Siberian irises: Of diploids and tetraploids

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Irises. The Opinionated Gardener grows three kinds: the “common” bearded irises that most refer to as German irises and the “less common” beardless Siberian and Japanese irises. She prefers the Siberian, finding the German and Japanese irises too much flower for her taste, whereas the overall appearance of the Siberian is delicate, like that of a butterfly. And the late spring garden doesn’t get much better than when the clear deep blues, purples and whites of the Siberian iris flit about on stage with the outrageous oranges of O’Keeffe’s Oriental poppy.

Appearance aside, the Siberian iris is also hardy. It grows in substandard acid, often droughty soil and spreads wildly and unassisted to distant corners of the garden. It’s the hardy independent kind of plant the OG prefers indoor and out. And when the spring show is over, unlike the pale green sword shaped leaves of the German iris, the Siberian’s strapping, narrow, grassy, green leaves continue on all summer. Last but not least, Siberian’s large bottle-shaped dark brown seed pods provide year-round pleasure when sharing a tall vase with the creamy pods of poppy and spiky teasel.

But as to be expected this column isn’t about preferences or cultivation. It’s about a “fairly recent” development in the 100 years plus history of breeding Siberian irises that took place in of all places Harpswell, Maine in the gardens of Currier McEwen, who for good reason is considered the “father of the modern Siberian iris.” (McEwen died at the age of 101 in 2003.)

Not only did McEwen develop some 100 new varieties – including the OG’s favorite purple Siberian -‘Teal Velvet’(McEwen, 1981), he was the first to breed the color fast “diploid” yellow Siberian iris ‘Butter and Sugar’(McEwen, 1977). Blue and white are the dominant colors in Siberian irises. Most important, he was the first to create a “tetraploid”

 Siberian iris.

Of chromosome numbers

Diploid? Tetraploid? To fully appreciate what McEwen achieved, one must go back to what they learned in grade school about those microscopic structures called chromosomes, which store the genes (genetic information) within the nuclei of cells.

Like ‘Butter and Sugar’ most plants and animals are “diploid” or “2n” meaning that each of their body (somatic) cells have two sets of chromosomes; one from each of their parents. The cells of ‘Butter and Sugar’ and of ‘Teal Velvet’

each have 28 chromosomes, or 14 pairs. The germ cells aka sperms and eggs are “haploids” or “n” with 14 chromosomes each.

But sometimes, Mother Nature “burps” during cell replication and creates “tetraploid” or “

4n” plants with twice the usual number of chromosomes or four sets of chromosomes. In the case of Siberian irises that would mean the plant cells would have 56 chromosomes or 28 pairs. While two sets is good, four sets is apparently better as tetraploids are known to produce bigger flowers, richer in color, stouter in stem, greener and bigger in leaves.

If you are still reading, you might want to switch to the term “polyploidy.” Cells are “polyploid”

if they have three (3n), four (4n) or more sets of chromosomes instead of the two (2n) in diploids. In the coursed of evolution, polyploidy has naturally occurred in most flowering plants including alfalfa, cotton, potatoes and wheat. Polyploid crops exhibit a similar vigor as found in hybrids. That giant watermelon you are about to eat… have you checked out the number of chromosomes in its cells?

Colchicine…a performance enhancing drug?

McEwen “introduced” his first tetraploids in 1970 after Orville Fay the Indiana plant breeder of day lilies taught him how to induce it in Siberian irises. While the German iris spontaneously doubles its chromosomes, it appears that Siberians do not; which is to say they need a bit of help from humans, whose cells by the way have 23 pairs of or 46 chromosomes.

To produce his impressive, “tetraploid” daylilies, Fay used the drug colchicine, an ancient drug derived from the autumn crocus and used for treating gout. McEwen was familiar with the drug. Before he retired and turned to plant breeding, he was a rheumatologist and used the drug for treating gout, a disease characterized by the painful inflammation of the joints. In later years McEwen confessed, it was the “

new use of an old friend” more than his fascination with Siberian irises that started his experiments with colchicine in 1962.

By 1970 McEwen introduced his first two Siberian tetraploids: ‘Fourfold White’ and ‘Orville Fay.’ With its wide flaring blue petal, ‘Orville Fay,’ won the prestigious Morgan Award of the Society of Siberian Irises in 1976. McEwen went on to create numerous other Siberian tetraploids over a period of 30 years, winning awards for them almost every year. Some of the more famous are his ‘Harpswell Haze,’ ‘Harpswell Happiness’ and ‘

Harpswell Halleluiah.’

As McEwen explains in his book “Siberian Iris”

(Timber Press, 1996), when a sprouted seedling of Siberian iris or any other plant material for that matter is treated with a solution of colchicine, the process of cell division ( mitosis) which leads to the normal growth of the plant is interrupted. During mitosis the chromosome sets double. Normally, the sets then move apart to opposite sides of the cell in preparation for cell division. Colchicine inhibits that movement. Cell division stops but not before the chromosomes have doubled. In an attempt to be concise, one could say that the cells are “stuck” with the double set of chromosomes (56).

And the plant? Because of all that excess genetic material, when grown, its petals are richer in color and larger in size and its leaves are larger and a deeper green. The letter “T”

in a nursery catalogue indicates the plant is a tetraploid. Before ordering, it’s best to see it in person.

Nurseries and Websites

Fieldstone Gardens. The local landmark for McEwen’s Siberian irises. Check their for a look at all the varieties in full color. Located in Vassalboro, Maine. Check out the plants at their June 29th open house. About one hour from Farmington.

Eartheart Gardens. Also specializing in McEwen’


s Siberian and Japanese iris, the nursery was founded in 1993 by Sharon Whitney, a colleague of McEwen’s in South Harpswell., McEwen country. Open Garden days June 15 and July 13. Check out the website

Tranquil Lake Nurseries. The best website for viewing Siberian irises by McEwen. Said to be the largest grower of daylilies and Siberian and Japanese irises in the northeast. More than 200 cultivars of Siberian irises. Located in Rehoboth, Massachusetts.

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