Rhododendron (calendulaceum)

John M. Englert USDA NRCS - Norman A. Berg National Plant Materials Center Bldg. 509, BARC - East, E. Beaver Dam Road Beltsville, Maryland 20705 (301) 504-8175 (301) 504-8741 (fax) john.englert@wdc.usda.gov http://plant-materials.nrcs.usda.gov/mdpmc/

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Family Scientific Name:	Ericaceae
Family Common Name:	Heath Family
Scientific Name:	Rhododendron calendulaceum
Common Name:	Flame azalea
Species Code:	RHOCAL
Ecotype:	Great Smoky Mountains National Park, Cumberland Gap National Historical Park
General Distribution:	Pennsylvania south to Ohio, Georgia and Alabama. Found in woodlands, old fields or pastures.
Propagation Goal:	plants
Propagation Method:	seed
ProductType:	Container (plug)
Time To Grow:	0
Target Specifications:	Stock Type: Deciduous container shrub. Height: Probably will be around 18" at outplanting. Root System: May not entirely fill larger pots. Healthy, defined root ball.
Propagule Collection:	Collected at Great Smoky Mountains National Park, Foothills Parkway, Wears Valley by National Park Service staff on 11/7/95.
Propagule Processing:	Seed Processing: By hand. Shake, rub or crush mature capsules and sieve material through a hand screen to separate seed. Seed was collected in November in the park. Seeds/Kg: Uncounted; probably several million per pound. Germination: Untested. Purity: Undetermined.
Pre-Planting Treatments:	Seed Treatments: Because of small size, 0.1 - 0.2 gram of seed is mixed with between 4 and 8 grams of talc and sifted over the surface of moist media. Sand was tried but didn't mix as well as talc.
Growing Area Preparation/ Annual Practices for Perennial Crops:	Propagation Environment: (Also see Kalmia latifolia) Production of Rhododendron calendulaceum in the greenhouse has been somewhat problematic in the past for several reasons. The seedlings are tiny and slow growing. Low winter light (in spite of extended day lighting) may slow growth from January - mid March. The peat-based media can become over saturated or too acidic causing ammonia toxicity, nutrient deficiencies, fungus gnat infestations and buildup of algae, moss, liverwort and fungus. Growing seedlings through the two-leaf stage has been difficult, even with bottom heat, careful watering, andhigh intensity lighting. We have had about a 40% survival rate with this species past the 2-leaf stage. However, flame azalea is still easier to propagate than Kalmia and the evergreen rhododendrons. We are now (March, 2001) attempting to grow small-seeded ericaceous plants (flame azalea included) in a propagation room rather than in the greenhouse, under fluorescent lighting, with hand-misting, periodic bottom watering and regulated day/night temperatures to avoid the climate extremes of the greenhouse. A huge number of seedlings can be germinated in a very small space with less inputs and better controls this way. To date, seedlings are growing fairly rapidly and have doubled in size in two weeks. Seed Propagation Method: Hand-sown over media. Container Type and Volume: Seed is sown into 4x4 trays which are placed in solid bottom 10 x 20 tray (to allow bottom watering.) Two-leaf seedlings are pricked off and spaced in new 4 x 4 trays with new media. When large enough (in approximately 4-5 months given good conditions) the tiny seedlings are transplanted to 2" pots. In subsequent years they are bumped up to quarts, 1/2 gallons and 1 gallon. The 1998 crop is presently in 1 gallon containers. Some of these may go up to 1.5 or two-gallon pots in spring 2001. Growing Media: Germinating mix: 4:2:1 mix of screened horticultural peat, sand and perlite. (Too much perlite will float to surface and may obscure the germinating seedlings.) Recommended pH of media is around 5.0. We have added dolomitic lime at about 40 grams per cubic foot mix to prevent excessive drops in pH. We have not added slow release fertilizer or micronutrients because of problems with toxicity should pH drop excessivelyBecause media deteriorates over time, we have found that periodic careful transplanting of tiny seedling clumps to new media appears to rejuvenate the seedlings. Transplant mix for seedlings: We currently use a transplant mix for seedlings going into 2" pots of 1:1 Sunshine #1 to peat. Transplant mix for quarts and up: 1:1:1 mix of Sunshine #1:peat:pinebark with supplemental soluble fertilizer for acid-loving plants.
Establishment Phase:	Sowing Date:Best results for first season establishment and growth were obtained by sowing seed in November and nursing the seedlings through the slow-growth phase which occurred from germination until March (about 3-1/2 months). % Emergence and Date: Seeds sown from November - January germinated well in about 2- 3 weeks. Sowing/Planting Technique: A fraction of a gram of seed is thoroughly mixed with 4 to 8 grams of talc and sprinkled over the surface of finely screened horticultural peat:sand:perlite mix in trays which have been bottom watered and sprayed with a fungicide. Seeds will remain on the surface of the media if it is very lightly compressed prior to sowing. Trays are set in solid-bottomed carrying trays (to allow bottom watering), on heating pads to keep media at about 72§F, and given continuous (24 hour) fluorescent lighting (4-8" above trays) during germination and early seedling growth. Surface of media should not dry out, crust over or become too hot or too cold during the germination period. Plastic covers on trays will help keep humidity high but shading from direct sunlight is necessary to prevent media from drying out or reaching excessive temperatures. Establishment Phase: Established seedlings were placed under high intensity lights set to give 14-16 hour days. Seedlings sown in January finally reached the 2-4 leaf stage and were transplanted in early April to 2" pots. Most mortality occurs during this slow-growth, 2-leaf period when seedlings can becomenecrotic and die. By mid March with longer day lengths, seedlings start to grow. Most were potted to quarts in September, 1998, to 1/2 gallons in 1999 and gallons in 2000. During July, 1998, some plants in the greenhouse were exposed to excessive temperatures which destroyed the growing terminals and caused witches broom effect. By September they had grown out of it and produced new terminals.
Active Growth Phase:	Rapid Growth Phase: Flame azalea is not as slow growing as Kalmia or evergreen rhododendrons. Seedlings that spent the summer outdoors in a shaded location grew the best. Problems occur if plants stay in one batch of peat-enriched media too long.
Hardening Phase:	Hardening Phase: Rhododendron appears to need a period of winter chilling to sustain new spring growth. Plants overwintered in the greenhouse did not flush in spring like plants over-wintered in a cooler.
Harvesting, Storage and Shipping:	Total Time to Harvest: The average for National Plant Materials Center -propagated flame azaleas seems to be 3 years to well-established gallon size. Storage Conditions: Smaller containers are overwintered in cold storage at 40§F. During winter, 2001, 1/2 gallon and gallon sized plants were overwintered under microfoam outdoors. Seed storage: Stored in plastic containers or paper collection bags in the National Plants Materials Center cooler at 40§F and 35% relative humidity. Seed dormancy: None known.
Length of Storage:	<b>Storage Duration: </b> November to late March.
References:	Woody Plants of Maryland, Brown and Brown, Port City Press, Inc., 1992. Manual of Vascular Plants, Gleason and Cronquist, D. Van Nostrand Co., 1963. Richard A. Jaynes. Kalmia, Mountain Laurel and Related Species. Timber Press, 1997.

Citation:

Davis, Kathy M.; Kujawski, Jennifer. 2001. Propagation protocol for production of Container (plug) Rhododendron calendulaceum plants USDA NRCS - Norman A. Berg National Plant Materials Center Beltsville, Maryland. In: Native Plant Network. URL: https://NativePlantNetwork.org (accessed 2025/02/24). US Department of Agriculture, Forest Service, National Center for Reforestation, Nurseries, and Genetic Resources.