Hot Weather Rhododendrons
Francis W. Mosher Jr., Woodacre, Calif.
Rhododendrons can thrive and bloom even in the hot interior valleys of Northern California. This has been proven conclusively by Charles C. Jensen, a member of the California Chapter, American Rhododendron Society, on his acreage located on Fair Oaks Boulevard, Carmichael, just a few miles southeast of the California state capitol building in Sacramento.
For the past seven years, this devoted rhododendron specialist has been blazing a new trail for gardeners in the Sacramento Valley, usually described previously as an undesirable location for rhododendrons. High summer temperatures coupled with freezing-low winter thermometer readings for many years have discouraged rhododendron enthusiasts in this region.
On retiring from business in Oakland, Alameda county, in 1958, Mr. Jensen moved from a city in the San Francisco Bay Area where he had recorded a low mark of 28 degrees Fahrenheit and a high record in the lower 90's to a suburban area inland where, during the past seven years, he has seen the thermometer drop to 18 degrees in 1963 and climb upward to 113. During the summer of 1962 there were 20 consecutive days over 100 degrees.
In preparation for the move from a moist Pacific Coast location to the dry interior, Mr. Jensen had all 75 of his rhododendron varieties planted in wooden containers where they had become well established. On reaching his new 3 acre retirement location, he immediately planted most of them. Since the soil at Carmichael had a high clay content, he used plenty of peat moss, along with sawdust and shavings (pine or fir). In some cases Mr. Jensen planted rhododendrons on top of the ground in raised beds which were surrounded by logs.
According to his records, 24 varieties failed to survive the move. They were:
'Alice' 'Dr. Stocker' 'Mars' 'Azor' 'Eldorado' 'Mrs. Donald Graham' 'Butterfly' 'Foresterianum' 'Mrs. W. C. Slocock' 'Cynthia' 'Jock' 'Unknown Warrior' 'Doncaster' 'Lady Alice Fitzwilliam' 'White Pearl' 'Dr. O. Blok' 'Lady Primrose' R. augustinii
Some could not survive the hot weather and others could not take the winter cold.
Fig. 60. 'Mrs. G. W. Leak' beside a path
on the Rhododendron Island at Portland, Ore.
Photo by Cecil Smith
Some Recommended Varieties
Based on his seven years experience in the interior valley area, previously not considered as good rhododendron growing country, Mr. Jensen lists his "Top Ten" recommended varieties as:
1. 'Vulcan' (has taken 113° heat in full sun) 6. 'Mrs. A. T. de la Mare' 2. 'Mrs. G. W. Leak' 7. 'Mrs. Tom Agnew' 3. 'Purple Splendour' 8. 'Mrs. Furnival' 4. 'Hon. Jean Marie de Montague' 9. 'Fabia' (and its hybrids) 5. 'Lucky Strike' 10. 'Broughtonii Aureum'
In September, 1965 Mr. Jensen records 105 varieties and species growing on his acreage in several locations. Until he has had more experience, it is not possible for him to rate them all as to heat and cold tolerance. However, he is enthusiastic about the prospects in the Sacramento Valley for the following 25 varieties:
'Anna Rose Whitney' 'Diva' 'Mrs. E. C. Sterling' 'A Bedford' 'Dawn's Delight' 'Ruby Bowman' 'Annie Endtz' 'Dr. Endtz' 'Pygmalion' 'Betty Wormald' R. griersonianum #550 'Loder's White' 'Cornubia' 'Jan Dekens' 'Antoon Van Welie' 'Bluebird' 'Margaret Dunn' 'Rainbow' 'Blue Diamond' 'Mrs. C. B. Van Nes'
Shade Tree Problems
When he first purchased the property, native live oaks and blue oaks furnished needed shade. Continuous watering during the summer months weakened and in some cases killed these shade trees. In place of them, Mr. Jensen has fast growing pecan trees. These volunteers grew from nuts carried by squirrels from neighboring orchards. They thrive on hot weather coupled with heavy watering.
Unable to get oak leaves, redwood litter or peat moss in sufficient quantities at reasonable prices, Mr. Jensen turned to local sources of supply for sawdust and shavings. These come mostly from wood shops and lumber yards using pine and fir lumber. In order to provide a balanced ration for his rhododendrons, Mr. Jensen adds cottonseed meal and ammonium sulphate, mixing in the additives before using the material.
In addition to his rhododendrons, Mr. Jensen has over 1,000 azaleas growing representing more than 75 varieties. The Gables, Pericats, Kurumes and Rutherfordianas grow well as do the Knaphill, Exbury and Mollis types. The Belgian Indicas and many of the popular florist shop varieties are on the tender side and often sustain split bark after a heavy frost. Dogwood, tree peonies, deciduous magnolias and skimmias also thrive.
One element which has contributed greatly to Mr. Jensen's success is the fact that his water supply is furnished by an irrigation district and comes from wells near the American River guaranteeing a neutral Ph rating. Alkalinity in the water supply should be avoided, if possible.
"Briefly, it seems that as far as Rhododendrons are concerned, the heat is no great problem provided they are planted properly," Mr. Jensen explains. "Of the many plants I have lost since moving to Carmichael, it has not been the fault of the variety but rather poor drainage in many cases."
"Damage by sunburn is more often due to insufficient moisture in the air. A good mulch is a big help in these cases and provides the needed humidity. In connection with hardiness, ratings of 5 or 6 must have protection."
Only experience can give rhododendron growers in hot summer climates definite recommendations as to the amount of sun, water, shade, fertilizer, mulch, exposure, and choice of varieties for any given location, Mr. Jensen believes. However, he does recommend informal personal visits to other rhododendron gardens in the vicinity which can result in considerable help in establishing guide lines for success.
He has not been able to find more than one rhododendron specialist in Sacramento county with a collection of any size. However, scores of home gardeners have successfully grown one or two rhododendrons as specimen plants. Elevation at Carmichael is about 160 feet. Mr. Jensen has checked with growers at Auburn, Placer County, and Placerville, El Dorado County with elevations 1,000 or more feet above sea level. He particularly mentioned the collections now growing for Mrs. Robert Adler at Auburn and Mrs. L. E. Alexander at Placerville, both members of the California Chapter, American Rhododendron Society.
Suggestions made by Charles A. Dewey Jr., of Charlotte, N.C., in his article entitled: "More About Rhododendrons in Southern Red Clay" (Quarterly Bulletin, April 15, 1964) and endorsed 100 per cent by Mr. Jensen for Sacramento Valley rhododendron growers include:
- Either have perfect drainage or use raised beds;
- Give as much sun as plants can stand;
- Use plenty of sawdust and shavings in planting mixture but add other fertilizers as needed;
- Don't over-fertilize, use sparingly;
- Mulch thoroughly in order to provide humidity during hot weather;
- Be careful not to over-water, it will kill plants.