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Iris chill requirements

  • To: "Iris-L" <iris-l@rt66.com>
  • Subject: Iris chill requirements
  • From: "Hall_Gigi" <gigi.hall@mtv.gtegsc.com>
  • Date: 23 Oct 1996 14:30:40 -0800
  • Return-Receipt-To: "Hall_Gigi" <gigi.hall@mtv.gtegsc.com>

Felt that I needed to chip in on this one.  This is a long message.  If you have nice cold winters, feel free to delete the message now, because most of the contents will not be of interest.

> ... what Iris require chill ...

As far as I know, all members of the genus Iris require chill to bloom properly.  The species from which our modern hybrids are derived come from native habitats in the temperate regions of the Northern hemisphere.  Take a Tall bearded Iris to the tropics, to the southern coast of Florida, or the shores of the Hawaiin islands and it will produce lovely sword shaped foliage, but few or no blooms.  So the simple answer is that all Iris to flower need some chill. (Note that I am speaking of the genus Iris only - the plant family Iridaceae does have tropical members).

The more complicated answer is that while all Iris require some chill to bloom, different types have different chill requirements and there is considerable variation within a type of Iris as to the chill requirements.  No one that I am aware of has done scientific, qualitative analysis of the type and level of chill required.  Also, some types of Iris require cold winters just to survive - if they never go dormant, they never break dormancy and produce new increase.  However,  based on my 20 years of experience of gardening in a "banana belt"  (bananas will grow and flower here) microclimate in the San Francisco Bay area in California, with briefer stints in the high desert of Arizona and the mediterrean climate of the central valley of California, I can offer the following ranking:

Greatest Chill Requirements to bloom:
Reticulatas (series of dwarf sized bulbous Iris)
Miniature Dwarf Bearded (MDB)
Siberian Iris (garden or 28-chromosome type)
Siberian Iris (Sino-Sibe or 40-chromosome type)
Species  Iris setosa (and relatives) - Alaskan and Canadian natives.
Iris missourensis (native of alkaline mountain meadows, above the snow line, throughout Rocky Mountains and connecting ranges)

The reticulatas, if planted in the ground, bloom one time and are then gone.  If grown in pots or tubs (colder in winter; warmer in summer than in the ground), they persist for two or three years. If I get a little extra cold during the winter, they will bloom every spring until they disappear.  This group includes Iris danfordiae, Iris reticulata, Iris histriodes, other closely related species and the named clones and hybrids from these.

Miniature Dwarfs just won't bloom if they don't get enough chill - they do grow handsome foliage.  Having grown over a hundred different varieties through the years, I have found only two reliable performers for bloom in my microclimate climate:  PUPPET BABY by Carl Boswell and SCRIBE by John Taylor (of England).  SCRIBE also wins the prize for the Iris with longest single season bloom - from before 22 February to 6 June in 1984. I don't know how long before 22 February it started - I came home from a six-month long installation job overseas on this date and my four-year-old clump was in full glorious bloom.

Siberian Iris show three responses to low chill:  bloom on shorter than "normal" stalks; no bloom; plants that never go dormant and eventually dwindle away (more common with Sino-Sibes than 28-chromosome type).  The only form of Iris setosa that will bloom in my garden after the first spring is the triploid form.

Iris missouriensis rarely blooms and is short lived in my mild climate.  Of course the first couple of times I killed it with kindness - having carefully acidified the soil it was planted in.  Later I would learn that it is a native of alkaline soils, wet in the winter and spring and dry in the summer.

Next greatest chill requirements:
Standard Dwarf Bearded (SDB)
Iris versicolor and interspecies hybrids from it
Japanese Iris (Iris ensata species and hybrids)

Only 20% of SDB cultivars I have tried bloom reliably after a typical winter in my climate (three to ten days of light frost in the winter; night time temperatures average in the mid-40's through January and February).  I had 97% SDB bloom after the record breaking cold winter of 1989-1990 (when there was a solid, four consecutive almost freezing days with night temperatures under 20 degrees Fahrenheit and nearly two weeks with hard frost surrounding that).

If an Iris versicolor cultivar goes dormant by November, it will probably grow and bloom the next spring.  Most don't go dormant until mid-January, never bloom, and the rhizomes and foliage eventually dwindle away.

Japanese, in general tend to bloom on shorter stalks than in climates with colder winters (about a foot shorter than in Sacramento, California).  My records for established clumps are not complete enough for me to give percent-of-varieties-blooming statistics.  As far as I know, I have never lost a Japanese because of lack of chill.  Transplant losses run between 3 and 7% of all plants - less on plants being transplanted from my garden into my own garden.

Certain Japanese cultivars, including every clump of species Iris ensata I have ever grown, tend to bloom out. I can't say that this is directly related to chill requirements or not. I disagree strongly with Clarence's statement that you won't get good JAP rebloom unless you get high chill.  A large number of Japanese Iris fall-rebloom reliably in the mild bay area climates (versus the more usual Japanese rebloom pattern of a second flush of bloom four to six weeks after the first spring bloom in cold winter climates).

Only moderate chill requirements:
Miniature Tall Bearded (MTB)
Hardy evansias (Iris tectorum, Iris cristata, etc.)
Iris laevigata (the species and related hybrids)
Regelia Arils and the arilbreds from regelias
historic (diploid) Tall Bearded
Dutch (bulbous florist) Iris

About 70% of MTB cultivars, historical diploid TB, regelia Iris,AND arilbred cultivars with regelia background will bloom here in atypical year.  About 30% of MTB cultivars exhibit strange growth (foliage after first year is short and distorted), don't bloom, and gradually dwindle away.

The hardy evansias either bloom profusely, or skip a year - the earlier I get my first real frost, the more likely that I will see blooms next spring. How hard the frost is doesn't seem to be as important as the total length of time they are dormant.

The laevigatas all grow fine, some varieties provide reliable bloom (provided I have poured enough water on them during early spring).  Other varieties produce little or no bloom after the first year.  I haven't grown enough different clones to give percentages.  If you leave the banana belt microclimate, these are favorite and dependable Iris for the ornamental pond gardener throughout the bay area.

There are two major strains of Dutch Iris - those bred for forcing and those bred for garden use (and most bulb catalogs don't tell you which strain a cultivar they list falls into).  The forcing Dutch start to grow as soon I plant them - will bloom one time and are then gone.  The "garden type" Dutch Iris will repeat bloom if I have a cooler than normal winter, just grow foliage after the first year if the winter is very mild.    The plants get disturbed after 2 to 3 years, so I can't say that I have ever naturalized these, but they perform more reliably than the reticulatas.

Iris requiring little chill:
Oncocyclus Arils and Arilbreds from Oncocyclus breeding
Intermediate Bearded (IB)
Border Bearded (BB)
Tall Bearded (TB)
Pacific Coast Iris (PCI) from Iris munii and Iris innominata background.
Spuria Iris (SPU)

Whether or not I get good bloom on these types of Iris does not seem to be connected to the type of winter I had.  However, even among these, there is variations between cultivars.  About 1 or 2 out of every 300 TB Iris will never, never, ever bloom in my climate.  I can take a nice big fat rhizome from one of these cultivars to my parent's home in the Sierra Nevada foothills (just below normal snow level) and it will bloom the following spring without fail.

Also, my "sample" of Arils and arilbreds is small enough that I probably don't see the full range of variation in this group.  My suspicion is that more experienced growers might place these in the prior group (requiring slightly more chill).

Requiring almost no chill:
Louisiana Iris
Pacific Coast Iris with Iris douglasiana background (majority of modern hybrids)
Iris virginica
Iris pseudacorus and the interspecies hybrids from it.
The tender evansias (Iris wattii, Iris confusa, Iris nana, etc.)

Not only do Iris in this category bloom reliably after my mildest winter, they are limited in range by their relative lack of cold hardiness.

Various folks I know in the colder climates of Oregon and Washington no longer grow the PCI because every tenth winter or so, they lose almost very cultivar to sub-20 degree Fahrenheit nights.

Lorena Reid (Laurie's Garden) of Oregon listed Louisianas for many years, but finally gave up because:  they never looked great in the garden (and she had people who came to see them specifically) and she doesn't ship unless she has seen the clump bloom - these did not bloom reliably after an Oregon winter that was colder than usual.

Iris virginica (native to south-eastern United States) does very well here (contrast to notes on Iris versicolor above).

Iris pseudacorus (including dwarf forms) and hybrids Roy Davidson and Phil Edinger are evergreen in my garden - the new foliage is quite short through the winter - under 12 inches - but quite green and healthy.  Well grown, Roy Davidson is a Queen of show contender with stalks with two or three lower branches, more than a dozen buds, and blooms that last three to four days (versus the one day for the species).

The tender evansias (and cymbidium orchids) are outdoor garden plants here.  They would be considered greenhouse plants over most of the United States.  Iris confusa (and very similar Iris nana) produce small clouds of white crested Iris in the spring in semi-shaded locations in my garden and other bay area gardens.

Iris types where my experience is too limited to count:
The junos (bulbous Iris with fleshy roots and plants like miniature cornstalks with the Iris flowers in the axils of the leaves)
Cal-sibes (crosses between the 40-chromosome Siberian and Pacific Coast Iris).
Most of the Europogon bearded Iris species.
Iris ruthenica (anyone have a source?).
Iris minuto-aurea and related cousins
Any other species not specifically discussed above.

Gigi Hall

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