JARS v57n2 - The Life Cycle of a Delp Hybrid Rhododendron

The Life Cycle of a Delp Hybrid Rhododendron
Joyce Delp Harris
Harrisville, Pennsylvania

Weldon E. Delp (1920-1999) spent forty-seven productive years as a hybridizer and had a talent for creating beautiful, hardy rhododendron hybrids for the climate of western Pennsylvania. His passion for rhododendrons and skill in culture and propagation won him the Bronze Medal from the Great Lakes Chapter in 1979. In addition, he was awarded the Gold Medal by the American Rhododendron Society in 1986 for his intensive research, innovative techniques, thorough analysis, meticulous record keeping, advanced hybridizing and propagation. The following is an explanation of the Delp process for hybridization and accelerated propagation of rhododendrons.


The process of hybridization begins in the spring. The first step is to collect pollen from the plant chosen to be the pollen parent. This step is accomplished by removing the stamen with pollen from the parent plant and placing it into an empty pharmaceutical capsule or simply shaking off the pollen into the capsule. The capsule is then sealed with thin painter's masking tape and labeled. If the pollen will be used in the current season it may just be placed in a container with a moisture absorbent substance (calcium chloride) and refrigerated. (The calcium chloride must be protected from touching the capsules directly with a layer of cotton balls.) However, if the pollen needs to be preserved for later use, it should be frozen in the same container.

To prepare the seed parent for pollination, first the stamens are removed before the pistil matures and becomes sticky. This leaves the pistil available for hand pollination without the possibility of self-pollination. To hand pollinate, the desired pollen is placed on the pistil once it has become sticky. The transfer can occur in a variety of ways including sprinkling from the capsule, patting on with your finger, or the unusual but highly effective use of the body of a dead bee. The later simulates the natural process. The individual truss of flowers is then labeled with the hybrid cross using a flexible metal tag. The cross and date is also recorded.

Finally, when the seed heads have matured and turned brown, the seed capsules are collected. Then the seed is extracted by breaking the pods open into a strainer and shaking the contents onto a sheet of white paper, separating the seed from the other debris. Once clean, the seed is stored in a paper envelope and stored in a cool, dry location until you are ready to plant it. Note: Do not store seed in plastic bags.


The process of germination begins in January with the preparation of the following mix:


Combine and dissolve the following in 1 gal. of hot water:

3 Tbs. dolomitic lime (magnesium to aid seed germination and early growth)

3 Tbs. super phosphate (phosphorous to promote root development)

1 Tbs. diammonium phosphate (phosphorous to promote root development)

Combine and add the following:

1 – 5-gal. bucket peat moss screened through ¼" hardware cloth

1 – 5-gal. bucket perlite screened through ⅛" hardware cloth

This mixture may be stored for use throughout the season. Rhododendron seeds need no stratification or cold period, so they can be sown directly from the storage envelope. However, the date for sowing is determined by referring to the chart in the Farmer's Almanac for root crops. The day before the sowing date, the mixture is firmly placed into 4" square pots, scalding water is poured over the mix from a sprinkling can, and the mixture is allowed to set overnight before planting.

The following morning, the seed is sprinkled on top of the mix (do not cover seed) and pots are labeled on two opposing corners with 8" plastic stakes. Then the pot is placed inside a freezer bag, which covers the tags and is closed with a twist tie. This process creates an individual green house for germination.

These individual green houses are then placed in a location where they will receive 70°F bottom heat and an 18-hour day length, which can be simulated with a combination of warm and cool fluorescent lighting on a timing system.

As soon as germination has occurred, gradually give the seedlings more air by opening the bag. First just remove the twist tie and then continue to open a little more each day. After one week, completely remove the bag and mist the seedlings to keep the medium moist. When the first true leaves are formed, it is time to transplant the seedlings into the potting mix.


The process of initial growth and maturity takes place between February and June. It begins with the preparation of the following mix:


Combine the following in 2 gallons of water:

2 oz. Granular AquaGro (water retention)

2 Tbs. wettable sulfur (sulfur to promote growth and maturity and as a natural fungicide)

2 Tbs. gypsum (calcium to promote early root growth and new top growth)

2 Tbs. 18% rock phosphate (phosphorous to promote root, flower and seed development)

1 Tbs. Epsom salts (magnesium to promote early growth and uniform maturity)

1 Tbs. Peters Acid Fertilizer (21-7-7) (nitrogen to promote rapid growth and vigor, phosphorous for root development, and potassium to improve winter hardiness)

Add the following:

1 – 5-gal. bucket perlite (to provide aeration)

1 - 5-gal. bucket coarse peat moss (to provide aeration, fiber content and acidity)

1 – 5-gal. bucket Michigan peat (to provide organic nutrients)

1 – 5-gal. bucket aged pro base/composted pine bark (to provide density for good drainage, organic nutrients and disease resistance)

Seedlings are transplanted into this mix as soon as the first true leaves are formed. Originally the seedlings were transplanted into wood flats; however, the current method is to transplant them into individual 3.5" square containers. Labels and records include the date and number transplanted.

The seedling containers are then placed under lights to continue to produce 18-hour daylight with night temperatures of 70°F in order to induce rapid growth. This time the 18-hour daylight is provided by placing under 60-watt light bulbs spaced 30" apart and 20" from the seedlings. Note: Use regular light bulbs, not cool white, etc. The seedling pots remain in this environment until June 21st, the longest day of the year.

After June 21st, the seedlings are transplanted into 5.5" square pots and allowed to grow naturally, without extra heat or light, and allowed to enter normal dormancy in the fall in a cool greenhouse (below 50°F). This process accelerates the bud production and often produces blooms in 18 months rather than the traditional 3 to 5 years.


The evaluation process begins in the spring when the seedlings bloom for the first time. This can be from 18 months to 5 years. Of course, each year the seedling plants are transplanted into larger containers to promote additional growth and development until blooming occurs.

The seedlings are evaluated for flower color, truss size and shape, and leaf appearance. The selection is based on individual merit or use in additional hybridization. If selected, the seedling is named and colored according to the Royal Horticultural Society Colour Chart and recorded for future possibility of registration. Naming of plants chosen for additional hybridization allows for ease in labeling and record keeping.

At this stage of the process, the selected seedlings are either repotted or transplanted directly into the field where they are tested for plant and bud hardiness and growth pattern for 5 years. Those selected for individual merit are recorded for registration according to the ARS registration form including the specifications of the plant and flower (e.g., truss formation, leaf shape, size of plant in a specific number of years). At this point, the mature seedling may also be used for propagation.

Cutting Propagation

The propagation process begins in June and can proceed through December. The first step is the preparation of the following for the cutting rooting bed:


Dissolve 1 oz. Liquid AquaGro in 2 gallons of water and add:

2 – 5-gal. buckets medium peat moss

2 – 5-gal. buckets perlite

The mixture is placed in the rooting bed with 70°F bottom heat at a depth of approximately 4", and watered well to settle and firm the mixture.

The time frame for cutting propagation differs between lepidotes and elepidotes as well as among the different hybrids. In general, lepidote cuttings are ready in June and elepidotes from August through December, with those hybrids containing Rhododendron maximum or red flowering trusses being the last to be collected. Cuttings are ready for collection when the new growth is firm, no longer has a sticky stem and on the days designated in the Farmer's Almanac for root crops. In addition, cuttings are taken before 11 a.m. while the highest concentration of nutrients are in the new growth and are refrigerated until ready to prepare for the rooting bed.

To prepare the cuttings for the rooting bed, first remove all but three leaves. For lepidotes, an angled stem cut is made at 4" below the leaves and additional cuts are made down each side from approximately 2" above the bottom cut. For elepidotes, the leaves are trimmed in half to conserve space, an angled stem cut is made at 4" and additional cuts are made down each side from 2" above to 1" above the bottom cut forming a ledge that holds the rooting solution. Both types of cuttings are dipped into a rooting solution. The preferred solution is Woods and the strength is dependent upon the condition of the stem. Softer cuttings (June) receive a 1-20 dilution while the harder cuttings receive a 1-10 (August) or even 1-5 (December) dilution.

Next the cuttings are stuck into the rooting bed in rows allowing 1"-2" between both cuttings and rows. They are firmed in place and misted either through a continuous system or twice daily by hand throughout the rooting period. The general rooting time is 2-3 months, although some elepidote hybrids may take as much as 4 months to establish ample rooting. This method has produced a 75% to 95% success rate.

When the cuttings are rooted into fist sized balls, remove and pot into 5.5" square pots filled with the potting mix previously mentioned and place them under the same conditions of light and heat as mentioned under growth of seedlings. When the second set of new growth begins to form, stems are pinched out by hand about 1" above the site of new growth, leaving the stub to initiate growth of axial buds. This begins with the second set of new growth and is repeated every other growth as necessary to produce a well-branched plant. The plants are also fertilized monthly with a high phosphate fertilizer (15-45-5) to enhance root development and flower bud production. They are also maintained in this environment until June 21st, then transplanted into 1-gal. containers and allowed to go into a normal cycle of dormancy. This method has produced plants that bloom the following spring.

Joyce Delp Harris, a member of the Great Lakes Chapter, is the daughter of the late Weldon Delp. She is the owner of Crystalaire Naturescapes.