Impact of Four Irrigation Treatments On Four Shrub Species

In response to a severe regional drought that afflicted much of Colorado in 2002, Colorado State University initiated a study in 2005 to determine the impact of irrigation treatments on some common shrub species. Irrigation treatments were based on the evapotranspiration (ET) of Kentucky bluegrass (KBG).  Seven species considered “low water use” or “very low water use” were evaluated 2005-2008.  Results of that study were reported at the 2009 ASHS conference.   In 2008, four shrub species considered to be more water demanding than the previously studied species were planted:  Redosier dogwood (Cornus sericea ‘Isanti’), Annabelle hydrangea (Hydrangea arboresens 'Annabelle’), Diablo® ninebark (Physocarpus opulifolius ‘Diablo’), and blue arctic willow (Salix purpurea ‘Nana’).  The 2008-2009 study was comprised of an in-field component and a lysimeter component.  The in-field component had four treatments based on the ET of KBG, (0%, 25%, 50%, and 100%) and the lysimeter component had three treatments based on the ET of KBG (25%, 50%, and 100%).  All four species were planted in the in-field component, and the Redosier dogwood and Annabelle hydrangea were planted in the lysimeter component.  Data collection in both components included height and width, visual ratings, predawn leaf water potentials, and end of season leaf area and fresh/dry weights.  The in-field component also included infrared readings of the KBG and soil moisture readings.  The lysimeter component required daily weight measurements during dry down periods and periodic leaf gas exchange readings.  Even after applying treatments that were roughly 0, 0.15, 0.3, and 0.6 gallons per shrub per week for the 0%, 25%, 50% and 100% treatments, respectively, irrigation treatments had little to no impact on all 4 species in the in-field component.  Above normal precipitation (~25.4 cm [10 in]) and cooler temperatures (17.9°C [64.2°F]) during the growing season from May – September 2009 may have contributed to the lack of differences.  The results of the lysimeter component revealed that both Annabelle hydrangea and Redosier dogwood will use more water if provided with more water.  The potted plants that received 100% of ET lost more water on a daily basis compared to the plants that received 25%, and the plants that received 100% tended to be less stressed than the 25% plants.  However, the hydrangeas in the 50% and 100% treatments were equal in pressure chamber readings, suggesting that greater than 100% of ET of KBG may be required to improve growth and performance on this species.