Evaluation of the native shrubs Spiraea alba Du Roi and Spiraea tomentosa L. for landscape use

Little is known about the cultural requirements of the native shrubs S. alba and S. tomentosa. If more were known about their light, water, soil pH, and pruning requirements, better decisions could be made about how to care for these plants and where to use them in the landscape. Winter pruning could improve the form of S. alba and S. tomentosa and encourage their use in more formal gardens. Pruning reduced the size of the plants in the following growing season and resulted in improved form. Spiraea alba produced the most inflorescences when pruned to 15 cm and the least when pruned to 3 cm. Pruning did not affect flowering in S. tomentosa. Moderate pruning is an effective way of improving form in S. alba and S. tomentosa, but it may not be necessary in plants that already have a desirable form. Growth, flowering, and symmetry were greatest in full sun or moderate shade, and least in deep shade in both species. Both species acclimated to shading by increasing specific leaf area. Spiraea tomentosa leaves were more chlorotic with increased shading, whereas the leaves of some ecotypes of S. alba were darker green in some shade treatments. While these species can survive in the shade, they are best suited to being grown in full sun or moderate shade. Growth, total leaf area, transpiration, and water potential were lower under water deficit conditions in both species, and both species osmotically adjusted to the lower soil water potential. Only S. alba had a lower photosynthetic rate under water deficit conditions. Spiraea alba also had the lowest total water potentials. Both species responded to drought stress by losing their older leaves. Spiraea alba and S. tomentosa are naturally found growing in acid soils. In greenhouse studies with media adjusted to a pH range from 4 to 7, plants of both species were smaller and chlorotic at pH 7. This indicates a pH mediated nutrient deficiency at pH 7. Spiraea alba and S. tomentosa are suitable for use in full sun or moderate shade, and moist acidic soils