QBARS - v5n1 Rhododendron Culture in Swiss Gardens and Parks

Rhododendron Culture in Swiss Gardens and Parks
Jacob Jenny, Solothurn, Switzerland

The following article by J. Jenny, a contributor to Revue Horticole Suisse and author of many garden articles was sent in the original German. Mr. Bacher has translated this paper on methods of growing rhododendron in a country where one would assume none but the very hardy would survive. The methods of culture employed there are very little different than tile best practices followed in this country or elsewhere in the world.- EDITOR.

When one tries to obtain in our gardens, private or public the utmost in flowers and growth one chooses readily the various types of roses, various early flowering shrubs, rockery plants, perennials, etc. Sometimes one reserves a large spot for annuals or the many forms of dahlias, dwarf and tall sorts, which by their diversity provide rich effects from early summer to late fall. All these plants thrive in our region if properly cared for, and produce colorful effects, a real treat to the mind and souls of the people. Nevertheless those who have had occasion to observe plantings of rhododendrons in full bloom will never forget the impression of supreme beauty they have enjoyed. They will understand why the English and their visitors speak so highly of the splendor of rhododendrons. They are beyond the comparison of other garden effects. For them the rhododendrons merit the most important space in the gardens. For this reason the following lines have been written, it is to be hoped to acquaint others somewhat better with this interesting shrub and its cultural needs. As to climate; England, enjoying a seashore influence with moderate temperatures, is somewhat more favorable than the harsher climate of Switzerland. However our continental climate is not by any means the only reason that rhododendrons succeed so rarely with us, for by and large it is the ignorance and lack of proper care and cultural requirements that account for many failures.

Those who have had the opportunity to observe the Rhododendron plantings in private parks on the left side of Lake Zoug (FIG. 4) will admit that good reason is there for the grand displays. This region like many others near Swiss lakes with moderate temperatures, much fog, cloudy days and year around ample rainfall, little wind or dry periods with absence of late spring frosts makes for an ideal region for the culture of rhododendrons.

Since the winter of 1929 the rhododendrons have not suffered any real frost damage. During the cold spells the evergreen sorts roll up their leaves in self protection against evaporation, while the ground is frozen. However it is necessary to plant them in a protected location free from strong winds or in the shelter of other plants. As to locations and conditions of soil, our native Rhododendron ferrugineum of the Alps give a clear indication of their needs, for they may be observed anywhere from 5000 ft. to even above the timberline growing fully exposed to sun and winds with their roots embedded in sandy humus and rocks. The species R. hirsutum is found on limestone rocks. Both species are frequently in company of pines, firs, heathers, junipers, huckleberries and a host of other acid loving soil plants where they form groups of unique charm. It is the peaty soil of the mountains that the gardeners of the Lake Zoug region try to imitate by hauling in the swampy soil from the mountains near the lake. Large quantities of such material are hauled in and the P. H. of such soil varies between 4.8 and 5.5. This soil is used at once or first composted but never screened for the fiberous roots and other woodsy parts are congenial to the rhododendron plants and their welfare.

One usually abstains from using clay or compost soil with manure. Imported heather soil (Terre de Bruyere), manure, peat or chemical fertilizers are avoided however really well rotted manure would not hurt them in any way. However it is the planting practice that most of the errors are committed even when the soil around the roots is ideal. It is absolutely inadvisable to dig holes deeply in our heavy soils of clayey nature with lime content. The plants would eventually find themselves potted into waterproof containers. The gardeners of this region de Zoug are careful to dig out hardly more than the sod where a planting is to be made. They put in a layer of brush, old wood or roots, preferably of decaying nature to a depth of 6 to 8 inches. This layer helps to provide drainage and interrupts the capillarity action of moisture from the limy subsoil to the root system. On this layer is placed the real soil prepared for the roots of the plants. It is essential that the earth and the roots of the plants be soaked by placing the balls into containers of water before planting. The planting soil is then placed over the balls of the plant and on it a thin layer of brush and the same covered lightly with soil. The rhododendrons have a shallow root system not requiring much depth, usually a foot is plenty. Plants set out in this manner will reveal a vigor of exuberance, while if set out in the ordinary manner into heavy soil they soon suffer from the tight soil of uncongenial nature, and after a few years it will be observed that the roots have not expanded, while those set out the proper way need transplanting to larger quarters. If replanting is required it is done in a similar manner by using larger holes, with brush for the bottom, and larger amounts of ideal soil. The brush layer will also prevent other nearby shrubs from encroaching into the root systems of the rhododendrons.

Early spring is considered the ideal planting period for rhododendrons. In this manner winter damage is avoided, which may occur if the plants have been set out in early fall, when the precaution of winter shelter is advisable. When transplanting it may be advisable to reshape the bushes or eliminate unshapely branches for rejuvenation. The rhododendrons of large blooms such as the R. catawbiense and its hybrids will tolerate thinning out to good advantage for they grow in width more than in height. At the park of Buonas there is one specimen with a crown of 36 ft. in diameter and 12 ft. tall. R. 'Cunningham's White' is very vigorous and the earliest flowering of the whites in large flowers. Its lower branches drape to the ground and take roots. This variety is also appreciated by growers for grafting of all the catawbiense hybrids.

The principal cultural requirements of rhododendron plantings consist in maintaining a continuous moisture in the soil, where rainfall is not adequate. In this case deep watering is advisable, taking care to use soft water only. The very light soil used with rhododendrons will absorb very large quantity of water, often double its volume. It is also desirable not to delay watering very long for the Heather soil (Terre de Bruyere) once dry becomes very difficult to wet again, for it is then almost water repellant. In regions where winter rains are deficient watering is to be carried out whenever needed so that periods of frost do not find the roots dry. Winter protection is not needed ordinarily except for new plantings, and may consist of fir branches spread out over the ground. Peat moss, leaves. etc., to a depth of 8 or 12 inches must be removed early in the spring as the root system might suffer from the extra cover. It is useless anyway to let ones fears for the safety of rhododendrons bother your mind as they can readily stand temperatures of 25° below zero, ponticum x Azalea mollis , the so called Kurume Azaleas are suitable. Opposed to this splendid display come the hybrids of evergreen foliage between R. arboreum and R. catawbiense. Depending on varieties, the flower of rhododendrons are more or less open, campanulate or in trumpet forms, clustering at the end of branches in bouquets more or less regular, half round, often attaining the size of a child's head. The color range runs from pure white to deep red, yellow to scarlet orange to pink of all shades of violet and rose to purple. A regular symphony of colors are present with some species and hybrids of multicolor type.

As in all plants there are enemies in the form of insects, and diseases which bother the plants slightly. One of them the tiger of the rhododendron (stephanitis rhododendri) is often noticed in quantity in the plantings at Lake Zoug. Usually during warm dry summers this insect makes the leaves turn prematurely yellow and drop. This parasite winters on the under surface of the leaves. It begins to spread from May to September and hatches 3 generations in one season. A dusting with DDT powder will give good results. Othiorynchus sulcatus , the strawberry weevil will occasionally eat the edges of the young leaves.

There are different varieties with small leaves, evergreen or deciduous, small and medium flowered. The Ghent Azaleas, hybrids of R. ponticum x Azalea mollis, the so called Kurume Azaleas are suitable. Opposed to this splendid display come the hybrids of evergreen foliage between R. arboreum and R. catawbiense . Depending on varieties, the flower of rhododendrons are more or less open, campanulate or in trumpet forms, clustering at the end of branches in bouquets more or less regular, half round, often attaining the size of a child's head. The color range runs from pure white to deep red, yellow to scarlet orange to pink of all shades of violet and rose to purple. A regular symphony of colors are present with some species and hybrids of multicolor type.