Amateur Rhododendron Grafting
When the amateur gardener acquires some new, rare choice hybrid rhododendrons, it is natural for him, as also the experienced collector, to want to propagate some extra plants from them. They are insurance against loss of the parent plant, make very nice gifts, and are good trading stock with another fan. And its a lot of fun for spare time.
Having a small greenhouse will facilitate the work of grafting. However, having a well built cold frame with bottom heat (electric cable) in a protected location will do nicely. The grafting bin should be 12 to 14 inches deep, about 30 inches wide and as long as desired. 1½ inches of coarse sand or fine gravel should be placed in the bottom, the heating cable laid, and ½ inch of sand on top of the cable. Then about 6 to 8 inches of fine peat moss, thoroughly moistened, should be placed on top of the sand. A window sash or light frame covered with glasstex should be used as a cover for the bin. If clear glass, cover it with paper or thin cloth.
For understock the kinds most suitable are decorum, fortunei, ponticum and discolor. They are usually raised as seedlings and are obtained at a very modest price at your nursery. Two year old plants seem to be the best size, or obtain one year old plants and grow them on. The best time here for grafting rhododendrons seems to be in February. This article is concerned only with hardy garden hybrids, and at first the amateur will probably do as I did, graft any hybrid on any understock obtainable.
Cut the scion wood from the previous years new growth, leave 1½ to 2 inches of stem and with a sharp knife cut a wedge shape point ½ to 1" long on the stem. Then make a slanting cut on the stem of the understock beginning about 1¼ above the soil, about the depth of the scion wedge. Insert the scion so that the cambium layer meets all around as nearly as possible. Try to match for size the stems of the understock and scions, and if that is impossible, have one edge meet nicely. Wrap and tic with a rubber grafting strip, leaving space between the strands. Do not at anytime bruise the cambium of scion or understock; to do so will court failure.
The understock root ball should be thoroughly moistened, then the grafted plant plunged in the peat in the grafting bin, at an angle of about 45° and so that there is a couple inches of peat around the root ball, and a small amount around the graft. If the foliage of the scion is large, cut it half in two, making more room in the bin, as the foliage should not lap. The top of the understock should be cut away down to the last leaf joint. The reason for the peat is to hold moisture, help keep temperature even, and during the callousing period the understock will make root in it.
For low growing varieties, or when it is desired to start the plant higher above the ground, the graft may be made higher up on the stem of the understock. Cut the top off at the desired height, preferably at a leaf joint, and having an "eye" on each side, slit the stem straight down from the top. The scion is pointed, inserted and tied as previously indicated. This type of graft is not covered by peat in the grafting bin and requires more care and watching during the first year out until it is well healed. It may need tying a second time and a little tree-seal applied.
Scions from the earlier blooming varieties naturally push their new growth earlier and, if your graft is going to "take," your plant (scion) will push earlier. Now if your understock is an early variety like decorum or fortunei, you will have a happy combination and a vigorous growing plant. The same principle applies to ponticum for midseason and discolor for late. In other words, the understock is activated for growth about the time the variety naturally makes new spring growth. If it is desired to space the grafting operation during the spring months, graft the early varieties about February 1st, midseason kinds about March 15, and the late ones about May lst. It requires 6 weeks to 2 months for the cut to callous, and by the above manner of timing, the plants do not lay in the grafting bin so long. For example, if you graft Tally-Ho in February it will not push its new growth until June, even though the graft is calloused, but lays in the bin 4 months.
June is also a good month for grafting rhododendrons by placing "green" on "green." New spring growth six weeks old or older may be grafted as above described on new growth of the same age on the understock. Set the scion deeply allowing no cut area on the scion exposed. The graft will usually heal in 30 days and may make one growth the same summer, or at least will start off exceptionally well the following spring.
Try to get good sturdy well-grown understock, as after the graft is made, the health and vigor of the plant depends on the root system under it. R. ponticum is generally recognized as the best all purpose understock, and the beginner may use it exclusively if obtainable. However other kinds are usable as heretofore indicated. I have used lots of discolor understock as it was easily obtainable at the time. It does not sucker as badly as ponticum and the wood stays green longer and therefore heals more rapidly. On account of it being a late variety, there is some discussion, though no conclusion, as to whether it retards the blooming date.
The grafts should be in the bin six weeks to two months. When the new growth is well started, take the plant out and bed it down in a shaded part of the greenhouse or lath-house. Leave all plants in the bin until new growth starts. However, after two months the cover can be gradually raised, making sure direct sun does not enter. Do not water until plant comes out of the bin. Raise the cover for ventilation and inspection for an hour or two a couple of times a week, evenings preferably. If moisture collects on foliage, raise cover slightly for ventilation more frequently. More grafts are lost from excess moisture or dampness than from dryness. Try to maintain a warm (70° average) moist atmospheric condition in the grafting bin. Extended periods of cold and wet, or hot and dry, will spoil your luck.
For lepidote rhododendrons, lepidote understock must be used. For the falconeris and grandes, understock of the same series should be used. For the maddeniis, R. ponticum may be used. Propagation of rhododendrons from seed and the rooting of cuttings is for the specialist and is not touched upon here.
For the amateur, I believe there is no more simple, sure and speedy way of propagating a few extra plants from his choice rhododendrons, than by grafting. The amateur gardener likes to make things grow, and next to seeing the plant in the glory of its bloom, the greatest fascination is in watching the plant he created start to grow and put out its new luxuriant foliage. The foliage of no hardy garden plant or shrub in the world is as varied and interestingly beautiful as the genus rhododendron, and the same might he said in quintuple measure of its blooms.