Does Propagation Method Impact Survival and Growth of Below Grade Planted Trees?

Rooted cuttings from four clonal taxa ranging from those that grow in dry well drained soils, Chilopsis linearis (Cav.) R. Sweet ‘White Storm’, to those that tolerate wet soils, Vitex agnus-castus L. ‘LeCompte’, and with intermediate soil requirements, Nerium oleander L. (unnamed large white flowered clone) and Lagerstroemia indica L. x Lagerstroemia fauriei B. Kohne ‘Basham’s Party Pink’, and seedlings from each of these same clones were propagated and growth concurrently in a container nursery under the same production conditions.  Factorial combinations of each taxa propagated as rooted cuttings and as seedlings were transplanted to a landscape site at four planting depths [7.6 cm (3 in.) above grade, at grade, 7.6 cm (3 in.) below grade, and 15.2 cm (6 in.) below grade] resulting in a 4 planting depth x 2 propagation methods x 4 species factorial in a randomized complete-block design with four blocks containing two plants of each treatment combination per block.  An unexpected rapid drop from growing temperatures to 18 °F (–8.8 °C) in Winter 2010–11 permitted assessment of cold injury in response to the planting depth treatments.  Propagation method interacted with planting depth and taxa where Lagerstroemia responses to cold damage were different among propagation method and planting depth, while Vitex and Chilopsis were not.  Exposure to low temperature had only minimal effects on the canopy survival of Lagerstroemia, Chilopsis, and Vitex, whereas Nerium canopy tissue survival was substantially reduced for plants planted above grade.  Planting at grade was the best treatment for most characteristics across species and propagation methods, but there were significant interactions involving both planting depth and propagation methods for some taxa.  For instance, the arid climate C. linearis improved survival with planting above grade, but planting below grade reduced the cross-sectional trunk area of C. linearis regardless of propagation method.  Growth indices of seedling propagated plants tended to be larger across species when planted at or above grade, but cutting propagated plants exhibited reduced reduction in growth relative to seedling trees compared to those planted at grade.  When evaluating the impacts of suboptimal planting techniques, data from this study supports the importance of knowing if the plants are cutting or seedling produced if it is a species that is sensitive to this effect.

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