not wet condition throughout the entire experiment and at no time during the experiment did the plants show any sign of being in need of water.
     Soil and Fertilizer. The soil used to pot the plants contained 50% well-rotted cow manure and 50% sandy loam by volume. One pound of bonemeal was mixed through each bushel of the soil. The plants have received a light feeding with superphosphate on a three-week schedule.
     Pots. The mature plants were placed in 10-inch puts, the lortetii rhizomes in 8-inch pots, and the seedlings ill 3- and 4-inch pots. All plants have been kept in the original puts and as of March 18th those examined are pot bound.

By far the most interesting results were those shown by the five mature tetraploid rebloomers and by the I. olbiensis from San Angelo. All of these plants continued in good growing condition, went on to ripen their seed pods, and then went on to develop more bloom stalks, to flower, to set seed, and to continue growth. None of these plants has had a resting period since the winter of 1961-62 and show no need for one. The 48-chromosome tetraploid plants had bloomstalks in June 1962, developed the rhizomes adjacent to those that bloomed and bloomed again in September or October 1962, again developed the adjacent rhizomes and bloomed in February or March 1963. Although the I. olbiensis had the remains of an old bloomstalk on the rhizome which was parent to the one that produced bloom in October 1962, it is not known that this particular plant bloomed in June. However, it did bloom in October, set and ripened seed, and then developed the adjacent rhizome to bloom in January 1963. This plant is again carrying a rapidly maturing pod and rapidly developing increase.
     The I. lortetii rhizomes showed growth above the soil two weeks after being taken to the greenhouse, went on to develop good growth, and to bloom in January. Flowers of these plants were selfed and one is developing a pod. Their pollen was used on the flowers of the tetraploids and of four such pollinations, two pods have resulted.
     The entire group of 86 new seedlings has shown excellent growth. These plants have the characteristics of plants grown under favorable conditions with good foliage in good proportion to the plant as a whole.


It would seem that the 48- and 40-chromosome plants used, after attaining their maturity, have no need for a period of darkness in order to achieve further growth.

It would seem that these plants require no period of cold temperatures between periods of bloom.

It would seem that these plants need no period of rest or dormancy between periods of bloom.

Concerning this last point, a bit of supportive evidence is to be found in the behavior of a number of rebloomers. Rather regularly such plants have been observed to carry podded stalks along with blooming stalks on the immediately adjacent rhizomes.

The I. lortetii plants provided less information but it does seem that this species may require no period of darkness in order to produce bloom once growth has started.

The general good health of the group of 86 new seedlings suggests that irises may benefit from much longer periods of light than are usually available to them.

Comment. A number of writers have described the iris as a "sun plant." On the basis of the few results reported here it would seem that a better name for the iris is light plant.

Questions. As suggested earlier, these results raise more questions than answers. Some of these questions are listed below because they indicate the need for further study.

Is the ability to do without a period of darkness a characteristic of the genus, or of certain species, or is it confined to some few varieties?

What relationships, if any, are there between 24-hour light and a relatively constant temperature (68°F) as influence on growth and maturing?

What is the significance of the fact that the irises used in this experiment were able to grow, flower, and mature seeds when provided with a white light that carries considerable blue light and very little red light?

Do the results obtained have any significance for those who wish to grow irises in warm climates?

Does the ability of a plant to use 24-hour light have any significance for the commercial grower?

Do the results obtained have a general significance for the culture of iris seedlings?

Is it possible that irises are more in need of some minimum exposure to light of some minimum intensity than for a minimum period at some maximum intensity? (What about the garden which receives little sun but a considerable amount of artificial light?)

What do these results suggest concerning the belief that irises require a resting period is well as a period of cold temperature?

*Garner, W. W., and H. A. Allard, "Effect of relative length of day and night and other factors of the environment on growth and reproduction in plants." Journal of Agr. Res., 1920, Vol. 18, pp. 553-606.

Dr. McGarvey is professor of psychology at the Oswego branch of the State University of New York. The following titles of articles in earlier issues illustrate his wide-ranging interest in irises:
"The median's tall bearded parent as test plant"
(April 1962)
"Pink Siberians and minor frustrations"
(October 1961)
"Acidity (pH) of the cell sap of irises"
(April 1961)