Budded Grafts as an Important Contribution
To Rhododendron Hybridizing
By David G. Leach, Brookville, Pa.
In 1952 Mr. H. L. Larson, a pioneer rhododendron nurseryman of Tacoma, Washington, wrote to me that he intended to send to me an assortment of small grafts bearing floral buds for experimental purposes in connection with my hybridizing projects.
At the time I was greatly interested but not particularly optimistic about the practical usefulness of these grafts. After they were received in the spring and the experiments had been concluded a few months later I was firmly of the opinion that Mr. Larson had made an important contribution toward the success of hybridists everywhere.
The grafts had been made the previous August by Mr. Larson on ponticum understock of the current year's growth. The understocks were cut off just above the top pair of leaves. The stems were then split down through their centers a distance of one inch and wedge-shaped scions bearing floral buds were inserted into the split and bound with rubber grafting strips. No wax or paraffin was used to cover the wounds.
The grafted plants in their pots were then placed on open greenhouse benches and the temperature of the greenhouse was kept as cool as possible by shading and by sprinkling the floor with water. The greenhouse was not equipped for automatic humidification. Mr. Larson says the shading should be of a degree that the shadow of the hand can be seen, or slightly more shade than is required for orchids. He found that the grafts knit in two or three weeks with the floral buds remaining in good condition.
In previous experiments with winter grafting Mr. Larson had determined the the excessive humidity of the enclosed grafting case which must be used at that season of the year caused the floral buds to mould and drop off.
I would like to insert here my own suggestion that amateurs without greenhouse facilities can be very successful with summer grafts if they will spray the plants immediately after grafting with one of the Vinyl resin preparations such as "Wiltpruf" which is sold by Rosedale Nurseries, Eastview, New York. This material coats the leaves and stems with a very thin plastic film which so reduces transpiration that the water loss from the scion is negligible and the little grafted plants can be handled outdoors in the shade in much the same manner as seedlings. The plastic coating on the plant gradually erodes with ordinary rainfall and at the end of several months it has disappeared, by which time the grafts have long since been firmly knit. The Vinyl resin spray may also have an important commercial application for professional nurserymen since it eliminates the need for expensive greenhouse space and the labor of shading and maintaining humidity.
In my own experiments, using R. maximum as an understock, the use of the spray increased the percentage of success enormously. In fact, failures became entirely a matter of mechanical error in aligning the scions with the understocks.
Returning now to the grafts as received from Mr. Larson: most of them were plunged in their pots in an open frame, half shaded, which had been filled with sawdust. Two were placed in cold storage at 34° F. to retard flowering so that they could be used as seed parents with pollen that was not available until later in the season.
When all of these grafts subsequently bloomed the flowers were undersize but normal in color and in the development of their floral parts. Each graft, of course, bore one terminal truss of blossoms.
Just before they came into bloom the plants were brought into the house for pollination. Various numbers of florets were removed from the trusses and the remaining florets were pollinated to determine the maximum number of seed capsules that the little grafts could support. The day after making the crosses the plants were returned to their outdoor frames.
The results were successful far beyond my expectations. By September it was apparent that not more than two or three florets should be pollinated in each truss. So restricted, the grafts bore capsules of exceptional size which yielded an abundant supply of seeds due in part, no doubt, to the pollination having taken place indoors under ideal conditions and protected from the vagaries of outdoor weather conditions.
The developing seed capsules on the little plants close to ground level proved to be an attraction to rodents, presumably chipmunks, and it was necessary to protect the grafts with a cage made of hardware cloth with a quarter-inch mesh.
I believe the development of grafts with floral buds is an important contribution by Mr. Larson to hybridists everywhere. Budded scions of new originations can be imported from England to serve as seed parents here years before mature plants of blooming age would be available.
In cases where it is desired to use the pollen from the blossoms of the grafts on plants which would normally bloom too early to make such crosses possible, it is a simple matter to force the little grafts into bloom prematurely either in a greenhouse or in a residence.
When the graft is wanted as a seed parent for pollen of late-blooming plants it is equally simple to defer its normal blooming period by holding it in cold storage until the desired time arrives. For my climate in Northwestern Pennsylvania, plants which are wanted in bloom in early June are removed from cold storage 31 days before the desired blooming date.
Other advantages in using grafts for breeding are the savings of time, labor, expense and cost of transportation as compared with plants of blooming age. The convenience of a graft in a four inch pot as against a mature shrub balled and burlapped is very great indeed.
To Mr. H. L. Larson, then, who originated and developed this idea, go my personal appreciation and gratitude, and I believe other hybridists will be equally indebted to his ingenuity in the future.