Factors affecting iris bloom season

Thanks to Graham Spencer and Diana Louis for responding to my question
about what may explain the fact that TB iris bloom at about the same time
at near sea level in the mild, moist maritime climate of southern England
(Zone 7b-8) and in a high altitude valley with a semi-arid, continental
climate in the central Rocky Mountains (Zone 4).

First, the question of Zones. These cannot explain all the variable factors
that affect plant growth and performance, of course. Technically, they are
Plant Hardiness Zones, and even for this purpose, they are a unidimensional
index, based essentially on minimum expected temperatures.

Graham mentions light levels, soil warmth, and ambient temperatures as
other factors that might come into play.
	Light levels: As S. England is 9 degrees of Latitude further from the
Equator than N. Utah, during the period between the Winter and Summer
Solstices, day length is increasing faster in England, and after the Vernal
Equinox (the main period of spring iris growth and bloom) absolute day
length is greater in England, too. Due to 
its lower Latitude, higher altitude, and probably clearer skies, the
insolation should be greater in Utah.
	Soil warmth: In Utah in the spring the soil must warm up from a frozen
state, which I doubt is the case in England. The rate at which soil absorbs
warmth depends a lot on its texture, sandy soils warming up quickly and
clay soils being notoriously "cold". My soil is a well-drained silt loam
that warms up relatively rapidly (I have earlier bloom by several days than
most other local irisarians, because they are mainly gardening in clay
soils).
	Ambient temperatures: Diana notes that extreme fluctuations in temperature
are untypical of England; they are the norm here in the spring. Not only
can we go from a period of daytime temperatures in the 70's to subfreezing
weather very abruptly, but, even when it does warm up in the daytime, there
is a lot of diurnal variation - 70's in the afternoon, 30's overnight. How
all this plays out in the way the iris respond to ambient temperature is
difficult to assess.

I do know that there is a tendency here for TB iris, particularly, to bloom
below registration height (for instance, in the 10 years for which I have
measurements, BEVERLY SILLS has never reached registration height (36") and
has averaged 27").
I think our typical weather pattern brings iris into bloom before they have
reached their full potential development. The one year we did have an
"Oregon" (or English) spring - early thaw, consistent mild temperatures and
cloudy weather with frequent gentle showers, the iris stalks shot up like
rockets, and many cultivars bloomed 8 to 12 inches taller than they ever
have other years. 


Jeff Walters in northern Utah (Zone 4)
cwalters@cache.net
"This is the Place" - Utah Pioneer Sesquicentennial:  1847-1997
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