Exbury Azaleas - From Seed, Cuttings and Stools
Lawrence L. Carville
Rhode Island Nurseries Inc.
Newport, Rhode Island
Our approach to the propagation of Exbury azaleas this morning will explore both sexual and asexual means and during my presentation we will discuss the three most commonly accepted methods. I will not attempt to state any preference for the method of propagation you should use since each grower will tailor the method to meet his specific goals of production.
Sexual propagation by seed offers the grower a simple and inexpensive method of producing large numbers of siblings at an extremely low unit cost. One of the primary factors to be considered in seed production is the source of supply. If we choose to purchase seed, we should buy only from a reputable seed house where quality and viability may be relied upon. Many growers and amateurs prefer to harvest their own seed from stock blocks or outstanding plants in their gardens. Seed capsules should be picked in late September or early October, labeled and placed in open containers in a cool, dry place to permit gradual ripening. When the capsules begin to open, shake out the seeds, package and store until ready to sow.
Seed may be sown indoors on flats beginning in early January. Wooden or metal seed trays may be used for starting the seeds. We prefer to use a media of sterilized potting soil with a depth of 2½ inches which is firmed and watered thoroughly prior to seeding. Seeds are scattered over the surface of the soil and covered lightly with "No Damp Off" sphagnum moss. Flats are watered, placed on seed racks in the greenhouse where temperature is maintained at 65 degrees F. and allowed to germinate. Covering the seed flats with a light of glass or with individual polyethylene caps to maintain a constant humidity over the flat will shorten germination time and reduce losses due to drying out of the media.
When seeds begin to germinate, usually within 7 - 12 days, glass or caps are removed and the young seedlings are gradually allowed to harden off. Pricking off begins when two pairs of leaves appear on the seedlings. Since we desire to produce 4" - 6" transplants by early May, the seedlings are pricked off into flats or Jiffy Pots containing sterilized potting soil. Seedlings are grown on in the greenhouse until May at which time we then transplant them into outside frames. After two growing seasons, the plants are transplanted into the field in three foot rows. When blooming takes place in the third year, we tag by color and select those plants which have outstanding qualities Vegetative propagation may be utilized to increase plants which we feel are worthy of specific production.
Asexual propagation from cuttings may be carried out either in the winter from forced new growth or in early spring from naturally produced growth.
Winter. If facilities permit, we may elect to propagate during the winter season when labor is less likely to be involved in other pressing duties. Stock plants are lifted in September and moved inside the greenhouse where night temperature is maintained at 60 degrees F. Supplemental lights are turned on over the plants from dusk to dawn and intermittent light is supplied at 15 to 20 second intervals. Growth commences after three weeks and we may begin taking cuttings six weeks after plants have been moved into the greenhouse. Successive crops of cuttings may be taken throughout the winter and grown on in the greenhouse under lights until spring. Although cuttings produced from this method are rooted without the use of mist or hormone, we find that the additional handling of plants contribute to a higher unit cost than when rooted from spring cuttings.
Spring. We prefer to propagate during early spring and normally begin taking cuttings the last week of May, when the new growth is still green, semi-soft but not sticky and before apical buds are evident. Cuttings are collected early in the day from stock plants which have been well watered at least two days previously. All cuttings are dipped briefly in a solution, drained and stored in plastic bags in a cool dark area until we prepare them for sticking.
Lower foliage is stripped from the cuttings, the top growth is pinched and the cuttings are dipped in Hormex #8 plus 5% Benlate. No wounding is given the lighter wood but we do slice the heavier cuttings and some of the yellow hybrids which we find more reticent to root. When cuttings have been treated, they are immediately stuck under mist in a medium of 100% European peat which has been previously watered with Aqua-Gro or some other suitable wetting agent.
The duration of the mist cycle is not important as long as we insure that the leaf surface is never allowed to become dry during the first few critical sunny days. Mist should be operated daily during warm, bright days but the frequency can be shortened during overcast or cloudy days. After three to four weeks in the cutting bench, the mist cycle should be gradually shortened and at the end of seven or eight weeks we can safely discontinue the automatic misting. Daily syringing is recommended during this period when cuttings are rooted and are allowed to slowly harden off prior to lifting.
Bottom temperature in the greenhouse propagating benches is maintained at 60-70 degrees F and after six to eight weeks rooting is sufficient to permit lifting of the cuttings. The rooted plants may be flatted or containerized in a peat-perlite growing media and placed under supplemental light at a temperature of 60 degrees. Light is continued from early August until mid-September. Low intensity light of approximately 35 foot candles is used to extend the day length and initiate shoot elongation. By mid-September the plants may be moved to cool houses and hardened off prior to winter storage.
In the spring, the flatted liners may be planted out in beds for growing on for two seasons. Containerized liners may be shifted into one gallon cans in mid-April and grown on for two seasons.
If mildew should appear on the foliage during mid growing season, we have found the use of Benlate 50% (DuPont) to be extremely helpful. Plants may be sprayed every two weeks beginning in late June with 8 ounces Benlate per 100 gallons of water. Two or three applications are generally sufficient to control mildew regardless of the size of the nursery operation.
Perhaps one of the oldest and least understood methods of asexual propagation is that of stooling or layering. This technique has been perfected in the British Isles and Holland where climatic conditions are more favorable for its application than in this country. Most of the Exbury azaleas which were exported from England during the 1950's were three year old layers and it is from these plants that stock has been propagated in this country. Although the process of stooling is not prevalent in this country a few selected nurseries in unique locations do commercially propagate dwarfing malus understocks and some varieties of magnolias by utilizing this unusual method.
Of primary consideration in any discussion of stooling is the importance that must be given to the care of the stock beds. Success of the entire operation can be directly related to the health, vigor and maintenance of the mother plants. The stock beds should be located in an open area of the nursery which can be easily watered but in soil having optimum drainage properties. Stock plants should be spaced at least three feet apart and duplicate varieties should be planted to permit alternate year layering. When establishing a stock bed, it is imperative that only own root plants be used.
Prior to the actual layering operation we should thoroughly hand spade or cultivate around the plants, at the same time incorporating two parts peat moss, one part coarse sand and one part well rotted manure. The addition of the compost will insure that we have a rich, well drained media in which to establish the layers.
Slender branches of current season wood are selected for layering in late April. The branches are twisted slightly and pegged into the compost media. A slight incision beneath a vegetative bud in the area which is to be buried may shorten the time required for the layer to produce roots.
Frequent attention to the stock beds is also required during the first season when cultivation and watering are requisites to success. If the layer has developed sufficient roots by the following spring, we may separate it from the stock plant and line out in nursery beds in early May when new growth commences. When it is apparent that a new layer is lightly rooted, we prefer to allow another season in the stock bed before severing the layer. Certainly in this method of propagation, more than in any other, patience and perseverance are the watch words.
Under normal growing conditions, a healthy stock plant can be expected to produce fifty to seventy-five layers every two years.
The important factors which the grower must take into consideration when determining what method of propagation he will apply are: production goals; available facilities; labor; geographic location and market potential. Our techniques may be simple or complex but whatever our decision, we should realize that our ultimate goal is to produce more Exbury azaleas, both for the beautification of the landscape and for the personal gratification of the beholder.
1. Cutting dip solution—Wiltpruf, 1 part to 20 parts water.
Add to above solution:
2 TB per gallon Sevin 50% WP
2 TB per gallon Captan 50% WP
REFERENCES The Quarterly Bulletin of the American Rhododendron Society: No. 4 1963 - Radcliffe Page 242 No. 2 1968 - Carville Page 102 No. 2 1969 - Smith & Mossman Page 85 No. 1 1970 - McGuire & Bunce Page 45 Proceedings of the International Plant Propagators' Society: Vol. 15 - Crossley Pages 327-334 Vol. 17 - Comerford Pages 178-180 Vol. 17 - Vermeulen Pages 236-241 Vol. 17 - Carville Pages 255-258 American Nurseryman 5-1-66 - Leach Propagation of Trees, Shrubs and Conifers, Sheat - IacMillan - 1953 Rhododendron & Camellia Year Book - 1968.