JARS v36n1 - More Notes on Growing Rhododendrons in a Hot Climate

More Notes on Growing Rhododendrons in a Hot Climate
Sandra Spencer, Ukiah, CA

One of the hazards of writing about gardens and gardening is that right after appearing in print, the writer makes new discoveries or changes opinion completely after reviewing experiences. My original article appeared in the January 1973 ARS Quarterly Bulletin during the middle of the most severe winter this area had seen. Our area of Ukiah, CA is a coastal inter mountain community of 700 to 900 feet elevation. As the 1973 winter passed and the plant corpses piled up, I felt I should revise the whole thing. Being a prize procrastinator with a heavy work schedule, I successfully postponed a sequel for nine years, with occasional guilt pangs. Finally, after reading George Ring's article in the Spring 1981 ARS Bulletin, I made the plunge and here is the result:

A. Temperature extremes: revise the original 16°F downward to 11-13°F, with a possibility of below 25°F temperatures at night for more than 10 days at a time. All else holds the same. (Just for the record, this winter we never dropped below 25°F once and the blooms were profuse.)

B. Rainfall: has been falling below average in the last few years. However, we have no water rationing and the water from the tap averages about pH 7.0; it's high in magnesium, no calcium. In fact, I have been considering putting dolomitic limestone on my lawn; but that is another story.

C. Soil: once upon a time l thought I had clay soil; then I visited my sister's garden in Alexandria, Virginia. I now describe my soil as heavy loam and after 17 years of work, it is well-fortified with humus and perlite...the envy of my neighbors. Adding humus via mulches will go on forever.

D. Water supply: see B.

E. Wind: in summer wind is a great ally but in winter the chief killer. By now my trees (mostly Pinus contorta and Acer palmatum ) effectively block the worst. Our one and only hurricane occurred a couple of years ago and demolished the south fence. Yes, it was rebuilt in a hurry to prevent sunburn. Surprisingly few branches were broken.

F. Pests: the weevils (all kinds) have discovered me. They prefer certain areas of the garden so far. Maybe I can keep them contained. On the other hand, I have finally controlled the grasshoppers. They much prefer sunshine and anyone desiring the secret weapon (ecologically sound but taking a strong stomach) let me know. Some minor damage is done to new foliage by snails and caterpillars. Rots, molds, etc., can be more easily controlled if the plants are growing in a slightly raised mound with well draining mulch.

Now to the meaty part, the plants...I repeat that the williamsianum hybrids and griersonianum hybrids including griersonianum itself are top-notch candidates for climates like mine. Now I'm going to add fortunei hybrids and the Triflorum series. In fact, Triflorums take sun better than others without scalding etc. Those who garden in hot areas with high humidity and high temperatures at night seem to fail with R. yakushimanum and its ilk. My yakushimanum is easily 20 years old and thrives in a large mound. It is getting rather large for an FCC form. (Maybe we should give 20 year size figures as well as 10 year ones). The yakushimanum has thrived so well that I added makinoi and it bloomed this year for the first time. It is a good thing and I really like the leaves.

Successful hybrids to date include: 'A. Bedford', 'Alison Johnstone', 'Carey Ann', 'Arthur J. Ivens', 'April Chimes', 'Arthur Osborn' (a real spreader), 'Beauty of Littleworth', 'Bluebird' (needs some shade), 'Blue Peter' (doesn't like to be moved), 'Bow Bells', 'Brocade', 'Chikor', 'Cornish Cross', 'Cornubia' (no blooms yet and it's past the six foot mark), 'Cotton Candy', 'Countess of Haddington' (flower-bud tender), 'Cream Crest', 'Crest' (protect from sun), 'Dame Nellie Melba', 'Day Dream', 'Diane', 'Dormouse', 'Earlybird', 'Emasculum', 'Elizabeth', 'Fabia Tangerine', 'Fastuosum Flore Pleno', 'Fragrans', 'Fragrantissimum' (protect from morning sun on frosty mornings), 'Ibex', 'Jim Drewry', 'Jock', 'Kim', 'Kerrigan's Yellow', 'Lady Rosebery', 'Lemon Mist', 'Little Bill', 'Loderi Venus', 'Mrs. G. B. Simpson', 'PJM', 'Purple Splendour', 'Queen of the May', 'Ramapo' (shade only), 'Seta', 'Small Gem', 'Snow Lady', 'Spinulosum', 'Susan', 'Tally Ho', 'Tiffany', 'Valaspis', 'Vanessa', 'Yellow Hammer', and 'Windbeam'.

The failures remain the same but in the name of research let me add some others: 'Conchita' (too tender but would be okay in pot moved to shelter), 'Damaris', 'Ethel', 'Hotei', 'Myrtifolium', 'Russautinii', and 'Tidbit'. Diagnosis of failure not always possible but educated guesses can always be made. I would like to experiment with cinnabarinum hybrids some more as so far results have been inconclusive.

If any of the above long winded lists is cold hardy enough for you, there is a reasonable chance that it will beat the heat too. So far I've been lucky with them. The above ground plastic pipe sprinkler system simplifies the watering and leaves soon hide the pipe. Changes can be easily made as the plants grow. To date I have never lost an evergreen azalea to cold although a few have suffered burned leaves (Pericats, 'Fielder's White', and 'Albert — Elizabeth') and they thrive on heat, even the Satsukis. The deciduous ones are more difficult as they seem to attract weevils.

If I were going to select species to use in hybridizing for heat tolerance, I would use R. hippophaeoides , R. carolinianum , Triflorums, williamsianum hybrids, griffithianum hybrids, 'Yellow Hammer', R. aberconwayi , certain arboreum selections and the Scabrifolium series.

I have recently had an opportunity to visit gardens in the Sacramento, Central Valley, and Orinda areas and find that Ukiah's shorter growing season (meaning fewer hot days and colder wetter winters) give different opportunities. The companion plants vary too. Basically, it's a case of citrus versus non-citrus growing areas. Ukiah lost the battle of the orange. I suspect Maddeniis go with the orange and while I miss the unique fragrance, I have less work in general to keep rhododendrons and azaleas going.

George Rings pointers on irrigation, shade, drainage, and wind are the deciding factors in climate modification. There is no rhododendron or azalea that could survive this climate in an exposed position without additional summer water. Only a tap-rooted plant could.

Heat tolerance is a continuing subject of interest to many of us. Perhaps a round robin letter or set of letters should be exchanged with/by interested members.

Revised statistics and comments:


R. aberconwayi : still a good choice.

R. arboreum : Rock seedlings very sturdy; Golden Gate seedling form too tender.
R. augustinii : only 'Marine' is supersensitive; seedlings especially sturdy but occasionally sustain frost damage on new growth.
R. auriculatum : lost this one to overwatering; should be okay otherwise.
R. brachyanthum : does well, nice in rock garden.
R. edgeworthii : frost tender, should be protected.
R. bureavii : be careful with watering.
R. burmanicum : not hardy here in any form tried to date.
R. caesium : good, needs some sun.
R. calendulaceum : the weevils love it; protect from wind.
R. calostrotum : good; watch drainage.
R. campylogynum : var. cremastum best so far.
R. canadense : appreciates moisture.
R. canescens : a martyr to weevils.
R. charitopes : need to try again.
R. chryseum : just about given up on this one; besides being tricky, it attracts natural disasters, i.e. moles, cats.
R. cinnabarinum : tricky; hates fertilizer and likes extra acidity.
R. chrysodoron : great pot plant; flower bud tender.
R. concatenans : very good, needs some sun for bloom.
R. concinnum : 'Chief Paulina': new in my garden; looks to be a winner.
R. coriaceum : lost to unknown causes and not replaced.
R. crinigerum : still hopeless.
R. cubittii : too tender.
R. davidsonianum : needs wind protection and some sun for best bloom.
R. decorum : a winner.
R. desquamatum : plan to try again.
R. dichroanthum : great grower, likes sun, shy bloomer.
R. sanguineum ssp. didymum : still hopeless.
R. drumonium : good of Lapponicum type.
R. fargesii : never replaced, terminal dieback.
R. fictolacteum : lost to unknown causes and not replaced.
R. formosum : only white form hardy; one of the easier Maddeniis.
R. forrestii : managed to kill my last one; too near the cement foundation I suspect.
R. glaucophyllum : not happy.
R. griersonianum : super.
R. gymnocarpum : must give it another chance in a better spot.
R. hanceanum var. nanum : good in all forms; outstanding in rock garden.
R. hemitrichotum : for good bloom recommend half sun at least.
R. hippophaeoides : most sun resistant of all; trying one of its hybrids this year.
R. hirsutum : died, cause unknown; not replaced to date.
R. impeditum : good of Lapponicum type.
R. japonicum : excellent.
R. johnstoneanum : too tender.
R. keiskei : good in small form, have not tried any other.
R. keleticum : great in rock garden but beware of smothering neighbors.
R. keysii : not satisfactory to date.
R. kiusianum : looks better every year.
R. lepídostylum : a newcomer doing well in rock garden.
R. leucaspis : blossoms can be nipped by early frost; a nice fuzzy one.
R. lutescens : needs sun to bloom, top ten.
R. luteum : shy blooming here but very sturdy.
R. macabeanum : simply beautiful, can't wait for it to bloom but foliage is a real 10.
R. makinoi : popular with leaf chewers.
R. megeratum : nice rock garden subject.
R. micranthum : thrives but fails to bloom — probably in too much shade.
R. microleucum : attractive but so small and slow-growing I've nearly lost it.
R. morii : died, cause unknown; not replaced to date.
R. nakaharai : very spreading late bloomer. Must give it more space.
R. occidentale : a swamp plant.
R. odoriferum : too tender.
R. oreotrephes : have variety exquisitum and it never fails.
R. pemakoense : too many failures, about to give up on it.
R. prostratum : good rock garden specimen. Chose neighbors carefully to prevent smothering.
R. racemosum : good with some sun.
R. rhabdotum : gave away, too straggly.
R. prinophyllum : good but watch out for weevils.
R. rupicola : adequate Lapponicum.
R. russatum : very good in all forms; some freer in flowering.
R. sargentianum : too fussy.
R. scabrifolium : a real standby in sunny spot.
R. scintillans : behaves like russatum.
R. serpyllifolium : good, try massing if you have space.
R. spiciferum : very willowy, makes a good espalier.
R. spinuliferum : still looking for a hardy dark colored form.
R. supranubium : too tender.
R. sperabile : must give it another chance in a better spot.
R. tephropeplum : like charitopes.
R. trichanthum : new in my garden but looks like a winner.
R. valentinianum : has settled down in rock garden; bud tender.
R. virgatum : nearly lost this in the winter of '72-3 but it has made a strong comeback.
R. viscosum : nice summer bloomer.
R. williamsianum : okay, but hybrids of this much easier.
R. xanthocodon : like concatenans.
R. xanthostephanum : too tender.
R. yakushimanum: top value.