Effects of irrigation volume and frequency on shrub establishment in the landscape. 2009. Gilman, E.F., C.L. Wiese, M.Paz, A.L. Shober, S.M. Scheiber, K.A. Moore, M.M. Brennan. J Environ. Hort. In Press

Irrigation frequency and volume effects were evaluated on recently installed shrubs of three taxa, Ilex cornuta Lindl. & Paxt. 'Burfordii Nana', Pittosporum tobira Thunb. 'Variegata', and Viburnum odorotissimum Ker Gawl. Irrigation frequency and volume had no effect on Pittosporum at any time for any measured root or shoot parameters. Irrigation frequency and volume had no effect on Ilex and Viburnum canopy biomass, root biomass, root dry weight:canopy dry weight ratio, and stem water potential at any time after planting. Canopy growth was affected by irrigation treatment only for Viburnum plants installed in May 2004, and growth response to more frequent irrigation only occurred while plants were irrigated with no lasting impact on growth once irrigation ceased. Root spread and root spread:canopy spread ratio for only one shrub, Ilex, were influenced by irrigation treatment. Applying excessive irrigation volume (in this case 9L) reduced root dry weight:shoot dry weight ratio for Ilex and could increase the time needed for plants to grow enough roots to survive without irrigation. Our study found only slight influences in shrub growth from irrigation frequency and volume regardless of the time of year when data was collected. This indicates that these shrubs can be established with 3 liters irrigation applied every 4 days under the conditions of this study. Applying more volume or more frequently did not increase survival or growth. Canopy growth and plant quality data combined with past research suggest that establishment of these shrub species may be more influenced by environmental conditions such as rainfall than by the irrigation frequency and volume used in this test.