Iris...and most plants, actually...will stand a "lower than freezing" temperature (in varying degrees, of course) because the natural sugars in the sap act as an 'anti-freeze'. My Re's will *usually* not be hurt by overnight temps of 29 degrees, 28 starts to get "iffy", and lower than that...it's over for the year . Of course, a lot depends on how long that temp. persists.....if it's just for several hours overnight....or stays that way for a lot of the next day.
. Of course, a lot depends on how long that temp. persists.....if it's just for several hours overnight....or stays that way for a lot of the next day.
