Re: TB: low bloom height this year
iris@hort.net
  • Subject: Re: TB: low bloom height this year
  • From: L* M* <l*@lock-net.com>
  • Date: Sat, 01 Jun 2013 20:07:30 -0400
The critical timing for stalks being stunted seems to be about 6 weeks <before> bloom in TBs here. This is before stalks are showing or can be felt in the fan.
So that's usually some time in March, well before last average freeze date. Critical temperature seems to be below mid-20s. Anything below about 20, especially if night time temperatures have been above freezing for ? a week?, will knock out just about all TB bloom the following month. Stunting can occur if temps are cold, but not cold enough to kill stalks before they emerge. & of course, all of this varies to some extent among different cultivars and effects are much worse if cold follows a really warm spell in early to mid March.
Those temperature thresholds are based on my experiences here for 40+ years growing irises - I'm in a low spot, shaded from morning sun by a ridge behind my house, so cold settles and stays.
Rotting terminals are another issue - on new plants, especially those coming from a milder climate, just plain cold temperatures seem to kill terminals over the winter.
On plants that are 'adjusted' to climate here, those late freezes that knock out terminals definitely set plants up for major rot problems. Some cultivars seem to rot more readily after that kind of damage than others.. 

Linda Mann east TN USA zone 7b

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