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Kelly M. Stanton, Sally S. Weeks, Michael N. Dana and Michael V. Mickelbart

Two native shrubs, meadowsweet (Spiraea alba) and steeplebush (Spiraea tomentosa), are currently grown for use in habitat restoration. Pruning could improve their form and encourage use in more formal gardens. Two seedling populations of each species were grown in the field or in containers outdoors. In Mar. 2009, plants were pruned to 3 or 15 cm or were left unpruned. By midsummer, there was no effect of pruning on plant height in field-grown plants. However, pruning did eliminate lateral growth in field plants and, therefore, improved overall form of the plants. Meadowsweet pruned to 3 cm had about half as many inflorescences as did meadowsweet pruned to 15 cm, with unpruned plants producing an intermediate number of inflorescences. Pruning container plants to 3 cm resulted in plants that were about half the height of unpruned plants, and total biomass was greatly reduced. In general, plants that were pruned had fewer inflorescences, although the total number of inflorescences in all plants was small. Pruning resulted in lower quality plant form in container plants. Species and seed sources within species responded similarly to pruning. Based on the data collected in this study, newly planted meadowsweet and steeplebush should be pruned to achieve good form in the first year. Despite the sometimes leggy growth habit in containers, these species should not be pruned before growing out in the spring, or maximum growth and good form will not be attained.

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Kelly M. Stanton, Sally S. Weeks, Michael N. Dana and Michael V. Mickelbart

Two native shrubs, Spiraea alba (meadowsweet) and Spiraea tomentosa (hardhack or steeplebush), have potential as landscape plants, but little is known about light requirements for these species. The performance of plants from four geographical seed sources of each species was evaluated in the field under six different light treatments: full sun; morning full sun; afternoon full sun; and 40%, 60%, and 80% shade. Provenance differences did exist for height, flowering, and leaf greenness. Growth, flowering, and canopy density were greater in full sun and 40% shade and least in 80% shade. Both species responded to shade with increased individual leaf area and higher specific leaf area. Relative leaf greenness decreased with shade in S. tomentosa but did not change in S. alba. Plants grown in morning or afternoon shade were shorter and smaller and had fewer inflorescences than did the full-sun plants. These species can survive in deep shade, but based on growth and appearance, they are best suited to full sun or light shade in the landscape.

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Michael V. Mickelbart, Michael J. Gosney, James Camberato and Kelly M. Stanton

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.