Some Notes on Grafting Rhododendrons
by Joe Klupenger
Fig. 8 Fitting the scion for a side graft.
Rhododendrons are known in all parts of the world and in many places considered as the king of shrubs. It rates its merits for many reasons in the home gardens, city parks and commercial plantings. Hybridizers of the past many years have developed new hybrids which range from one foot high and one foot in diameter and are many years old compared with plants of the same age measuring three to four feet tall and wide. The flower trusses are unique in their sizes, varying from two inches up to eight and ten inches in height with a wide range of color.
The rhododendron gives us its greatest bounty if it can get partial shade from the afternoon sun. It has been discovered in recent years that it can be grown successfully in much dryer climates with low humidity by giving it a little extra care.
This beautiful shrub has been a question to many garden lovers as to how it can be reproduced, yet there are many known methods; mainly by seeds, layerage, air rooting, cuttings, budding and grafting. Hybrid rhododendrons reproduced by seed will not come true to color, many will revert back to the parent species. Layering is successful but a very slow process as only a limited number of new plants can be obtained from the parent at a time. Air rooting can be done but is used more by the amateur than the commercial grower. Propagating by cuttings from current wood well matured is commonly used today by commercial growers and also amateurs, but many varieties are hard to root. Some of these hard to root varieties will strike a good percentage one season and a small percentage the next. Countless methods have been tried in rooting such hybrids as 'Britannia' and the 'Earl of Athlone' but the results are usually negative. Possibly the most practical and certain method of propagation is by grafting which is one of the oldest practices used in the reproduction of rhododendrons.
Before grafting can be started the kind of understock to be used must be determined as some scions are not compatible with certain understock. Possibly the most commonly used varieties are; ponticum, decorum, fortunei and discolor seedlings. These are usually grown under lath to encourage vigorous growth with a softer bark thus making the grafting process much easier. It takes from two to three years to grow the seedlings large enough to get the proper stem size to graft. The stock of the seedling at the base or near the ground line should be about the size of an average pencil, ¼ to ⅜ of an inch.
The most common type of graft used is the side graft. In using this method the straightest side of the seedling is selected about 1½ inch above the root ball, then with a good sharp grafting knife a side cut is made down toward the base of the plant about 1 inch long and not over one third to one half of the way through the stock.
The scion having been taken from the current growth is cut approximately four inches long with a tapered cut on the end of about one inch. The scion is now ready to place in the incision made on the understock. Place the scion in place firmly to the bottom of the cut of the understock making certain that the cambium layer is perfectly matched at least on one side of the graft. (FIG. 8)
Fig. 9. A newly made graft showing the scion tied in place.
A 4" or 5" rubber grafting band or budding strip is thin wrapped around the graft. Starting from the bottom of the graft wrap it securely to the top of the splice pulling the rubber band firmly as you wind around the graft. At the top of the splice, tuck the end of the rubber band under the last wrap and pull firmly. This will hold the band from coming loose. (FIG. 9 )
Now that the graft is complete, proper care is necessary until the union shows signs of growth. Different methods are used, one, the grafted plants can be placed in deep frames called sweat boxes setting them in damp peat moss. The frames are covered with glass sash to hold the humidity until the grafts start to grow. The temperature should be maintained at about 68 to 70 degrees. Another method is the open bench system whereby the graft is set in Peat Moss and by the use of over-head humidifiers the humidity is kept in the area of the graft at 80 or 90 per cent to prevent the scion from drying out.
After six weeks to two months the scions will show new growth. At that time cut the understock back about half way. This is done to force more strength into the scion. If the grafts are in sweat boxes with glass covers, the covers should be removed at this time to let more air into the grafts.
After another 30 to 45 days the understock should be cut back further and the scions would show a rather good growth.
Grafting is normally done during the winter months when rhododendrons are in the dormant stage. This times them about right to be moved out from the greenhouse and placed under shade lath after the spring weather has warmed a bit. The rubber band which was used in securing the scion to the understock should be cut loose at this time to let the sap flow more freely and prevent the plant from being girdled.
Now that the grafted rhododendrons are out in the open air under partial shade and in beds well mulched with peat moss they are ready to grow into well formed plants.