Rhododendron (macrophyllum)

Lee Riley Horticulturist USDA FS - Dorena Genetic Resource Center 34963 Shoreview Road Cottage Grove, Oregon 97424 541-915-7324 541-767-5709 (fax) lee.riley@usda.gov

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Family Scientific Name:	Ericaceae
Family Common Name:	Heath family
Scientific Name:	Rhododendron macrophyllum
Common Name:	Pacific rhododendron
Species Code:	RHMA3
Ecotype:	Forest margin and understory, 1200 m, Mt Hood National Forest, Oregon
General Distribution:	Rhododendron macrophyllum occurs on the Pacific coast from British Coumbia to central California, and is widely distributed in the Coast and Cascade Ranges in fairly dry, open forests and forest edges.
Propagation Goal:	Plants
Propagation Method:	Seed
ProductType:	Container (plug)
Stock Type:	444 ml (27 in3) container
Time To Grow:	18 months
Target Specifications:	Stock Type: Container seedling Height: 26 cm Caliper: 10 mm Root System: Firm plug in container.
Propagule Collection:	Seeds are hand collected in late summer to early fall when capsules turn from green to brown and prior to capsule opening. Capsules are collected in paper bags and air-dried to allow for opening.
Propagule Processing:	Seed longevity is 5 to 7 years at 2 to 4 °C in sealed containers. Seed dormancy is classified as physiological dormancy. % Germination: 50 to 90%
Pre-Planting Treatments:	Seeds are placed into a 1% hydrogen peroxide soak for 24 hours, rinsed, and placed in water for an additional 24 hours. Seeds are placed into a 30-day cold, moist stratification (at 1 to 3 °C). Seeds can either be placed in fine mesh bags in a covered container (for Direct sowing method) or sown into trays filled with stabilized medium plugs (Q-plugs) and sealed in plastic bags (for Transplant method). Containers or trays are then placed in refrigeration at 1 to 3 °C. It is very important to rinse stratified seeds in mesh bags or moisten plugs weekly. If mold is evident, seeds should be treated with 1% hydrogen peroxide.
Growing Area Preparation/ Annual Practices for Perennial Crops:	Greenhouse growing facility. Direct Sowing: Seeds are directly sown into containers from mesh bags. Seeds are lightly covered with nursery grit. Transplant Method: Q-plugs are lightly covered with nursery grit. Seedlings are transplanted to target containers when primary leaves have formed (approximately 3 weeks following removal from stratification). Growing medium used is 40:20:20:20 peat:composted fir bark:perlite:pumice with Apex controlled release fertilizer (16N:5P2O5:10K2O with minors; 6 to 7 month release rate at 21C) at the rate of 2 gram Apex per 444 ml container. Seedlings remain in the greenhouse for 18 months, and are then moved to an outdoor growing area to induce dormancy. Average growing season of nursery is from mid-March until early October.
Establishment Phase:	Germination is fairly uniform and is usually complete in 3-4 weeks. True leaves appear 3-4 weeks after germination.
Length of Establishment Phase:	3 to 4 weeks
Active Growth Phase:	Seedlings are very slow growing in the first year, but fairly rapid growth occurs during the second growing season. Following germination, plants are fertilized with soluble 12-2-14-6Ca-3Mg at 75 to 100 ppm for 3 weeks. During the growing season, fertilization depends on weather and physiological needs. Soluble 20-9-20 NPK, 20-18-18 NPK, or 17-5-24 NPK at a range of 100 to 150 ppm is applied weekly throughout the first growing season. In the spring on the second growing season, seedlings are hand-fertilized with Apex controlled release fertilizer (16N:5P2O5:10K2O; 6 to 7 month release rate at 21C) at the rate of 2 gram Apex per 444 ml conetainer. Throughout the remainder of the growing season, seedlings are fertilized weekly with soluble 20-9-20 NPK or 20-18-18 NPK at a rate of 150 ppm.
Length of Active Growth Phase:	18 weeks
Hardening Phase:	No dry-down is done to induce dormancy. Seedlings are moved to an outdoor growing area in mid-September.
Length of Hardening Phase:	3 to 4 weeks
Harvesting, Storage and Shipping:	Total Time To Harvest: 19 months Harvest Date: Late October Storage Conditions: Seedlings are usually outplanted in fall to early winter. No storage except in outdoor growing area. Plants are well irrigated prior to shipping and shipped in containers.
Other Comments:	Rhododendrons are very susceptible to root weevils. It is recommended to treat plants with beneficial nematodes several times, particularly during the second growing season.
References:	Hitchock CL, Cronquist A. 1973. Flora of the Pacific Northwest. Seattle (WA): University of Washington Press. 730 p. Bonner FT, Karrfalt RB, Nisley RG, editors. 2008. The woody plant seed manual. Washington (DC): USDA Forest Service. Agriculture Handbook 727. 1223 p. Young JA, Young CG. 1992. Seeds of woody plants in North America. Portland (OR): Dioscorides Press. 407 p. Dorena Genetic Resource Center Propagation Records, unpublished.

Citation:

Riley, Lee E.. 2018. Propagation protocol for production of Container (plug) Rhododendron macrophyllum Plants 444 ml (27 in3) container; USDA FS - Dorena Genetic Resource Center Cottage Grove, Oregon. In: Native Plant Network. URL: https://NativePlantNetwork.org (accessed 2025/04/20). US Department of Agriculture, Forest Service, National Center for Reforestation, Nurseries, and Genetic Resources.