Best Practices for Managing Weeds in South Carolina Fall Broccoli Production

Growing Broccoli in the Southeast

Broccoli (Brassica oleracea L., Group Italica) has emerged as a prominent vegetable in the last fifty years with per capita consumption increasing dramatically from less than one pound per person per year in 1965 to almost nine pounds per person per year as of 2014.¹ The US broccoli crop was almost exclusively supplied by western producers in California and Arizona from the late 1960s through the early 2000s, with the two states accounting for 90% or more of the crop during this timeframe.² A recent noticeable increase in acres dedicated to broccoli on the east coast of the United States can be attributed to the increased popularity of the crop, interest by consumers in purchasing and eating a relatively local or regional crop, and the lower costs associated with shipping the vegetable shorter distances along the eastern seaboard.³

In South Carolina, warmer temperatures from late summer into mid-fall provide ample opportunity for young broccoli to rapidly put on mass prior to the arrival of cooler temperatures in late fall to mid-winter.⁴ In order to capture the market window corresponding to peak economic returns during the holiday seasons, growers typically have multiple plantings as early as mid-August in upstate South Carolina to early September in coastal South Carolina, which continue about every ten days lasting into early October. Earlier plantings typically require germplasm that is more heat tolerant. The arrival of cooler temperatures in late fall allows for vernalization and flower bud setting. Varieties that perform well in South Carolina include Emerald Crown, Eastern Crown, Lieutenant, and Iron Man. One of the difficulties with growing broccoli in the Southeast is competition with weeds during the growing season.

Weeds Are Very Active During the Optimal Time to Grow Broccoli in South Carolina

Figure 1. Yellow nutsedge, Cyperus esculantus (perennial). Image credit: Matt Cutulle, Clemson University.

During the warmer months of the fall growing season, weeds can be problematic for earlier plantings. One of these problematic weeds is yellow nutsedge (Cyperus esculentus L.). Not only is this sedge species hardy due to the numerous subterranean tubers and sharp, fast-growing leaves, but a single mature plant is also capable of producing over 1,000 seeds in a single growing season, meaning a field of several hundred thousand flowering plants can introduce hundreds of millions of seeds into the weed seed bank of the soil. These characteristics make yellow nutsedge one of the more problematic weeds during the fall broccoli establishment period in the Carolinas. Other weeds that are problematic as they can reduce yield and quality in South Carolina broccoli production include swinecress and purslane (figures 1–3).

Figure 2. Lesser swinecress, Coronopus didymus (annual or biennial). Image credit: Matt Cutulle, Clemson University.

Figure 3. Common purslane, Portulaca oleracea (annual). Image credit: Matt Cutulle, Clemson University.

Managing Weeds in South Carolina

There are limited post-emergent herbicide options available for controlling weeds in broccoli. Therefore, growers need to rely on pre-emergent (PRE) herbicides and cultural practices to reduce weeds. Effective PRE herbicides include Devrinol, Dual Magnum, and Goaltender. Cultural practices include (1) in-row mechanical cultivation from a rolling cultivator or sweep-type cultivator to uproot weeds and aerate the soil, (2) initiating the stale seedbed technique, which includes allowing weeds to come up and burning down with a post-emergent herbicide or flame weeder multiple times prior to planting, and (3) planting a spring cover crop or other marketable crops to help reduce weeds such as yellow nutsedge and summer annual grasses prior to the fall broccoli planting.⁵

It appears cultivation three weeks after transplanting broccoli is necessary during the fall as PRE herbicides themselves will not be able to hold back weeds for the entire growing season. The best herbicides for use in fall broccoli production are Dual Magnum and Goaltender applied five days pre-transplant followed by cultivation three weeks after transplanting (tables 1 and 2). In a fall broccoli study, Devrinol was effective at controlling grass weeds but was not as effective as Dual Magnum and Goaltender at controlling purslane. Weed pressure significantly reduced the quality of the broccoli crop and yield; therefore, it is necessary to use both cultivation and herbicides.

Table 1. The effect of PRE herbicides and cultivation eight weeks after transplanting on weed control in Charleston, South Carolina. Green represents excellent weed control, red represents unacceptable weed control, and orange represents marginal control or suppression.⁴

Table 2. The effect of PRE herbicides and cultivation on fall broccoli crop quality and yield in Charleston, South Carolina. Combining Goaltender with in-row mechanical cultivation resulted in the highest quality broccoli and greatest yield.⁴

Recommendations

Cultivate in row three weeks after transplanting (figures 4 and 5).

Apply PRE herbicides such as Goaltender (oxyfluorfen) or Dual Magnum (S-metolachlor) prior to transplanting broccoli in mid to late September. This creates a scenario where weeds are controlled, and the broccoli plant is growing vigorously in warm weather. Control is extended through the application of in-row cultivation, which will reduce weed competition until the cooler temperatures arrive in November.

Maintain good weed control practices in the offseason, including implementing the stale seedbed technique, performing seed head destruction of weeds after broccoli harvest, and spring cover crop plantings.

Figure 4. Cultivation in Mid-October provides adequate weed control. Image credit: Matt Cutulle, Clemson University.

Figure 5. Nutsedge encroaching on a plot that was not cultivated. Image credit: Matt Cutulle, Clemson University.

References Cited

1. Farnham MW, Grusak MA. Assessing nutritional changes in a vegetable over time: issues and considerations. HortScience. 2014 Feb:49(2);128–132.
2. USDA Agricultural Statistics Service. 2017 [accessed 2019 Dec]. https://www.nass.usda.gov/.
3. Atallah S, Gomez S, Bjorkman T. Localizations effects for a horticultural supply chain: Broccoli in the Eastern United States. Food Policy. 2014 Dec; 49:151–159.
4. Cutulle MA, Campbell HT, Couillard D, Ward B, Farnham M. Pre-transplant herbicide application and cultivation to manage weeds in Southeastern broccoli production. Crop Protection. 2019. https://doi.org/10.1016/j.cropro.2019.104862.
5. Farmaha BS, Sekaran U, Marshall MW. Cover crops for weed and nutrient management. Clemson (SC): Clemson Cooperative Extension, Land-Grant Press by Clemson Extension; 2020. LGP 1088. http://lgpress.clemson.edu/publication/cover-crops-for-weed-and-nutrient-management.