Impact of Four Irrigation Treatments On Seven Shrub Species

In 2005, Colorado State University initiated a study to determine the impact of irrigation treatments on several common shrub species.    Irrigation treatments of 100%, 50%, 25%, and 0% were based on the evapotranspiration of Kentucky blue grass.  Four species considered “low water use” and three species considered “very low water use” were chosen for the study.  Low water use species included serviceberry (Amelanchier alnifolia), blue mist spirea (Caryopteris incana), Meyer lilac (Syringa meyeri ), and common lilac (Syringa vulgaris ).  Very low water use species included fernbush (Chamaebatiaria millefolium ), Russian sage (Perovskia atriplicifolia), and three leaf sumac (Rhus trilobata).  At the end of the 2007 growing season, none of the species showed signs of physiological stress associated with insufficient soil moisture.  In 2008, irrigation treatments were applied over a 15-week period; during this time 22.0, 11.0, 5.5, and 0.0 gallons of water were applied weekly to each shrub in the 100%, 50%, 25%, and 0% subplots, respectively.  Predawn leaf water potentials indicated that serviceberry in the 100% subplot were less water stressed than their counterparts in the 50%, 25%, and 0% subplots; irrigation treatments had no impact on the water relations in three leaf sumac; and common lilac in the 100%, 50%, and 25% subplots were less stressed than their counterparts in the 0% subplot.   Irrigation treatments had no effect on size of serviceberry, blue mist spirea, or common lilac; and treatments had no effect on visual appeal of serviceberry, Meyer lilac, or common lilac. Visual appeal of blue mist spirea decreased as irrigation increased.   Size of three leaf sumac increased as irrigation increased.  Meyer lilac shrubs in the 100% subplot were larger than their counterparts in the 0% subplot.  By the end of the 2008 growing season, none of the species displayed signs of physiological stress associated with insufficient soil moisture.  Leaves of serviceberry in the 100% subplot were larger than those in the 50%, 25%, and 0% subplots; leaves of three leaf sumac and common lilac in the 100%, 50%, and 25% subplots were larger than their counterparts in the 0% subplots; and irrigation treatments had no impact on the leaves of blue mist spirea.  Findings from this study indicate that although some irrigation may be necessary for the long-term health of established common lilac, established serviceberry, blue mist spirea, three leaf sumac, and Meyer lilac can survive with little or no irrigation.