Effect of Previous Flood Exposure On Subsequent Flood Tolerance and Growth of Three Native Landscape Shrubs

Plants in rain gardens encounter repeated periods of flooding.  Research was conducted to determine if flooding pre-conditioning during production could affect subsequent flood tolerance outdoors.  Plants of <i>Ilex glabra</i> (L.) A. Gray. ‘Shamrock’ (inkberry holly), <i>Itea virginica</i> L. ‘Henry’s Garnet’ (sweetspire), and <i>Viburnum nudum</i> L. ‘Winterthur’ (possumhaw) were potted in 2.5 L (trade gal) pots in 5:3:1 pine bark:peat:perlite (PB) or sterilized field soil, placed in a greenhouse in Feb. 2009, and subjected to 0 (non-flooded), 3, or 6 days of flooding.  Following flooding, plants were allowed to drain for six days and received no irrigation.  Flooding followed by draining was repeated a total of 7 and 5 flood times for plants in the 3 and 6 day flood lengths, respectively.  Non-flooded plants were hand-watered as needed.  Flooding was discontinued in the greenhouse, and the plants were then planted outdoors in 171 L (45 gal) tubs.  Plants in PB in the greenhouse were planted in the same substrate outdoors.  Plants in soil in the greenhouse were planted in calcined clay outdoors.  Three plants (same taxon), one each that had been flooded for 0 (non-flooded), 3, or 6 days in the greenhouse were planted per tub.  Outdoor flooding treatments were the same as greenhouse flooding treatments and were randomly assigned to each tub.  Thus outdoor flooding treatments were in a factorial combination of greenhouse flooding length and outdoor flooding length.  Relative growth indices were calculated separately for the greenhouse (RGIg) and the outdoor (RGIo) portions of the study.  RGIo of <i>I. glabra</i> ‘Shamrock’ was similar among all flooding length combinations in both substrates suggesting tolerance of flooding at any stage regardless of preconditioning.  RGIo of <i>I. virginica</i> ‘Henry’s Garnet’ was higher in PB if plants had been previously flooded in the greenhouse but was not affected by any flooding treatments in CC.  RGIo of <i>V. nudum</i> ‘Winterthur’ was only affected by outdoor flooding length.  RGIo of <i>V. nudum</i> ‘Winterthur’ in PB was highest for plants that were non-flooded outdoors; in CC RGIo was highest in plants that were flooded for 6 days outdoors.  Flooding during production does not appear to be critical to flood tolerance for these taxa, particularly since all plants continued to grow throughout both phases of the experiment, and survival rate was 100%.  Visual quality of all plants was also high, suggesting that all three taxa would be suitable for use in rain gardens.