QBARS - v16n3 There are Also Lime Tolerating Rhododendrons

There Are Also Lime Tolerating Rhododendrons
Friedrich Heiler, Munich, Germany
Translated from the German by Alfred A. Huber

In my report of lime tolerating rhododendrons I have divided the report into three parts:

  1. The base for the grafts determines the ability of lime tolerance.
  2. Lime tolerating rhododendrons can also be developed by crossing wild lime tolerating rhododendrons with our own hybrid garden rhododendrons. Here is a list of such crossings which already have been accomplished by rhododendron growers in Germany.

  3. Part 3 is a critical report, "How about growing rhododendrons by grafting only?" Example: If rhododendron hybrids, of which one parent is lime tolerating, i.e. R. fortunei x thomsonii are grafted on R. ponticum the inherited lime tolerance will not be effective. A buyer should therefore ask for the kind which have been grafted on the stock of R. fortunei , decorum or discolor , or which are of genuine rootstock, or have been grown from cuttings or layers, as it is customary nearly in all nurseries in the United States.

I am certain that in the U.S.A. and in England there are many other rhododendrons which have been crossed with lime tolerating kinds. Suggestions: Perhaps some lover of rhododendrons who has alkaline soil and water available is willing to undertake the task, according to my suggestions, to plant a part of his garden with cuttings or layers from lime tolerating hybrids. It should be noted, however, that the soil be properly prepared. In our Bavarian limestone mountains there are R. hirsutum growing in coarse humus between cliffs, that never suffer from the lime in the water. One should consider that fruit trees also are planted in soil which has been improved with compost or peat moss.
The enclosed printed articles A and B were printed in the first issue of the "Immergrunen Blatter" (Evergreen Leaves), December 1961, the publication of the German Rhododendron Society. The articles C and D will be published in the coming yearbook.

The Stock for the Graft Determines the Tolerance for Lime

Tests over two decades were necessary to discover those rhododendrons which would grow in parts of the country with non-acid or alkaline soils and hard water to longevity and would yearly gladden the heart of the gardener with their strong growth and profuse flowers.
My love for this plant genus, the rhododendrons and other ericaceous plants, was already awakened in the years 1910 and 1911, when I enriched my knowledge as gardener in the beautiful gardens and parks in England. How could it be otherwise than that I eagerly wished to make the rhododendrons more and more popular in my native country. More than 60 years ago my father had planted exactly according to the rules some R. catawbiense and R. 'Cunningham's White' in acid soil but after 8 to 10 years the plants had to be removed because they made a poor showing. The newly planted rhododendrons did not develop roots, the leaves and flowers became smaller and smaller every year, till the plants finally died and were discarded.
Being a botanical gardener I noticed that the native alpenrose in our Bavarian mountains, R. hirsutum , also could be found in limestone mountains; the other, R. ferrugineum , also called the rust-colored alpenrose, grows only in limestone soil.

R. catawbiense hybrid on rootstock of R. ponticum R. 'Mrs. Butler x williamsianum, a lime tolerant cross
Fig. 28. R. catawbiense hybrid on
unfavorable rootstock of R.
. After 10 years there
was no new formation of roots
even though it had been planted
in acid soil.
Fig. 29. R. 'Mrs. Butler x
williamsianum, a lime tolerant
cross with strong root formation
and luxuriant growth.

New root system being formed in an oak leaf mulch
Fig. 30.  New root system being formed
in an oak leaf mulch.

My determination was now to find varieties of rhododendrons which grow in limestone formations of the Himalaya and which would be suitable for stock for our improved R. catawbiense hybrids. From different nurseries in Germany and foreign countries I purchased wild rhododendron varieties 25 years ago. Unfortunately all large leaved varieties were not on their own rootstock. I had to improvise by planting these grafted rhododendrons deeper into the ground to force them to develop roots above the graft. I was successful, several varieties formed roots above the graft. Out of the ball of earth below the graft there were no new roots. (Fig. 28) The problem of finding lime tolerating varieties had been solved. At first it was the R. decorum , fargesii , fortunei , oreodoxa and rufum .
Occasionally I talked to a progressive thinking rhododendron grower, our honorary member Mr. Hobbie about it, and he suggested to graft these rhododendrons with genuine stock large flowered hybrids. After he had made the necessary bases available, he grafted several thousand pieces on them many years ago, which developed beautifully.
The worst lime tolerating base is R. ponticum . R. catawbiense stays healthy on its own rootstock, although it cannot be called the best. R. 'Cunningham's White' is doing the best on its own rootstock. It remains to be seen, whether grafts on such rootstock will live longer than grafts on stock mentioned earlier.

Hybridizing For Lime Tolerance

Nature can be a teacher for the grower, if he observes and studies what takes place. Nature showed also in her own way, how to breed rhododendrons for lime tolerance. As mentioned above, there are growing in our Bavarian mountains R. hirsutum on the limestone mountain side, and R. ferrugineum on primeval rock soil or coarse humus soil. Where both varieties are growing close together, hybridization develops, brought about by the busy mountain bumble bee looking for nectar. The seeds of these crossings which fall on the limestone soil, sprout and grow nicely. The most important factor of these hybrid varieties is therefore that the cross R. 'Intermedium' had inherited the lime tolerance of R. hirsutum .
It was this perception which led to the crossing of our big flowered R. catawbiense hybrids with wild lime tolerating rhododendrons. After more than 20 years of tests in lime bearing soils and irrigating with hard water, many of the wild large leaved rhododendron varieties proved themselves to be lime tolerating. These lime tolerating big leaved varieties were then crossed with our improved R. catawbiense hybrids; in this manner hybrids were produced with R. decorum , fargesii , fortunei , insigne , \ oreodoxa , scyphocalyx , traillianum , wardii , williamsianum and many more.

R. williamsianum
Fig. 27. R. williamsianum
C. Smith photo

The medium to small leaved lime tolerating R. ambiguum , augustinii , ciliatum , didymum , hirsutum , impeditum , hippophaeoides , lutescens , were crossed with reliable garden hybrids. Culture experiments in the rhododendron test garden proved that the lime tolerance had been inherited in all cases. The ultimate result is that finally we have rhododendron hybrids that will root well in lime containing soils and have a remarkable healthy growth.
I believe that this points to the possibility of breeding lime tolerant hybrid varieties by grafting lime tolerating rhododendrons on suitable stock. It is first of all, the knowledge of this fact that gives the gardener with less favorable soil conditions the possibility of consulting nurseries to select for him, rhododendron varieties grafted as above.
Now that the starting of lime tolerating rhododendrons has been thoroughly explained earlier, the selection is much easier. The garden name R. 'Oldenburg', for instance, doesn't mean anything, if we don't know the origin of this cross. This variety is, after all, a hybrid of the varieties R. discolor x williamsianum . R. discolor and williamsianum are therefore the basis of the lime tolerance.
The selection is made easier, if the crossing parent is mentioned first, and then the name of the hybrid.
Of the varieties which are presently sold commercially in Germany, the name of the parents is given first, and then the name of the crossing partner. Many of the crossings have no common name as yet.

Selection of Rhododendrons For Alkaline Soils
Rhod. decorum x brachycarpum montanum =
" astrocalyx =
" wardii = Ightham Yellow
" williamsianum =
Rhod. didymum x dichroanthum = Black Strap
" kyawii = Exburiense
" williamsianum =
Rhod. insigne x Brittania =
" A. Welii =
" griersonianum = Ingre
" L. Pasteur =
" wardii =
" williamsianum =
Rhod. fargesii x Loderi = Farola
" morii = Morfar
" thomsonii = Rosalind
Rhod. fortunei x decorum = Thomas Bolas
" discolor = Avocet
" brachycarpum mont. =
" orbiculare = Fortorb.
" thomsonii = Luscombei
" wardii = Prelude
" Doncaster = Ronsdorfer Hybr.
" williamsianum = Caroline Spencer
Rhod. oreodoxa x Britania =
" Catawb.-Doncaster = Ronsdorfer Frilhe
" orbiculare =
" repens = Mme. Omsi
" williamsianum =
" W. C. Slocock =
Rhod. scyphocalyx x Louis Pasteur =
" March. J. Brooks =
" Prometheus
" wardii = Yoyance
Rhod. traillianum x repens =
" williamsianum =
Rhod. wardii x Ammerlandense =
" astrocalyx =
" didynum =
" decorum = Lightham Yellow
" discolor = Inamorata
" fortunei = Prelude
" Mr. Butler = Bength M. Schalin
" scyphocalyx = Yoyance
" souliei = Dortmund
" venator = Isme
Rhod. wardii Oldenburg =
" vernicosum =
" williamsianum = Cowslip
Rhod. williamsianum Britania = Ammerlaudense
" discolor = Oldenburg
" Doncaster = Gartendirektor Glocke
" Dr. V.H. Rutgers = Weser
" Duke of York = Lerigau
" Essex Scarlet = Norderney
" Fagetters Favourite = Libelle
" fortunei = Caroline Spencer
" Mrs. Butler = Psyche
" haematodes = Humming Bird
" Linswegeanum = Dr. H. Karl Forster
" litiense = Oxlip
" L. Pasteur = Merkur
" Mme de Bruin = Wiekhoff
" orbiculare = Temple Belle
" Staring = W. S. Reuthe
" wardii = Cowslip
" Mrs. Butler = Wega
Small Leaved Rhododendrons Which Are Lime Tolerant
Rhod. ambiguum augustinii = Candida
" concatenans = Mozari
Rhod. augustinii ambiguum = Candida
" intricatum = Blue Bird
" Intrifast = impeditum Blue Diamond
" impeditum = Blue Tit
Rhod. ciliatum dauricum = Praecox
" carolinianum = Dora Amateis
" lutescens = Quiver
Rhod. hirsutum ferrugineum = Intermedium
" minus = Myrtifolium
Rhod. hippophaeoides racemosum =

Can Breeding of Large Leaved Rhododendrons be Started by Grafting Only?

The dendrologists J. Berg and G. Krussmannwrite in their splendid book "Freiland Rhododendron", a publication of the "Rhododendron-Gesellschaft", on pages 47 and 48, about multiplying large leaved rhododendrons by cuttings in New Jersey, U.S.A. After 10 years of research good results of from 84-40 per cent of rooting have been obtained. Only the dark flowered varieties showed 30-10 per cent lesser rooting. Furthermore they state the important fact that grafting will attain the purpose, and that plants will grow best on their own roots. The same good results in propagating rhododendrons can also be obtained by layering, as it is practiced by the rhododendron nurseries in England.
Why prefer this method of rhododendron propagating, if it assures a greater percentage of rooting? Berg-Krussmann writes on page 56 that plants with their own rootstocks are considered hardier, will live longer and grow better than grafted plants. In addition, they are free of runners and wild shoots.
Another factor, which has not been mentioned, has to be considered. Many of the rhododendron hybrids are tolerant of lime, if their origin came from a crossing whose parents were lime tolerating and stand on their own roots. Thus the offspring of lime tolerating wild varieties R. decorum , discolor , fargesii , fortunei , insigne , oreodoxa , wardii and williamsianum have, as experimental tests proven, that lime tolerance is inherited, also the crossing of the wild R. hirsutum and R. ferrugineum grow well in lime containing soils.