This tropical, perennial weed, which is on the federal noxious weed list, is expanding across the southeastern United States.
⋅ By Mike Marshall ⋅
Extension Weed Specialist, Edisto Research and Education Center, Clemson University
Invasive weeds are troublesome in agronomic cropping systems and nearby non-crop environments. Benghal dayflower (Commelina benghalensis L.), also known as tropical spiderwort, was first detected in South Carolina by the Clemson University Department of Plant Industry in 2013. Infestations have now been documented in agronomic fields in South Carolina.
Once established, this invasive weed reproduces prolifically by producing both aboveground and belowground flowers and fruit, increasing the number of seeds in the soil seedbank. This article discusses Benghal dayflower identification, biology and control options in peanut.
Benghal dayflower is a tropical, perennial herbaceous weed listed on the federal noxious weed list and is expanding in distribution across the southeastern United States. Benghal dayflower is native to the tropical areas of Africa, India and the Pacific islands. It has been documented in Georgia, Florida, North Carolina, Virginia and California.
In the early growth stages, Benghal dayflower resembles other Commelina spp. found in South Carolina including Asiatic dayflower (Commelina communis L.) and spreading dayflower (Commelina diffusa Burm. f.).
Identifying characteristics that visually separate Benghal dayflower from other dayflowers include the presence of red or white trichomes or hairs on the apex of the leaf sheath and leaf margins, while the other two dayflowers have smooth, hairless stem, sheaths and leaf margins. Also, Benghal dayflower has tuber-like white spathes or subterranean flowers borne on stolons in the root system.
Each aerial fruit can produce four to five seeds and each subterranean fruit can produce two to three seeds. In one study, aerial seed production was 4.6 times higher than subterranean seed production. As a result, one Benghal dayflower plant can potentially produce up to 1,600 seeds per season. Currently, eight counties in South Carolina have confirmed populations of Benghal dayflower.
In cases where infestations are small and isolated, such as in nurseries or urban gardens, hand removal or spot spraying with an herbicide is generally very effective in controlling Benghal dayflower. However, small or initial infestations in agronomic fields typically remain undetected for several years before the grower realizes the severity of the problem. In that time, Benghal dayflower has spread across the landscape, usually by rooting of stem fragments and seed, and added considerable quantities of seed to the soil seed bank. Benghal dayflower seed can remain viable for up to four years in the soil.
Control Options In Peanut
May is the primary planting window for peanuts in South Carolina; this is close to peak Benghal dayflower emergence (June-July). During this period, peanuts are small with large gaps between rows, resulting in rapid emergence, establishment and spread of Benghal dayflower. Non-selective herbicides, such as Liberty and glyphosate, are not registered for use in peanuts. Postemergence choices for Benghal dayflower control in peanut are limited. In 2016, Strongarm (diclosulam) was registered for postemergence suppression in peanut when applied at the 1- to 2-inch Benghal dayflower growth stage. Strongarm preemergence activity on Benghal dayflower was limited.
The key to managing Benghal dayflower in peanut is overlapping soil residual herbicides during the season. Consult the product labels for seasonal application limits on each herbicide, and practice resistance management by rotating herbicide modes-of-action.
Benghal dayflower seed typically germinates and emerges later in June to July and continues to grow until frost. Soil residual herbicides tank-mixed with postemergence herbicides are a critical tool for long-term control during the growing season. For example, Dual Magnum (s-metolachlor) is a highly effective preemergence herbicide that provides 96% to 99% control six weeks after application. In contrast, Benghal dayflower control was less than 70% with the residual herbicides diuron, Valor (flumioxazin) and Staple. Other herbicides in the same family as Dual Magnum, which include Warrant (acetochlor), Zidua (pyroxysulfone) and Outlook (dimethanemid-p), can provide similar levels of preemergence control, although the length of residual control may be shorter.
Control Options In Rotation Crops
Cultural, biological and mechanical options are often under-utilized in weed management programs for Benghal dayflower. There are no available biological agents for control. However, modification of crop row spacing, seeding density and planting date are highly effective weed management tools, especially when deployed in combination. For example, the weed-free period (i.e., the time when weeds must be absent to achieve the maximum crop potential yield) for cotton planted on 21-inch rows was only six weeks. In contrast, cotton planted on wide rows (e.g., 31 and 42 inches) needed a significantly longer weed-free period of 10 to 14 weeks for optimum yield.
Crop seeding density can reduce the incidence of weeds. With the same herbicide program, Benghal dayflower control increased from 85% to 96% as seeding density increased from 0.7 to 2.1 plants per foot in single row cotton. In addition, cotton planted on a twin-row pattern resulted in 96% to 97% Benghal dayflower control averaged over seeding density and herbicide program 18 weeks after emergence.
Early planting dates (early May) are recommended to minimize Benghal dayflower competition in cotton. Burial through inversion tillage greater than 6 inches can also prevent Benghal dayflower germination. This drastic form of tillage should only be considered when infestation levels are extremely high due to its negative impact on soil structure and the environment.
Herbicide Options In Corn, Cotton and Soybean
Herbicides are the management tool most often preferred by growers. Corn, cotton and soybeans planted in South Carolina are glyphosate-tolerant. Glyphosate herbicide efficacy on Benghal dayflower decreases rapidly as weed size increases. For example, glyphosate alone in glyphosate-tolerant cotton only provided 53% control of 1- to 4-inch Benghal dayflower. The lower control value of 53% was attributed to the larger Benghal dayflower (3- to 4-inch) plants. Similarly, Liberty (glufosinate) provided 68% control, which is below the acceptable economical threshold of 70%. The tank-mix combination of Staple (pyrithiobac) plus glyphosate in glyphosate-tolerant cotton increased Benghal dayflower control compared to glyphosate alone.
In 2017, cotton and soybean varieties were introduced with tolerance to 2,4-D or dicamba herbicides. Benghal dayflower control was 99% and 94% with Liberty plus 2,4-D and Liberty plus dicamba, respectively. PG