QBARS - v35n2 Rhododendron Growing in California

Everett E. Farwell, Woodside, CA

Thirty years ago California Chapter of the American Rhododendron Society came into being. It was common at that time for visitors to San Francisco wishing to see rhododendrons to drive through Golden Gate Park, possibly to older homes in the East Bay foothills, the Peninsula and occasionally Monterey Bay, Carmel area or Fort Bragg.

In Fort Bragg, Dr. Bowman was hybridizing maddeniis and other tender forms, working closely with Golden Gate Park in the advancement of new varieties. The main commercial source was a grower in Eureka who sold you what size, kind and quantity he would allow each nursery to have - they were all grafted plants.

Cultural habits were limited to the north side of a house or building, peat moss and sand (sic) and of course, if it was not in the fog belt, it was felt best to avoid planting them.

Thanks to the American Rhododendron Society, California fast became a mecca for maddeniis, vireyas, dwarfs (including species) and many other collectors plants.

As a testimony to what has occurred in 30 years, the accompanying articles were written to assist those attending the Conference in San Francisco to appreciate the progress California has attained since 1951.

Eleanor Philp, Fort Bragg, CA

Rhododendron loves living in and around Fort Bragg, California, like to claim that they have one of the best areas in the U.S.A. for growing rhododendrons. The high humidity, cool summers, and mild winters provide the climate so beneficial to growing them in the open. Located on the Mendocino Coast in the Redwood forests, Fort Bragg is surrounded by miles on end of R. macrophyllum as well as islands of R. occidentale , with soil, as well as the climate, ideal for growing our favorite shrub.

Because of our mild winters, the early blooming rhododendrons do especially well in Fort Bragg. An occasional extra cold spell will possibly damage the blooms, but if planted with a minimum of overhead protection they will survive our normal midwinter cold. Hybrids of the species moupinense , racemosum , spiciferum , ciliatum , arboreum and dauricum are but a few that present flowers for us to enjoy in December and January. From then until August, with R. auriculatum and its hybrids, our gardens are ablaze with color. Color is brighter than in most areas because our coastal fog keeps the flowers fresher.

With the above advantages, it is no wonder that the Fort Bragg area has more rhododendron growing nurseries than any other area in California, or, as stated before: Fort Bragg rhododendron lovers claim to have the best growing area in the country.

Parker Smith, Sebastopol, California

Growing rhododendrons and azaleas in the varying soils and climates of Sonoma County presents several challenges. Very heavy clay soils (adobe) are present in most valley areas, restricting rhododendrons to raised planters containing well-draining acid soil mixes. Some of the tougher hybrids and a few species survive in the adobe soil if its character is greatly improved by incorporating at least 50% organic amendments such as nitrolized redwood sawdust or fir bark and shavings. If these areas of improved soil are not naturally sloped, mounding is required to insure adequate subsurface drainage.

Low winter temperatures (±20°F) are a problem only for the most tender species and hybrids such as the vireyas and some maddeniis. High summer and fall temperatures combined with low humidity in areas away from the coast limit the selection to heat tolerant varieties. The amount of shade required by rhododendrons in this area is usually in direct proportion to the distance from the coast that the garden is located. Most plants can be grown in full sun within a few miles of the coast because of the influence of the ocean and fog. Inland, almost all plants require filtered shade or exposure to the morning sun only.

All plantings depend on supplemental irrigation to meet their watering needs during the summer and fall when there is no significant rainfall. The quality of this water is generally adequate, but in some areas high concentrations of boron or other salts make growing rhododendrons virtually impossible. If these areas are avoided and the above soil and exposure considerations recognized, a very wide range of rhododendrons and azaleas can be successfully grown.

Fred Cummings, Orinda, California

In Orinda, on the inland valley side of the Oakland-Berkeley hills, in summer we often watch the fog from the San Francisco Bay come just over the hills, then evaporate 2 or 3 miles away from us, leaving the sun beating down on our rhododendrons in 90-100° F heat. In winter we often have frost (18°F rarely, 20° F often) while the area immediately surrounding the bay is frostless. Many tender rhododendrons are thus borderline or impossible for us, even though happy 4 miles away on the San Francisco side of the hill. In the summer we water every day and even then have low humidity and need more shade than is needed in "town".

Our native shade trees are mostly live and deciduous oaks. They are deep rooted and great for rhododendrons. My experience indicates that if they are started from acorns or very small trees with regular watering they will be happy with rhododendrons underneath; otherwise (starting with a mature tree) watering must be confined to the outer edge of the drip line. The most common imported tree is Monterey Pine. It provides good shade, but the roots are very invasive and I find it difficult to grow rhododendrons under, though I do.

Our soil is heavy clay. Patient gardeners spade gypsum into this 2 feet deep in a very large area (not just in a planting hole) for 4 or 5 years, adding peat moss or fir bark or some other slow-decaying humus, then plant their rhododendrons. This works fine. Impatient gardeners (I am one on this issue) build hills on top of the clay, of pure humus or humus and sand. Some think the roots will never penetrate the clay if planted in this loose soil to start. It's logical, but I don't believe it (perhaps because I don't want to believe it.)

Bob George, Cupertino, California

To make rhododendrons happy in our sunny valley, we must shade, either by tree or lath. To keep them cool during the long warm summer, we spray the foliage, and mulch the root area. To maintain a healthy plant the heavy black adobe soil is blended with compost, or replaced with a lighter mix.

Thorough watering is a must in the summer months to keep the salt in our local well water from becoming overly concentrated. Good drainage is a must, so our local "Oakroot fungus" doesn't strike...is it worth it? You bet it is!

Allan Korth, Monterey Bay Area

Because of the benign coastal climate, growers in the Monterey Bay and other central California Coastal areas can grow perhaps the greatest variety of rhododendrons in the Western Hemisphere - tropical varieties that bloom from late fall to winter, followed by 'Cornubia' and some of the maddenii species and hybrids in the very early spring, as well as the usual spring and summer blooming types.

Unfortunately, 'Cornubia' requires a few years to start blooming profusely, but those with patience are rewarded by glorious scarlet trusses as the first chronicle of spring. A goal for local hybridizers should be a breeding effort with 'Cornubia' for early flowering pinks and reds that set flower buds at an earlier age.

Soil conditions are perhaps a bit on the heavy side in many areas, but the availability of both redwood and fir sawdust for incorporating in rhododendron beds makes this an easily overcome obstacle.

Along the coast temperatures are mild, with occasional light frost in winter and cooling fog in summer, but as we move inland the local temperatures will vary considerably, with some areas receiving too much winter frost to grow maddeniis and vireyas outdoors, with hot, sunny summers, especially on the mountain tops. Obviously, growing on the north slopes of the mountains is preferable to the south facing slopes; however, there are growers who persevere in both locations.

Even though temperatures are generally moderate, one of the worst periods for heat usually comes in the fall during September and October, when the thermometer can go to a sizzling 105° F and up, depending on location. Even coastal areas receive some 90° F weather and occasionally 100° F plus. Considerable foliage and bud damage can occur at this time. Beating the heat can be accomplished by a systematic placement of frost sprinklers of the type used in grape vineyards, which will cool the growing area to 80° F while applying only about one gallon of water per hour. These sprinklers can be run for hours or until the heat subsides without over watering the plants.

The one shortfall of the area is that the rainy season, which generally lasts about six months (and sometimes less) would be more kindly to rhododendrons if it were longer. This calls for a lot of watering, and water quality and abundance vary widely from one small area to another, depending upon the source, which may be local springs, streams, rivers and wells, the latter frequently having excess mineral content.

There is probably no such thing as the perfect rhododendron area, but we in the Monterey Bay area have a lot going for us.