Preemergence Herbicide Applications
to Native Deciduous Azaleas.
By: Dr. Anthony Lebude, N.C.S.U.
Preemergence herbicides have been used sparingly in production of native azaleas for a number of reasons. On the Snapshot label, both Rhododendron catawbiense ‘Roseum Elegans’ and Rhododendron carolinianum are listed as cultivar/species that are harmed by applications. However, Rhododendron calendulaceum is listed as unaffected when applications are made to field grown plants, and no other native southeastern deciduous azaleas are listed. Most seedling azaleas are grown in containers for some portion of production and many more than flame azalea have enjoyed popular success lately in the public’s renewed interest in native plants. Therefore, preemergence herbicides were applied to seedling flame azalea (Rhododendron calendulaceum), swamp azalea (R. viscosum), and coastal azalea (R. atlanticum) directly after potting in June 2008 and again in June 2009 (See table 1).
Table 1. Preemergence herbicides and rates applied to deciduous azaleas.
Treatment |
|
|
|
Name |
Conc. |
Rate |
Unit |
Non-Treated |
|
|
|
Ronstar G (oxadiazon) |
2 |
2 |
lb ai/a |
Ronstar G |
2 |
4 |
lb ai/a |
OH2 (oxyfluorfen and pendimethalin) |
3 |
3 |
lb ai/a |
Regal OO (oxyfluorfen and oxadiazon) |
3 |
3 |
lb ai/a |
Freehand (dimethenamid-P and pendimethalin) |
1.75 |
1.75 |
lb ai/a |
Freehand (dimethenamid-P and pendimethalin) |
1.75 |
3.5 |
lb ai/a |
Snapshot (trifluralin and isoxaben) |
2.5 |
2.5 |
lb ai/a |
Snapshot (trifluralin and isoxaben) |
2.5 |
5 |
lb ai/a |
Pendulum (pendimethalin) |
2 |
3 |
lb ai/a |
Pendulum (pendimethalin) |
2 |
6 |
lb ai/a |
In 2008, no foliar damage was noted on any of the azaleas. In 2009, some of the herbicides were applied to plants with wet foliage. As a result, within a day or a week, leaf speckling, burning and tip dieback occurred on plants treated with Ronstar, OH2 and Regal OO. Symptoms persisted for 4-6 weeks, but new growth was unaffected by herbicide application. Plants treated with Snapshot or Pendulum were unaffected.
Note: Carolina Native Nursery would certainly like to thank Dr. Lebude and Joe Conner, both from the N.C. Mountain Agricultural Research Center, for the work on this project, their unparalleled help and advice, and true friendship to all of us at Carolina Native. This will save us time and money. Thanks guys!