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Rootstocks for “Difficult”Plants: Rhododendrons,Azaleas and Grevilleas

Michael S. Reid, Wesley P. Hackett andJason S. Julian

We are now entering the third and finalyear of funding in our project, which is aimed atproviding an economically viable means ofproducing grafted rhododendrons, azaleas andgrevilleas on rootstocks able to tolerate high-pHsoils. Ultimately, we hope to develop a reliableprocedure by which these beautiful acid-lovingshrubs can be produced for introduction intoareas with naturally high soil and/or water pHvalues.

For the production of graftedrhododendrons and azaleas, we have chosen anecotype of the western rhododendron (R.occidentale) for use as a rootstock. Our plantmaterial has been taken from an individual that iscurrently performing very well under the highpH conditions in Davis. For our research on thegrevilleas we have chosen ‘silky oak’ (G.robusta) seedlings as our high-pH tolerantrootstock. Specimen trees can be found inseveral locations throughout the city of Davisand are performing excellently.

In addition to traditional graftingtechniques, we have been experimenting with theestablishment of material from both groups intissue culture. We plan to use this material totest the feasibility of producing graftedindividuals through in-vitro mini-grafts.

Progress to dateOur research into producing grafted

rhododendrons and azaleas has been primarilyimpeded by difficulties in the production of largequantities of rootstocks. Time of year anddevelopmental stage have been found to effectthe ease of rooting of cuttings used to producerootstocks. Currently, we have found that itrequires almost two months on the mist benchbefore rooted cuttings are ready to betransplanted and grown-on (Figure 1).

Figure 1. Rooted R. occidentale cutting

We are also working to identify theoptimal hormone concentrations and mediumcomposition for rooting of the westernrhododendron. We have had little success inrooting through the use of the aero-hydroponicsEGS (Ein Gedi System); many of the cuttingswere infected with fungi before rooting couldoccur. We are currently using the optimal rootingprotocol to increase the number of rootstocks sothat a large quantity of grafts can be completed.

Using Anderson’s revised tissue culturemedium, we have been able to establish nodalexplants of the western rhododendron in culture(Figure 2). More efficient methods for divisionand multiplication of the rootstock for use inmicro-grafting are currently being determined.Work is also being done to establish severalcommercial cultivars in culture.

Figure 2. R. occidentale in culture

We have met with much greater successin our attempts to generate large numbers ofgrafted grevilleas. We have found the productionof G. robusta seedlings from seed purchasedthrough a wholesale seed-farm to be a relativelyfast and inexpensive way to produce a goodnumber of rootstocks (Figure 3). While

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germination percentages for these seeds havebeen found to be quite low, the seeds are fairlyinexpensive and we have sown extra in order toproduce the number of seedlings we require.Seedlings took approximately 45 days to reachtransplantable size and were ready to be graftedafter another 2-3 months in the greenhouse.

Figure 3. Grevillea robusta seedlings

Attempts to produce rooted cuttings foruse as rootstocks were not as successful.Repeated experimentation using the EGS aero-hydroponics system resulted in what appeared tobe some callus formation at wound sites in mostG. robusta cuttings but no rooting. All cuttingsdied before rooting could occur, usuallyaccompanied by leaf drop and dehydration(Figure 4). Traditional cutting production withhormone application and misting was alsoattempted multiple times, all producing resultssimilar to those found in the EGS experiments.

Figure 4. G. robusta cuttings in EGS

Using seedling rootstocks we haveproduced a good number of grafted plants usingscion material from several cultivars (Figure 5).Those attempted so far include ‘Red Hooks,’‘Ivanhoe,’ ‘Ruby Clusters,’ and ‘Constance.’

Figure 5. G. 'Red Hooks' on G. robusta after 2.5months

We have collected another large group ofgrevillea cultivars from Bay Area nurseries to useas scion material. These will be tested forcompatibility as soon as the next group ofseedlings is ready for grafting. So far we haveonly used simple ‘cleft’ grafts on the grevilleas.The unions formed through this procedure occurfairly quickly (3 to 4 weeks) and are holding upquite well (Figure 6).

Figure 6. G. 'Red Hooks’ on robusta union

During our grafting experiments we havetested for the effects that light and humidity haveon the speed and success of the graft unions.Plastic bags with and without aluminum covershave been used to test the effect of light on thegrafts while the inclusion or exclusion of arootstock leaf inside the bags has been used to testthe effects of relative humidity (Figure 7). So farit appears as if the complete shading provided bythe aluminum covers slows down the process. Onaverage, grafts that have received light haveleafed out faster and have developed fewer fungalinfections. The effects of humidity in the grafting

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enclosure are still unclear.

Figure 7. G. 'Ivanhoe' grafts with leaf addition

In addition to bench grafting grevilleaswe have also been working on establishing themin culture. The goal is the same as for therhododendrons, in that we would like to useestablished cultures of G. robusta and othershowy hybrids and cultivars to attempt mini-grafts. As with the rhododendrons, we are stillin the in-vitro establishment phase with thegrevilleas. Initial explants of G. robusta havebeen successful so far, but techniques and mediafor subdivision are still being developed (Figure8).

Figure 8. G. robusta in culture

During this final year of funding weplan to complete several goals with regards toour work on the grevilleas. First, we willcontinue to prepare grafted plants in thegreenhouse using the new cultivars that we haverecently collected. Once the grafted plants arewell established they will be planted out in testplots on campus, primarily within the AustralianNatives section of the U.C. Davis Arboretum.Here we will be able to monitor their progressand watch for any signs of nutrient disorders orincompatibilities. Also, once we have determined

the best procedures for preparing and maintainingthe grevillea explants in culture, we will test thecommercial applicability of using mini-grafts inthe production of grafted grevilleas.

With the rhododendrons and azaleas, ourprimary concern is to determine the mostsuccessful and efficient means of propagating R.occidentale rootstock. Once we have enoughrootstock plants to conduct sizable graftingexperiments we will test scion material fromseveral commercial cultivars for compatibility.Again, established grafted plants will be plantedin our test gardens and monitored. Tissue cultureexperiments will continue with the hope ofestablishing a reliable division and multiplicationsystem, from which we will be able to test theviability of micrografting in the production ofgrafted rhododendrons.

Michael S. Reid is professor, Wesley P.Hackett is professor emeritus and JasonS. Julian is graduate student in thedepartment of environmental horticultureat the University of California, Davis.