Effect of Repeated Short Interval Flooding Events On Growth of Five Native Shrub Taxa

Plants in rain gardens must be able to tolerate repeated short intervals of flooding.  Research was conducted to screen five native landscape shrub taxa for tolerance to repeated flooding events.  On 13 Aug. 2009, thirty 3.8 L (1 gal) plants each of <i>Fothergilla</i> x <i>intermedia</i> L. ‘Mount Airy’ (dwarf witchalder), <i>Ilex verticillata</i> L. A. Gray ‘Winter Red’ (winterberry), <i>Clethra alnifolia</i> L. ‘Ruby Spice’ (summersweet), <i>Callicarpa dichotoma</i> Lour. K. Koch ‘Early Amethyst’ (purple beautyberry), and <i>V. nudum</i> L. ‘Bulk’ (BRANDYWINE possumhaw) were removed from their containers and potted into 11.3 L (3 gal) containers in 5:3:1 pine bark:peat:perlite substrate and placed in a greenhouse in Auburn, Ala.  Beginning on 28 Aug. 2009, plants were flooded to the substrate surface for 0 (non-flooded), 3, or 6 days (flood length).  Following the flooding period, plants were allowed to drain for 6 days.  During the draining period, no water was added to containers.  The flood-drain process was repeated for 6 weeks.  Non-flooded plants were hand watered as needed.  The experiment was terminated on 16 Oct. 2009.  Flooding length had no effect on growth index (GI), root dry weight (RDW), or shoot dry weight (SDW) of <i>I. verticillata</i> and <i>V. nudum</i> ‘Bulk’.  Flooding length did not affect RDW and SDW for <i>C. alnifolia</i> ‘Ruby Spice’, however GI was similar and higher in plants flooded for 0 and 3 days than in plants flooded for 6 days.  GI and RDW of <i>C. dichotoma</i> ‘Early Amethyst’ were not affected by flooding treatments, however SDW decreased with increasing flood length.  GI of <i>F.</i> x <i>intermedia</i> ‘Mount Airy’ decreased with increasing flood length, and RDW and SDW were higher in plants flooded for 0 or 3 days than in plants flooded for 6 days.  All taxa, with the exception of <i>F.</i> x <i>intermedia</i> ‘Mount Airy’, maintained good visual quality, had no reduction in root dry weight, and exhibited minimal effects of flooding on shoot growth.  <i>F.</i> x <i>intermedia</i> ‘Mount Airy’ exhibited poor visual quality, and its growth was adversely affected by flooding, suggesting this taxon would not be a good choice for use in a rain garden.  Conversely, all other taxa appeared tolerant of flooding and would be appropriate native shrub selections for rain gardens.