Light quantity and timing are often variable in urban and suburban landscapes (Kjelgren, 1995). Buildings often cast shadows to the north (in the northern hemisphere) and east and west sides of buildings provide only morning or afternoon sun, respectively. Trees can also cast considerable shade. Based on these varying light patterns, there is demand for shade-adapted flowering plants in urban landscapes (Schwartz, 1997).
The native shrubs S. alba (meadowsweet) and S. tomentosa (hardhack or steeplebush) grow to be 1 to 1.5 m tall with long-lasting terminal inflorescences present through most of the summer. Spiraea alba produces white flowers and S. tomentosa produces pink flowers. They are native to eastern North America and Canada and can be found growing from Canada to the Gulf States (Smith, 2008; U.S. Department of Agriculture, 2007). Spiraea tomentosa was used in the landscape as early as 1736 (Symes, 1983). Today, however, both species are mainly used in habitat restoration. Many species within the genus Spiraea are valued for their form and flowering habits, and species not previously used in formal landscapes are being evaluated (Li and Zhang, 2008).
Light intensity can affect plant form, flowering, leaf size, and color in both herbaceous (Jeong et al., 2009; Vendrame et al., 2004) and woody species (Hampson et al., 1996). Shade-tolerant plants have both morphological and physiological adaptations that allow them to adapt to low-light conditions (Boardman, 1977). Phenotypic response to light can vary within a species, suggesting that selection may allow for development of cultivars with enhanced shade tolerance (Kitajima et al., 2006; Siemann and Rogers, 2001).
Spiraea alba and S. tomentosa are usually found growing in full sun and populations of these species are not maintained when shaded after forest regeneration (Darbyshire, 2003; Flaccus, 1959). This suggests both species are adapted to full sun. However, there are no published reports documenting the degree of shade tolerance of these species. Furthermore, response to light can vary among genotypes within a species (Jeong et al., 2009; Johnson and Cartwright, 2005). Therefore, we felt it was important to base general recommendations on evaluations using a wide range of germplasm rather than a single genotype. Growth and flowering of S. alba and S. tomentosa plants produced from seed of different geographical origin under several light regimens were evaluated to determine landscape potential of these plants under varied light conditions. Potential shade-adaptation mechanisms in both species were investigated.
Hampson, C.R., Azarenko, A.N. & Potter, J.R. 1996 Photosynthetic rate, flowering, and yield component alteration in hazelnut in response to different light environments J. Amer. Soc. Hort. Sci. 121 1103 1111
Kitajima, K., Fox, A.M., Sato, T. & Nagamatsu, D. 2006 Cultivar selection prior to introduction may increase invasiveness: Evidence from Ardisia crenata Biol. Invasions 8 1471 1482
Netto, A.T., Campostrini, E., de Oliveira, J.G. & Bressan-Smith, R.E. 2005 Photosynthetic pigments, nitrogen, chlorophyll a fluorescence and SPAD-502 readings in coffee leaves Sci. Hort. 104 199 209
Pounders, C.T., Blythe, E.K., Fare, D.C., Knox, G.W. & Sibley, J.L. 2010 Crapemyrtle genotype × environment interactions, and trait stability for plant height, leaf-out, and flowering HortScience 45 198 207
Reich, P.B., Tjoelker, M.G., Walters, M.B., Vanderklein, D. & Buschena, C. 1998 Close association of RGR, leaf and root morphology, seed mass and shade tolerance in seedlings of nine boreal tree species grown in high and low light Funct. Ecol. 12 327 338
Steingraeber, D.A., Kascht, L.J. & Franck, D.H. 1979 Variation of shoot morphology and bifurcation ratio in sugar maple (Acer saccharum) saplings Amer. J. Bot. 66 441 445
U.S. Department of Agriculture 2007 Spiraea L. The PLANTS Database National Plant Data Center Baton Rouge, LA 7 Nov. 2007 <http://plants.usda.gov/java/profile?symbol=SPIRA>.
Vendrame, W., Moore, K.K. & Broschat, T.K. 2004 Interaction of light intensity and controlled-release fertilization rate on growth and flowering of two New Guinea impatiens cultivars HortTechnology 14 491 495