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Welcome to The Turf Zone Podcast. This episode covers Bosc’s Mille Graines – An Emerging Weed Challenge in North Carolina Turfgrass Systems. Written by Venkateswar Reddy Yelkur, Navdeep Godara, Ray McCauley – North Carolina State University.

Bosc’s mille graines [Oldenlandia boscii (de Candolle) Chapman] is becoming an increasingly problematic broadleaf weed in turfgrass systems in North Carolina. Native to the southeastern United States, Bosc’s mille graines is a perennial species in the Rubiaceae family and is commonly found in moist or seasonally wet turfgrass areas, roadsides, and other disturbed habitats. It has a prostrate growth habit and can form dense, mat-like patches within the turfgrass canopy. Bosc’s mille graines can be identified by its linear to elliptic, opposite leaves; highly branched, smooth, green to reddish stems; and deep, woody taproot. The inflorescences have sepals that are visible from aboveground and produce four tiny, white petals from July through September before the plant enters dormancy during the winter months. Bosc’s mille graines can invade turfgrass rapidly because mature plants can produce more than 15 shoots, with each shoot producing multiple seed capsules containing more than 100 seeds per capsule. Consequently, weedy plants that escape control measures can contribute substantially to replenishing the soil seedbank for subsequent growing seasons. Once established, the deep taproot and prostrate growth habit allow Bosc’s mille graines to persist and regrow following routine mowing.

These biological characteristics can aid in successful invasion of Bosc’s mille graines in sod production fields and lawns, where infestations can reduce turfgrass aesthetics, quality and sod marketability. Recently harvested sod fields and other disturbed areas may be particularly vulnerable because soil disturbance exposes previously buried weed seeds to light, creating favorable conditions for germination and establishment. In the past two years, several sod producers across North Carolina have reported increasing infestations of Bosc’s mille graines following sod harvest. Traditional programs utilizing a single application of synthetic auxin herbicides failed to provide acceptable control of Bosc’s mille graines in warm-season turfgrasses and led to weed seed-bank buildup following escapes. Although native to this region, Bosc’s mille graines is a relatively new weed in North Carolina turfgrass systems and its appearance can be confused with other weeds of the Rubiaceae family.

Several other problematic Rubiaceae weeds common in turfgrass settings are Florida pusley, old world diamond-flower, poorjoe, and Virginia buttonweed. Among these species, Bosc’s mille graines can be easily confused with poorjoe because of their similar appearance. The two species can be distinguished by several characteristics. Bosc’s mille graines has smooth stems, whereas poorjoe has noticeable hairs on its stems and fruit. Bosc’s mille graines flower sepals are visible from above and located close to the petals, while poorjoe has smaller sepals that are positioned at the base of the flower. Misidentification can allow Bosc’s mille graines to establish, replenish the soil seedbank, and make future management efforts more challenging.

Factors Affecting Weed Germination

Bosc’s mille graines germination is dependent on soil temperature, soil moisture and light. Most seedlings emerge from seeds near the soil surface, where they are exposed to light. Weed germination is limited when seeds are buried more than 1 inch deep. Warm temperatures favor germination, with emergence occurring when day and night temperatures are above 77 and 50 degrees Fahrenheit, respectively, and increasing as temperatures rise. Soil moisture also plays an important role in germination. Bosc’s mille graines can germinate when soil moisture is greater than 20% of field capacity, suggesting that even relatively small amounts of rainfall or irrigation may provide enough moisture for germination. Understanding these temperature and moisture requirements may help turfgrass managers improve the timing of preemergence (PRE) herbicide applications. Based on historical weather patterns, late February may be an ideal time to apply PRE herbicides before conditions become favorable for Bosc’s mille graines germination. Rubiaceae weeds can be difficult-to-control in turfgrass systems because of limited preemergence (PRE) and postemergence (POST) herbicide options. Effective weed control often requires multiple herbicide applications, increasing management costs.

PRE Herbicides

Bosc’s mille graines has the capacity to produce large quantities of seed, preventing new seedling establishment is critical for long-term management. Effective use of PRE herbicides can reduce weed establishment, limit the development of new infestations, and minimize replenishment of the soil seedbank. NC State University Turfgrass Weed Science Program conducted field studies to assess the efficacy of different PRE herbicide programs for Bosc’s mille graines control in bare ground conditions. Sequential applications of Coastal (active ingredients – simazine, prodiamine, imazaquin) at 32 fluid ounces per acre, Gallery (active ingredient – isoxaben) at 31 fluid ounces per acre, Pendulum AquaCap (active ingredient – pendimethalin) at 49 fluid ounces per acre, Pennant Magnum (active ingredient – S-metolachlor) at 41 fluid ounces per acre, Ronstar (active ingredient – oxadiazon) at 3 pounds of active ingredient per acre, Specticle Flo (active ingredient – indaziflam) at 6 fluid ounces per acre, and Tower (active ingredient – dimethenamid-P) at 32 fluid ounces per acre provided excellent control of Bosc’s mille graines (> 90% control) up to 12 weeks after treatment. These herbicide products can be used on established turfgrass; however, users should consult the product label for specific species and cultivar tolerance and application rate restrictions, particularly for oxadiazon-based products. Preemergence herbicides should be irrigated with 0.25 to 0.5 inches of water following application. In our research studies, sequential applications were made at 6-week interval with initial application during first week of March; however, users should consult the individual product label for the recommended re-application interval, sod harvest and establishment interval. Because these studies were conducted under bare ground conditions, herbicide efficacy may differ in established turfgrass due to greater microbial degradation and high organic matter content affecting herbicide availability.

POST Herbicides

Field trials were conducted to evaluate POST herbicide programs for Bosc’s mille graines control in ‘Empire’ zoysiagrass and ‘Sola’ St. Augustinegrass. Sequential applications (first application in June followed by another application in July) of Avenue South (active ingredients – 2,4-D, dicamba, penoxsulam, sulfentrazone) at 64 fluid ounces per acre, Sublime (active ingredient – dicamba, mesotrione, triclopyr) at 32 fluid ounces per acre, Trimec Classic (active ingredient – 2,4-D, dicamba, mcpp) at 64 fluid ounces per acre, a tank mixture of Trimec Classic at 32 fluid ounces per acre with Quicksilver (active ingredient – carfentrazone) at 2.1 fluid ounces per acre controlled Bosc’s mille graines 80% or greater. A single application was insufficient to provide season-long weed control, and sequential applications at 4-week interval were required. Herbicide applications should be avoided when turfgrass is under heat or drought stress, as these conditions can increase the risk of phytotoxicity.

Sequential applications of Avenue South at 64 fluid ounces per acre, Trimec Classic at 64 fluid ounces per acre and Trimec Classic at 32 fluid ounces per acre tank-mixed with QuickSilver at 2.1 fluid ounces per acre can provide effective POST control of Bosc’s mille graines in established bermudagrass and zoysiagrass. In St. Augustinegrass, Avenue South at 64 fluid ounces per acre and Trimec Classic at 29 fluid ounces per acre tank-mixed with QuickSilver at 2.1 fluid ounces per acre can be used for POST control but will cause transient injury to turfgrass. Turfgrass will recover from transient herbicide injury by 8 weeks after application. Sublime herbicide can be used on zoysiagrass and St. Augustinegrass, when temporary turfgrass bleaching is acceptable for up to 8 weeks following application.

Conclusions

Successful management of Bosc’s mille graines begins with proper identification, early detection, and limiting the spread of the weed in turfgrass systems. New infestations should be removed or controlled before plants produce seed, thereby limiting further additions to the soil seedbank. Because this weed is currently more prevalent in some sod production fields, clean mowing and harvesting equipment before moving between fields to prevent the spread of weed seeds. When purchasing sod, sprigs, or other plant material, ensure that the material is free of Bosc’s mille graines.

Practitioners should integrate PRE and POST herbicide programs when weed infestations are observed. PRE herbicides can reduce seedling establishment and should be considered as a preventive strategy in areas with a history of heavy infestation. PRE applications should be timed before weed germination period, as these herbicides will not control established weeds. Where feasible, rotating herbicides with different modes of action can help reduce the risk of herbicide resistance. When relying on POST herbicides to control established weed populations, sequential applications may be necessary to improve control and reduce the potential for weed escapes.

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