QBARS - v27n1 Some Notes on Growing Rhododendrons in a Hot Climate

Some Notes on Growing Rhododendrons in a Hot Climate
Sandra Spencer, Ukiah, California

Only 25 miles as the crow flies from the almost ideal rhododendron climate of Ft. Bragg, California is Ukiah, a coastal intermountain community on Highway 101. The elevation ranges between 684 and 864 feet depending on where you live in the valley.

A. Temperature extremes during the year can be 16°F to 115°F (more likely to be 22° to 110°F however). The thermometer only hits these extremes for a few hours at a time but daytime heat in the summer can and has stayed above 100°F for weeks on end. Twenty to fifty degree temperature changes from day to night are generally the rule. Early and late frost are not unusual, meaning late April or late October. Heat can be expected in June, July, and August and sometimes the first ten days or so of September.
B. Rainfall averages 40 inches per year but easily varies 15 inches either way. Little or no rain can be expected from June through October, with very low humidity during these months. Once the rains start no supplemental winter watering is necessary. Ukiah is almost fog-free. Snow is rather rare except on the hillsides and never lasts.
C . Soil pH is about 6.6-6.4, depending on the proximity to the Russian River. Soil is inclined to be on the heavy side...easily modified however. Humus is always needed. Adequate magnesium is available but potassium seems low and usually has to be added. Pears and wine grapes are raised commercially in the valley.
D. The water supply for summer watering is usually from wells and ranges in pH from 6.8-7.2; hardness depending on the time of year, is principally caused by magnesium.
E. Another factor is wind, almost always present. When it is a dry wind, it presents a terrible hazard especially in the spring. On the other hand it helps in cooling, and in preventing damaging frosts. The worst insect pest is large green grasshoppers in summer. They defoliate whole plants in hours. Strawberry root vine weevils are not too common (I think they prefer the damp coast) but the brown woods weevil is occasionally seen. Armillaria and water molds are endemic. Petal blight is unknown so far. Deer are pests in unfenced areas.

After reading of the above challenges, one might decide to forget about trying to grow rhododendrons here. Actually it is not as difficult as it might seem. It is a better camellia than rose area after all. Heat, by itself, almost never kills a rhododendron. But in combination with too much sun, wind, frost or poor drainage, it can be lethal. With adequate shade, water, soil preparation and protection, a surprising range of rhododendrons can be grown here. Experimentation on individual property to provide the right microclimate is necessary.
The so-called ironclads of the eastern U. S. do quite well and so do deciduous azaleas. With wind protection, Loderis flourish. The yellow flowered varieties have a much better color than those grown in the San Francisco Bay region. The williamsianum hybrids are easy and so are the griersonianums . The maddeniis and their hybrids have the most trouble adapting to the temperature changes, but in quite protected spots, can do well. 'Cotton Candy' is the most resistant variety to root rot that I have found so far. Azaleodendrons do well and have good foliage all year long.
Many species rhododendrons have succeeded, R. yakushimanum , aberconwayi , impeditum and spiciferum being standouts. R. impeditum and hippophaeoides will take full sun and heat if well-supplied with water.
Blooming time is about 4 to 6 weeks behind San Francisco. February warm spells bring out early bloomers, so I bring 'Seta' inside to enjoy it as it opens. A couple of Malesians winter over indoors nicely.
Due to the lack of humidity in the summer, leaves are not always as big and lush as in the fog areas. I think misting could help here and I'm planning to try it eventually. right now I'm installing a sprinkler system and I foresee a much easier time with the summer watering problem. I tend to place my plants in too much sun as my trees are small yet. As they grow the conditions will improve and sunburned leaves will vanish.

Statistics and Comments : (Have tried others but not long enough to assess.)


  • aberconwayi : easy and takes quite a bit of sun. No frost damage, leaves sturdy.
  • arboreum : careful on watering. augustinii: very wind and sun sensitive.
  • auriculatum : fine in light shade; grasshoppers love leaves of this one. brachyanthum: does well in light shade; rapid growth.
  • bullatum : frost makes this one a shapely plant.
  • bureavii : rather slow-growing. burmanicum: sensitive to sun and frost, not particularly bud hardy, but will not replace until I have more shade.
  • calendulaceum : prefers some wind protection and extra acidity. canadense: great in sun if sufficiently watered.
  • canescens : appreciates protection from sun and wind.
  • carolinianum var. album : takes more sun and cold than most; appreciates sharp drainage.
  • charitopes : does well if protected from early and late frosts.
  • chryseum : shade and water required.
  • cinnabarinum var. roylei : needs extra potassium and acidity, attention to drainage; attractive to grasshoppers.
  • concatenans : very sun enduring, rather easy if given sharp drainage.
  • coriaceum : must have protection from sun and wind;
  • crinigerum : hopeless so far...still trying for the right spot.
  • cubitti : beware of freak frosts.

  • davidsonianum : prefers a shady spot.
  • decorum : easy, blooms well, quite resistant to root rots, responds to TLC most encouragingly.
  • desquamatum : not as sun-resistant as reported; may be susceptible to root troubles.
  • dichroanthum : prefers some sun.
  • didymum : see crinigerum .
  • drumonium : likes light shade and neighbors.
  • fargesii : subject to die-back.
  • fictolacteum : see coriaceum .
  • formosum : frost damaged; white form most resistant.
  • forrestii: shade and drainage essential; subject to die-back.
  • glaucophyllum : shade only; needs water.
  • gymnocarpum : slow growing but endures sun.

  • hanceanum nanum : easy and good, leaves scorch if overexposed, no frost trouble.
  • hemitrichotum : takes sun, very easy.
  • hippophaeoides : a breeze, loves water, wish flower color was deeper, re-blooms reliably.
  • hirsutum : doing very well in light shade, rapid growth.
  • impeditum : has taken 114° in full sun. Very compact. Some times a full bloom. Dies in full shade.
  • japonicum : does well in sun but heat. can spoil flowers.
  • johnstoneanum : see cubitti.
  • keiskei : does well in light shade, better foliage and flowers than in Bay area.
  • keysii : sun and frost sensitive.
  • kiusianum : easy but could probably use more sun than I could give it.
  • leucaspis : beware of early frosts, otherwise fine.
  • lutescens : early frost sometimes catches it; likes more sun than other triflorums, blooms well, insects appreciative of flavor.
  • luteum : sun resistant, slow to bloom. macabeanum: fine with protection but leaves not as lush as in damper climates.
  • makinoi : does well, could probably take more sun; leaves get nibbled.
  • megeratum : touchy.
  • micranthum : likes some shade, watch drainage.
  • morii : slow growing
  • moupinense : frost tender, blooms at unpredictable times, cannot be neglected for a second, susceptible to root rot.
  • nudiflorum : takes some time to get established, lovely in sun with sufficient watering; likes crowding at roots.
  • occidentale : needs lots of water
  • odoriferum : see cubitti .
  • oreotrephes : takes sun and sharp drainage without wind.
  • pemakoense : see glaucophyllum .
  • racemosum : likes sun, does rather well but is slower growing than spiciferum.
  • rhabdotum : has taken 18° F without damage; late to start into growth. Very straggly.
  • rupicola : needs shade and wind protection.
  • russatum : will take some sun if sufficiently watered.
  • sargentianum : likes neighbors, water and shade.
  • schlippenbachii : cannot take sun or wind at all! Or else I've had two poor forms.
  • scintillans : requires shade and wind protection.
  • spiciferum : grows vigorously in light shade. Wind and frost do not seem to faze it.
  • spinuliferum : orange form tender but will re-sprout from base. Avoid wind.
  • supranubium : seems to be the hardiest of all Maddeniis tried. Likes shade.
  • tephropeplum : beware of early frosts, sometimes puts out several sets of growth.
  • valentinianum : Needs protection from everything, but grows rather well for a dwarf.
  • viscosum : does well, loves water.
  • williamsianum : needs protection from sun and frosts. Good drainage essential.
  • xanthocodon: insects delight; avoid wind.
  • xanthostephanum : more frost hardy than burmanicum - avoid too much sun.
  • yakushimanum : best all around plant. Can not understand why it can't take hot summers in Japan...humidity factor?

Successful hybrids to date include: 'A. Bedford', 'Arthur J. Ivens', 'Arthur Osborn', 'Beauty of Littleworth', 'Blue Peter', 'Bluebird', 'Bowbells', 'Conchita', 'Cotton Candy', 'Countess of Haddington', 'Crest', 'Damaris', 'Dame Nellie Melba', 'Dancing Lady', 'Diane', 'Dido', 'Earlybird', 'Ethel', 'Fabia Tangerine', 'Fatuosum Flore Pleno', 'Fragrantissimum', 'Helene Schiffner', 'Hummingbird', 'Ibex', 'Jock', 'Lady Chamberlin', 'Lady Roseberry', 'Little Bill', 'Loderi Venus', 'Olympic Lady', 'Penjerrick', 'Royal Flush' (orange form), 'Sapphire', 'Seta', 'Small Gem', 'Snow Lady', 'Spinulosum', 'Tally Ho', 'Tidbit', and 'Yellowhammer'. Others are being tested.
Spectacular hybrid failures to date: 'Blue Diamond', 'Little Bert', 'Golden Horn', 'Eleanor', 'Eldorado', 'Else Frye', and 'Saffron Queen'. I will try some of these again under different conditions to find out what the problem is.