Preemergence Herbicides for Container Nurseries – old, new, and bright yellow too.

Joseph C. Neal

Department of Horticultural Science

NC State University

Container nursery crop producers typically make between three and six applications of preemergence herbicides each year.  Yet, weeds continue to emerge and establish in containers, necessitating frequent hand weeding.  Selecting the most appropriate herbicide(s) for the site may reduce the time spent hand weeding and save money.  Fortunately, for most woody nursery crop producers, broad spectrum herbicides are available.  Standard nursery herbicides such as Scotts OH2 and Rout will control most nursery weeds and are safe on most woody nursery crops.  However, these herbicides cannot be utilized in all situations.  Injury to plants that can trap the granules is common.  Also, as reported in the 2004 nursery short course, new label changes will limit the total number of applications of herbicides such as these, containing oxyfluorfen as an active ingredient, to 3 applications per year.  Furthermore, these herbicides have not provided adequate control of eclipta or doveweed, and their efficacy on other newly introduced weeds has not been determined. 

The most commonly used preemergence herbicides control a broad spectrum of weeds and are safe on most woody plants.  In Table 1 and Table 2 are a few of the most commonly used preemergence herbicides.  Preemergence herbicides must be applied to the surface of the soil and incorporated by rain or irrigation into the zone of weed seed germination.  Consequently, preemergence herbicides must be applied before weeds germinate.  Apply herbicides as soon as possible after potting.

A prime factor in herbicide selection is safety to the ornamental crops.  Herbicides containing oxyfluorfen or flumioxazin can burn tender foliage of ornamentals.  If the crop has leaf architecture that catches herbicide granules (such as hosta or daylily), severe injury can result.  One alternative is to use a herbicide that contains isoxaben.  The herbicide formulation used can also impact crop safety, particularly with herbaceous crops.  Liquid applications of Surlan, Pendulum, Barricade, or Gallery applied soon after bud break have been shown to stunt the growth of several woody nursery crop species by as much as 20% and severely damage herbaceous ornamentals.  However, when applied in a granular formulation these herbicides are much safer to woody and herbaceous crops.  Table 1 lists the most common preemergence herbicides used in container nursery crops and general comments on efficacy and crop safety. 

Choosing the most appropriate herbicide for your nursery requires some knowledge of the weed species and the relative effectiveness of preemergence herbicides on these species.  Table 2 compares the efficacy of preemergence herbicides on a few common nursery weeds. Each preemergence herbicide has a fairly broad spectrum of weeds controlled; however, no herbicide controls all weeds.

Table 1.  Preemergence herbicides commonly used in container nurseries.

Table 2.  Relative effectiveness of nursery herbicides on some common weeds.

Regardless of which herbicide is selected, in order to achieve the maximum level of weed control the herbicide must be applied uniformly at the recommended dose before weeds germinate.  Easier said than done!  When we measured preemergence herbicide applications in container nurseries we discovered up to 250% variation, from less than one-half to twice the target dose, within a small production block.  Application uniformity can be improved by calibrating spreaders to apply one-half the labeled dose, then treat the beds twice moving in different directions.  Remember, the herbicide cannot control weeds if it is not applied as uniformly as possible.  When making applications to individual pots, the herbicide must be spread over the surface of the potting substrate as uniformly as possible.  Irrigate immediately after application to wash the herbicide off the foliage and to incorporate the herbicide into the surface of the potting substrate or soil where weeds will germinate.   In North Carolina premergence herbicides provide only between 6 and 10 weeks of residual weed control, depending on the weed species and herbicide.  After which time, the pots must be hand weeded and preemergence herbicides reapplied.  Frequent hand weeding between herbicide applications will reduce secondary weed populations and reduce overall weed management costs.

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