The Effect of High June Temperatures on Some Rhododendron Foliage
by Josephine Polivka, Portland, Oregon
I thought it might be of interest to write about what last week's two hot days and nights (June 2-3), did to our rhododendrons. Temperature (down town) reached 103 degrees Tuesday, 107 degrees Wednesday, with a hot night intervening. At our house, the maximum Tuesday (11 July) was 100 degrees. and next day it was 102 degrees, hotter, of course, near the house with the reflection of the brick walls.
Resistance to sunshine is a trap for the unwary buyer, as I have found out. At any rate, you never see it mentioned in our publications, at least, not as often as cold hardiness. I will list the damage to our plants below and I am sure it will be a surprise. Plants in shade, all getting two hours of midday sun, showed no damage at all. These were: R. 'Moonstone', R. haematodes, R. 'Idealist', R. camtschaticum.
Plants in partial shade (morning sun only): R. 'Bow Bells' (not burned); R. hemitrichotum (not burned).
Plants in full sun, all day long: R. keleticum (not a single leaf burned); R. leucaspis (not burned); R. pemakoense (not a single leaf burned); R. williamsianum, several leaves burned, all old leaves, this year's growth not affected; R. discolor or fortunei hybrid, not even so much as a scorch on a single leaf.
These plants do not get full sun until about 11 a.m.: R. 'Arthur J. Ivens', not more than one leaf to a bush burned; R. 'Conemaugh' and 'Tessa', no damage; R. 'Rose Elf', no damage.
Fig. 48. R. 'Blue Tit'
R. Henny photo
R. forrestii var. repens, about ¼ of all the leaves burned, (full sun all day). R. 'Carmen', one in full sun, some damage, one in partial shade, no damage. R. 'Jock', no damage, full sun almost all day. R. 'Blue Tit', no damage, full sun almost all day. (FIG. 18)
R. 'Moonstone', gets sun until about 1:30 p.m. (daylight time), undamaged. R. 'Cilpinense' in full sun, very badly damaged, scarcely a healthy leaf left on the plant. R. 'Bow Bells' in partial shade, sun from about noon on every day, very seriously damaged, every leaf burned in toto or in part.
This is the first summer we have had such general leaf burning on our rhododendrons, though we have lost several individual plants from too much sun and heat-all these were within 5 feet of the south brick wall of our home and had to take reflected as well as direct sunshine and heat. This condition is general through the city from what I hear from other gardeners. Even our camellias, which are much more hardy than rhododendrons, had lots of leaves cooked right off.