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Rhododendron (maximum)

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)

Family Scientific Name: Ericaceae
Family Common Name: Heath Family
Scientific Name: Rhododendron maximum
Common Name: Rosebay rhododendron
Species Code: RHOMAX
Ecotype: Cumberland Gap National Historical Park
General Distribution: Nova Scotia south to Ontario and Ohio, south to Georgia and Alabama. Found in mountain zones, moist or wet woods, often along creeks or in ravines.
Propagation Goal: plants
Propagation Method: seed
ProductType: Container (plug)
Time To Grow: 0
Target Specifications: Stock Type: Container shrub. Height: Projected approximately 18-24". Root System: May not entirely fill larger pots. Healthy, defined root ball.
Propagule Collection: Collected at Cumberland Gap National Historical Park, Tri-State Trail, near top of Chadwell Gap Trail by J. Englert on 10/94 and 10/96.
Propagule Processing: Seed Processing: By hand. Shake, rub or crush mature capsules and sieve material through a hand screen to separate seed. Seed is collected in October in the park.
Seeds/Kg: Uncounted; estimated at between 2,000,000 and 5,700,000 per pound.
Germination: Untested; a fraction of a gram (0.16) produced over 500 seedlings in 1998.
Purity: Undetermined.
Pre-Planting Treatments: Seed Treatments: Because of small size, .1-.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 maximum in the greenhouse has been problematic in the past for several reasons. The seedlings are extremely tiny and slow growing. Low winter light (in spite of extended day lighting) will stop the growth of many ericaceous seedlings from January-mid March. Lower temperatures, and over-saturation can result in deterioration of the peat-based media causing ammonia toxicity, algae, moss, liverwort and fungal growth, fungus gnat infestations, excessive drops in pH and subsequent nutrient deficiencies or toxicity. Growing seedlings through the two-leaf stage has been difficult, even with bottom heat and watering, and high intensity lighting. We have had about a 20%-30% survival rate. Luckily, a tremendous number of seeds germinated.
We are now (March, 2001) attempting to grow small-seeded ericaceous plants 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.

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 4 x 4 trays again, then, when of sufficient size, transplanted to 2" pots, quarts, 1/2 gallons and 1 gallon. The 1998 crop is presently in 1 gallon containers. Some of these will 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 obscure the tiny 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 excessively. Seedlings grow slowly and may remain in the same media for months.
Because 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 3 weeks.

Sowing/Planting Technique: 0.1 to 0.2 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 (8-12" above trays) until germination has occurred in about 3 weeks.

Surface of media must 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: Placed under high intensity lights from 4:30-10:30 p.m. and bottom watered when possible. Seedlings sown in January had 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 become necrotic and die. By mid March with longer daylengths, seedlings start to grow. Most were potted to quarts during the summer, 1998 and were overwintered in the greenhouse. They were potted to 1/2 gallons in 1999 and gallons in 2000.
Leaves of plants at all sizes frequently are discolored or have necrotic edges. This is probably cultural, as samples sent to a pathologist did not reveal disease.
Active Growth Phase: Rapid Growth Phase: Rosebay rhododendron does not appear to have a rapid growth phase. 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. During summer, 2000, gallon-sized containers were held in our shade house with overhead irrigation. They flushed beautifully in the spring but by late summer, leaf edges on many plants were necrotic and leaves were off-color. Suspect problems with the overhead irrigation and media deterioration.
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: Average for National Plant Materials Center propagated rosebay magnolia seems to be 3 years to well-established gallon size. Many of those started in 1998 and in gallons now will be bumped up to larger pot sizes this spring.

Storage Conditions: Smaller containers are overwintered in cold storage at 40186;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 Plant Materials Center cooler at 40§F and 35% relative humidity.

Seed dormancy: None known.
Length of Storage: <b>Storage Duration:</b> November - 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.


Davis, Kathy M.; Kujawski, Jennifer. 2001. Propagation protocol for production of Container (plug) Rhododendron maximum plants USDA NRCS - Norman A. Berg National Plant Materials Center Beltsville, Maryland. In: Native Plant Network. URL: http://NativePlantNetwork.org (accessed 2021/09/19). US Department of Agriculture, Forest Service, National Center for Reforestation, Nurseries, and Genetic Resources.

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