propagation techniques to overcome Propagation of Stewartia: Past Research the propagation problem. Various as- Endeavors and Current Status pects of propagation such as seed scarification, cutting type, rooting hormone, rooting medium, nutrition, Ajay Nair1,2 and Donglin Zhang cold acclimation, and micropropaga- tion have been specifically targeted and show promising results. Stewartia ADDITIONAL INDEX WORDS. rooting, overwintering, KIBA, stratification, semi-hardwood has been a plant of interest not only to the propagation experts, but also to SUMMARY. Stewartias (Stewartia spp.) are prized for their camellia (Camellia taxonomists and molecular biologists. japonica)-like flowers, intense fall color, and exfoliating bark. In spite of having Researchers have conducted genetic outstanding ornamental value and features, these plants are not readily available for linkage studies to track their center of landscaping in the horticulture trade. The primary reason stated is the difficulty of origin and also to establish relation- its mass propagation and production. In the last two decades, considerable research has been conducted on various aspects of stewartia propagation such as seed ship between native and introduced germination, cutting type, light, rooting medium, rooting hormone, cold species. With close to a dozen species acclimation, and tissue culture. In this article, we discuss factors that directly and cultivars in different hardiness influence propagation of stewartia and we highlight results of published studies to zones and appealing morphological propagate stewartia. The evidence indicates success in adventitious rooting of characteristics, stewartia certainly has cuttings but at the same time recognizes the continuing challenge associated with a remarkable potential of being a front overwinter survival. Sexual propagation has also been studied, but its commercial yard ornamental tree. This article application is limited. To date, there is lack of concrete information on why summarizes past research endeavors stewartia remains under-represented in our landscapes. It still remains unclear if it is undertaken to propagate stewartias the lack of consumer demand or existing propagation difficulties that is the cause of and also focuses on techniques and under utilization of stewartia. Given the information from most published studies, we suggest further research on the aspect of overwinter survival in addition to approaches to increase commercial a survey of the nursery and greenhouse industry to accurately determine the cause availability. behind the absence of stewartia in horticultural trade. Seed propagation One of the fundamental prob- here has been constant de- white camellia-like flowers and lems associated with sexual (seed) mand in the horticulture trade smooth mottled bark (Hohn, 1994). propagation in stewartia is that it is Tto produce plants that have The bark flakes in patches to reveal a slow and unreliable process mainly unique features such as short stature, buff, tan, cinnamon, and plum colors because of complex dormancy mech- prolific summer growth, intense fall during winter (DeWolf, 1969). With anisms in the seed (Curtis et al., color, winterhardiness, etc. Stewartia, their beautiful and appealing flowers, 1996). Irregular and infrequent seed- with its remarkable traits and features, magnificent exfoliating bark, spectac- ing habit, low seed viability, and short qualifies to be in that category. Stew- ular fall colors, and low insect and longevity impede large-scale nursery artia comprises of eight to 21 species, disease problems, stewartias are an all production. Seeds of stewartia require depending upon the author (Li, season high-demand landscape tree certain warm and cold stratification 1996; Yang, 1997). Stewartias are (Spongberg and Fordham, 1975). treatments to germinate successfully. distributed on both sides of the In spite of having such outstand- The standard nursery practice for pro- globe, mostly dwelling in the wood- ing ornamental values and features, duction of stewartia seedlings is to lands. Stewartia comprise both tem- stewartia is not readily available for sow seeds outdoors in the fall and to perate (deciduous) and tropical landscaping in the horticultural trade. allow 2 years for seedling emergence. species (evergreen). One of the most The primary reason stated is the lack Under normal conditions, seeds that important stewartia species in the of efficient propagation techniques fall on the ground germinate not the trade is japanese stewartia (Stewartia and the propagation challenges asso- following spring, but the one after pseudocamellia), which was introduced ciated with stewartias (Struve and (Hohn, 1994). To resolve this dor- into the United States in 1866 by Lagrimini, 1999). Much work has mancy problem, experiments have Thomas Hogg, Jr. to a private green- been conducted to overcome propa- been conducted on mechanical and house in New York (DeWolf, 1969). gation difficulties and to develop spe- chemical seed treatments to break Stewartia grows well in U.S. Depart- cific techniques, but the commercial seed dormancy and enhance germina- ment of Agriculture hardiness zones availability of stewartia is still low. tion percentages. Oleksak and Struve 5 to 8 (Dirr, 1998). Most of the ste- Researchers have, to a large extent, (1999) demonstrated that seed ger- wartias are small trees reaching 30 to focused on various sexual and asexual mination of japanese stewartia is 40 ft in height and are easily distin- guishable by their numerous creamy Units Department of Plant, Soil, and Environmental Sci- To convert U.S. to SI, To convert SI to U.S., ences, University of Maine, Orono, ME 04469 multiply by U.S. unit SI unit multiply by 1Current address: Department of Horticulture, Michigan 0.3048 ft m 3.2808 State University, East Lansing, MI 48824 1 ppm mgÁL–1 1 2Corresponding author. E-mail: [email protected]. (°F – 32) O 1.8 °F °C(1.8·C) + 32 • April 2010 20(2) 277 REVIEWS promoted by desiccation avoidance, 1982)], azalea [Rhododendron spp. carbohydrate storage, cold acclima- gibberellic acid (GA3) treatment, and (Smalley and Dirr, 1986)], and japa- tion, and overwinter storage temper- warm and cold stratification. If the nese stewartia (Wilson and Struve, ature. The following sections will look seeds are exposed to ambient room 2004). Stem cuttings of stewartia into each of these aspects individually. temperatures for 48 h or 80 °C drying typically root well and develop healthy conditions for 24 h, they lose viability root systems in their first season, but Type and characteristic completely (Oleksak and Struve, 1999). are prone to die during the following of cutting The study also indicated that expos- winter or shortly after the onset of The type of cutting chosen for ing seeds to a 24-h aerated water soak growth the following spring (Dirr and propagation significantly affects root- in 1 mM GA3 and 3 months warm Heuser, 1987; Wilson and Struve, ing and subsequent overwintering of stratification (25 °C) followed by 7 2004). The symptoms associated with cuttings (Hartmann et al., 2002). Soft- months cold stratification (4 °C) sig- overwinter mortality are diverse. Cut- wood cuttings ensure higher chances of nificantly increased seed germination. tings put forth frail new growth that obtaining post-rooting growth, which Final germination was 70% when withers off and, ultimately, the cut- will increase the production of photo- seeds were given a 3-d aerated water tings die off (Waxman, 1965). In synthates (Smalley et al., 1987). How- soak in 1 mM GA3 followed by 3 months some cases, rooted cuttings fail to ever, softwood is not always the warm, moist stratification (25 °C) and 4 break bud after winter dormancy preferred source of propagation, as months cold stratification (7 °C) (Struve (Curtis et al., 1996). softwood cuttings tend to wilt easily et al., 1999). Germination in this case Even though some success has and need special care and attention was not synchronous. Depending on been reported (Perkins and Bassuk, (Hartmann et al., 2002). the source of the mother plant, germi- 1995), the overwinter survival prob- Rooting ability and overwinter nation varied from 30% to 90% for seeds lem of stewartia has been largely un- survival is influenced by the type of givena3-daerated1mM GA3 soak resolved. To avoid this overwintering cutting in many genera such as camel- followed by alternating warm stratifica- problem in tall stewartia (Stewartia lia (Macdonald, 1974), sugar maple tion temperature (20/12 °Conan monadelpha), Dirr and Heuser (1987) (Donnelly and Yawney, 1972), witch- alternating 12-h cycle for 3 months) suggested layering as an alternative hazel [Hamamelis spp. (Dirr and before cold stratification for 10 method. Hohn (1994) cited the pos- Richards, 1989)], and oak [Quercus months. Cold stratification alone for sibility of grafting in stewartia, using spp. (Drew et al., 1993)]. For moun- 9 months, without prior warm strat- tall stewartia and japanese stewartia as tain camellia, cuttings taken early in ification, resulted in poor germina- understocks. He further suggested the season (softwood cuttings) are tion percentages (less than 1%). The that mountain camellia (Stewartia an excellent source of propagation optimum temperature for warm ovata) and chinese stewartia (Stewar- (Curtis et al., 1996). Softwood and moist stratification was 15 °C. tia rostrata) can be successfully lay- semihardwood stem cuttings of stew- Although success has been ered, but layering cannot become artia rooted when treated with