Preemergence herbicides, especially members of the dinitroaniline class, are commonly applied to container-grown nursery stock. Dinitroaniline herbicides inhibit the development of new roots in susceptible plants, and can injure nursery crops. The effect of pendimethalin on root and shoot growth of ‘Tradition’ azalea was evaluated at 4 and 8 weeks after treatment. Pendimethalin was applied to azalea tops (shoot exposure), the growing medium (root exposure), or to the tops plus growing medium. Initial plant heights and root volumes were used to calculate percent increase in growth as affected by application method. Four weeks after treatment, plant height of shoot-only and shoot plus root treatments increased by 17 and 25%, respectively, while height in root-only and untreated control plots increased by 34%. Eight weeks after treatment (WAT), plant height of shoot-only and shoot plus root treatment increased by 61 and 63%, respectively, compared to root-only and untreated control plots, which increased by 105 and 108%, respectively. Greatest reductions in root growth were found in the shoot plus root and root-only treatments at 4 and 8 WAT, likely due to pendimethalin movement into the upper 3 cm of the growing mix. Pendimethalin can directly affect azalea shoot growth following exposure to only the foliage, and directly reduce root development following application to the growing medium. Any stunting of growth following over-the-top application of a sprayable pendimethalin formulation likely results from impacts on both the root and shoot systems.

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Author notes

2Professor and Research Assistant, respectively.