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  • Author or Editor: Julie Guckenberger Price x
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The need for reliable planting techniques that encourage posttransplant root growth in adverse conditions has prompted research into planting above soil grade (above-grade). Container-grown Morella cerifera (L.) Small (syn. Myrica cerifera L.) (wax myrtle), Illicium floridanum Ellis (Florida anise tree), and Kalmia latifolia L. (mountain laurel) plants were planted in Horhizotrons (root observation chambers) in a greenhouse in Auburn, AL, on 1 Mar. 2006, 6 June 2006, and 3 Jan. 2007, respectively. The experiment was repeated with all three species being planted 18 June 2007. Horhizotrons contained four glass quadrants extending away from the root ball providing a nondestructive method for measuring root growth of the same plant into different rhizosphere conditions. Each quadrant was filled with a native sandy loam soil in the lower 10 cm. The upper 10 cm of the quadrants were filled randomly with: 1) milled pine bark (PB); 2) peat (P); 3) cotton gin compost (CGC); or 4) more native soil with no organic matter (NOM). Horizontal root lengths (HRL, length measured parallel to the ground from the root ball to the root tip) of the five longest roots visible along each side of a quadrant were measured weekly for M. cerifera and I. floridanum and biweekly for K. latifolia. These measurements represented lateral growth and penetration of roots into surrounding substrates on transplanting. When roots of a species neared the end of the quadrant, the experiment was ended for that species. M. cerifera had the fastest rate of lateral root growth followed by I. floridanum and then by K. latifolia. In most cases, roots grew initially into the organic matter rather than the soil when organic matter was present. In general, HRL and root dry weight (RDW) of I. floridanum and K. latifolia were greatest in PB and P, whereas for M. cerifera, these were greatest in P. Differences in root growth among substrates were not as pronounced for M. cerifera as for the other species, perhaps as a result of its rapid increase in HRL. Increased root growth in PB and P may be attributed to the ideal physical and chemical properties of these substrates. Results suggest that planting above soil grade with organic matter may increase posttransplant root growth compared with planting at grade with no organic matter.

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Planting shrubs above-grade with organic matter has shown potential for improving landscape establishment. To further investigate this technique, wax myrtle [Morella cerifera (syn. Myrica cerifera)] (3 gal) and mountain laurel (Kalmia latifolia ‘Olympic Wedding’) (5 gal) were planted on 30 Oct. 2006 (fall planting) and 12 Apr. 2007 (spring planting) in the ground in a shade house in Auburn, AL. At each planting date, plants of each species were assigned one of four treatments. Three of four treatments used a modified above-grade planting technique in which shrubs were planted such that the top 3 inches of the root ball remained above soil grade. Organic matter, either pine bark (PB), peat (PT), or cotton gin compost (CGC), was applied around the above-grade portion of the root ball, tapering down from the top of the root ball to the ground. In the fourth treatment, plants were planted at-grade with no organic matter (NOM). In general, both species had higher shoot dry weight (SDW) and root spread (RS) when planted in the fall than when planted in spring. Among all treatments, plants also typically had larger RS when planted above-grade with PB or PT. For easy-to-transplant species (such as wax myrtle) and especially for difficult-to-transplant species like mountain laurel, fall planting using this modified above-grade planting technique with PB or PT may improve post-transplant root growth and speed establishment in the first growing season.

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Green roofs are becoming increasingly prevalent in the United States due to their economical and environmental benefits as compared with conventional roofs. Plant selection for green roofs in the variable climate of the southeastern United States has not been well evaluated. Shallow substrates on green roofs provide less moderation of temperature and soil moisture than deeper soils in traditional landscapes, necessitating empirical evaluation in green roof environments to make informed recommendations for green roof plant selection. Nineteen species and cultivars, including succulents, grasses, and forbs, were evaluated under seasonal irrigated and non-irrigated conditions in experimental green roofs. Plants were planted on 26 Oct. 2009 and each evaluated for survival and increase in two-dimensional coverage of the substrate during establishment, after overwintering, and after the first growing season. The winter 2009–10 was colder than normal, and some plants, such as ice plants (Delosperma spp.), considered to be cold-hardy in this climate did not survive through the winter. Irrigation influenced survival for the summer period and only succulent plants like stonecrops (Sedum spp.) survived without irrigation. Irrigated experimental green roofs had significantly lower summer substrate temperatures (up to 20 °F lower) and plants survived in irrigated conditions. Plants that survived both winter and summer under irrigated conditions include pussytoes (Antennaria plantaginifolia), mouse-ear tickseed (Coreopsis auriculata), eastern bottlebrush grass (Elymus hystrix), glade cleft phlox (Phlox bifida stellaria), and eggleston's violet (Viola egglestonii). Irrigation is recommended on extensive green roofs to increase the palette for plant selection by protecting against plant mortality due to drought and extreme soil temperatures.

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