Soil pH Effects on Growth and Foliar Nutrient Concentrations of Spiraea alba Du Roi and Spiraea tomentosa L.

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The North American native plants Spiraea alba and S. tomentosa have potential as landscape plants because they are small- to medium-sized shrubs with showy flowers that persist from early to late summer. However, the cultural requirements of these species are not well documented. Both species grow in slightly acidic soils in their native habitats. The objective of this work was to determine if the growth and/or appearance of these shrubs is affected by the neutral to slightly alkaline soils common throughout the Midwest. Spiraea alba grown from three seed sources and S. tomentosa grown from a single seed source were grown for 2 years in soils of different pH. Elemental sulfur (S) was incorporated into a Drummer silty clay loam with a pH of 7.2 to bring the pH to 5.8 and 6.4 2 years after incorporation, thus establishing plots with three pH levels for assessing growth and appearance. The soil pH increased to 6.4 and 6.8 in the amended plots between Years 2 and 3 after S incorporation. Soil concentrations of exchangeable magnesium (Mg) and calcium (Ca) were lowered with added S. Height and width varied among S. alba from different seed sources, and height and width of all plants were reduced when grown at the highest soil pH. Leaf greenness, specific leaf weight, and individual leaf area were unaffected by pH. Leaf concentrations of nitrogen, potassium, zinc, and manganese were higher in lower soil pH, whereas Mg was lowest at the lowest soil pH. These results suggest that S. alba and S. tomentosa can be grown in neutral pH soils without an effect on plant appearance, but plant size will be less than at pH levels closer to the native soils for these species. Although some micronutrients were present at lower concentrations at the neutral pH, leaf greenness was unaffected, again suggesting that these plants may perform suitably outside of their native habitat pH range.