Organic Weed Management Strategies for Apple Trees

Four weed management strategies were established in 2009 and continued into 2011:  1) periodic mowing and herbicide applied once in 2010 and 2011; 2) Herbicide 2, herbicide twice; 3) Herbicide 3, applied three times; 4) bark mulch applied once in 2009 with herbicide applied once in 2011; and 5) herbicide followed by bark mulch applied once in 2009.  Treatment 1 was intended to be an untreated control, but herbicide was accidentally applied once in 2010 and 2011.  The herbicide GreenMatch™, which contains the active ingredient d-limonene, was applied at the labeled rate in a one-meter band under the trees.  Bark mulch was also applied in a one-meter band.  The orchard was planted in 2007 with 'Honeycrisp' and 'Snowsweet' on M.26 rootstock. In 2010 and 2011, mulch resulted in the greatest duration of weed biomass reduction followed by herbicide applied three times.  Soil moisture tension was highly variable in this unirrigated orchard and fluctuated from 9 to 72 kPa in 2010 and was as high as 87 kPa in 2011.  Soil moisture tension was occasionally lower in mulched plots and with herbicide applied three times compared to other treatments.  Midday stem water potential did not differ between treatments, but was measured only 3–4 times each season.  Mulch increased foliar levels of potassium in 2010 and 2011, manganese in 2010, and zinc in 2011 compared to herbicide applications.  Foliar levels of nutrients were similar among the different herbicide treatments in both years.  Shoot growth was greater with mulch compared to herbicide in 2010, but not in 2011.  Weed management method did not affect the amount of bloom or yield.  Trunk cross-sectional area was greater with mulch compared to herbicide, but no difference occurred between the different herbicide treatments or between the mulch treatments.