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Although spring is not considered the optimal time for herbicidal control of most cool-season broadleaf weeds in turfgrass, spring applications are often required. Most new postemergence broadleaf herbicides combine several active ingredients, possibly resulting in synergistic, antagonistic, or additive effects. Therefore, as new herbicides become available, information is needed about their performance when applied in the spring. The objective of our study was to determine the effect of spring application timing on dandelion control with seven commercially available postemergence herbicides. Products were applied at their lowest labeled rate for dandelion control at three spring application timings, which coincided with dandelion anthesis stages (pre-, peak-, or post-bloom). A grid was used to determine percent dandelion control at several rating dates. The 2010 site had a denser turfgrass stand with smaller dandelions and was irrigated more frequently compared with the 2011 site. In 2010, all herbicides gave 98% or greater control at 30 days after treatment (DAT) when applied post-bloom; when applied preor peak-bloom, control was 80% or greater for all herbicides except for two products applied peak-bloom. At preand peak-bloom, products combining a protoporphyrinogen oxidase (PPO) inhibitor with a 2,4-D ester formulation were superior to most other herbicides. When evaluated at the end of the growing season in 2010, all herbicides provided 89% or greater control at all three timings. In 2011, with a less dense turfgrass stand, larger dandelions, and less frequent irrigation, control was more variable and shorter-lived among herbicides. When applied pre-bloom, all products containing 2,4-D provided 87% or greater control 60 DAT. Post-bloom application generally gave similar control to the pre-bloom timing. Peak-bloom application resulted in the poorest overall control at 60 DAT, but products combining a PPO inhibitor with a 2,4-D ester formulation performed better than most other herbicides. By the end of the season, dandelion regrowth caused reduced overall control at all timings, but overall control was poorest when applied at peak-bloom. In summary, peak-bloom applications should be avoided, especially if dandelion pressure is high. Products combining PPO inhibitors with ester forms of 2,4-D were most effective across all spring application timings. Products containing amine forms of 2,4-D may provide effective control if applied preor post-bloom. Common dandelion (Taraxacum officinale Weber) is one of the most widely recognized weeds of turfgrass. It is a simple perennial, forming a basal rosette of leaves and bright yellow flowers (Uva et al., 1997). Dandelions can reproduce through wind-dispersed seeds or from taproot fragments (McCarty et al., 2001). Dandelions are apomictic, so pollination is not needed to produce viable seeds (Uva et al., 1997). The wind-dispersed seeds can travel long distances and infest previously weed-free areas. Mechanical control of dandelion is difficult because the taproot must effectively be removed or destroyed. A study comparing dandelion control with 2,4-D and mechanical treatments found no measure of control was achieved with hand-weeding alone (Mann, 1981). Many turf managers have achieved acceptable dandelion control using selective herbicides (Christians, 2007; Gardner, 2009; Loughner and Nolting, 2010). Most products used for broadleaf weed control in turfgrass are combination products that contain several active ingredients, which allow turf managers to control a wide array of broadleaf weeds with a single product. However, because the interaction between active ingredients may be additive, synergistic, or antagonistic when combined (Zhang et al., 1995), it is difficult to predict the performance of a new combination herbicide based on the performance of its components applied singly. Many of the active ingredients found in these products such as 2,4-D, dicamba, mecoprop, clopyralid, fluroxypyr, and triclopyr are in the synthetic auxin class of herbicides (Senseman, 2007). Synthetic auxin herbicides are highly selective because most grasses can inactivate these compounds by conjugation, whereas broadleaf weeds cannot and thus are controlled. In recent years, some products have also included an active ingredient from the PPO-inhibiting class such as pyraflufen-ethyl, carfentrazone-ethyl, or sulfentrazone (Senseman, 2007). These compounds inhibit PPO, an enzyme in chlorophyll synthesis needed for catalyzing the oxidation of protoporphyrinogen IX to protoporphyrin IX (Cobb and Reade, 2010). Inhibition of PPO causes an increase in both triplet and singlet state oxygen, resulting in cellular leakage through lipid peroxidation (Senseman, 2007). One reason PPO inhibitors may be included in tank mixes with synthetic auxin herbicides is to provide more rapid necrosis of foliage. Many researchers have reported acceptable control of perennial broadleaf weeds using such combination products (Haley et al., 1995; Loughner and Nolting, 2010; Olson and Hall, 1988; Reicher and Weisnenberger, 2007; Watschke and Borger, 1999). However, previous research has shown that PPO inhibitors can reduce the efficacy of some non-phenoxy herbicides. Ashigh and Hall (2010) reported reduced glyphosate activity when combined with the PPO inhibitor saflufenacil and hypothesized the rapid contact activity of saflufenacil limited the translocation of glyphosate. Similarly, a study conducted by Breeden and McElroy (2006) found that carfentrazone significantly reduced white clover (Trifolium repens L.) control when tank-mixed with foramsulfuron compared with foramsulfuron alone. Fall is considered the best time for herbicidal control of cool-season perennial broadleaf weeds in turfgrass (Branham, 1990; McCarty et al., 2001; Reicher and Weisnenberger, 2007; Wilson and Michiels, 2003). Branham (1990) posited that herbicidal control in the fall is effective because plants are moving carbohydrate reserves to underground storage structures, which is believed to aid in the movement of xenobiotics to their site of action. However, although fall applications may be optimal, turf managers frequently need herbicidal options in the spring to meet clients’ expectations. Spring herbicide applications coincide with one of three dandelion anthesis stages: pre-bloom, peak-bloom, or post-bloom. To the casual observer, the extent of a dandelion infestation may not be apparent until the flush of bright flowers occurs during the bloom stage. Gray et al. (1973) reported less than one flowering dandelion stem/m in the first 13 weeks of the growing in a home lawn in Kentucky; however, in the fourteenth week (4 to 10 Apr.), flowering increased to 68 stems/m. Unfortunately, dandelion control may be reduced if herbicides are applied during the Received for publication 2 Mar. 2014. Accepted for publication 15 July 2014. Mention of a trademark, proprietary product, or vendor does not constitute a guarantee or warranty of the product(s) by Kansas State University and does not imply its approval to the exclusion of other products or vendors that also may be suitable. Graduate Research Assistant. Professor. To whom reprint requests should be addressed; e-mail zane12@ksu.edu. 1212 HORTSCIENCE VOL. 49(9) SEPTEMBER 2014 production of flowers and seeds (Schleicher, 1997). The movement of auxinic herbicides in plants is largely controlled by the physiochemical properties of the herbicide such as pKa and log Kow values (Cobb and Reade, 2010). Once absorbed into the shoot of a susceptible plant, auxinic herbicides such as 2,4-D (pKa = 2.8) typically exist as a free acid because cellular pH is 5.0 (Sterling, 1994). The non-ionized 2,4-D molecules can readily pass through the cell membrane and into the phloem. However, phloem pH is higher ( 8.0), and incoming 2,4-D molecules are immediately ionized and essentially ‘‘trapped’’ because anions have reduced membrane permeability (Sterling and Hall, 1997). Once trapped in the phloem, the movement of the 2,4-D anion is passive and primarily determined by the strength of source-to-sink partitioning (Cobb and Reade, 2010). In perennial weeds, underground storage structures may become a strong sink for surplus photosynthate produced in mature leaves (Coble et al., 1970), which is believed to enhance the efficacy of herbicide applications by increasing basipetal movement of weak-acid herbicides to underground storage organs (Tworkoski, 1992). Oppositely, during anthesis, the production of flowers and seeds is likely a major sink for photosynthate; therefore, basipetal movement of auxinic herbicides may be limited, resulting in reduced control. Turf managers need information on herbicide performance in the spring, and our literature search revealed no reports in the refereed literature concerning the influence of spring application timing on dandelion control with postemergence broadleaf herbicides currently used on turfgrass. Furthermore, because these herbicides typically contain multiple active ingredients that may be additive, synergistic, or antagonistic, each combination herbicide itself should be tested rather than testing its individual components. Therefore, the objective of our study was to determine the effect of spring application timing on dandelion control with seven commercially available postemergence broadleaf herbicides. Materials and Methods Site characteristics and experimental design. Field studies were conducted in 2010 and 2011 on adjacent sites at the Rocky Ford Turfgrass Research Center in Manhattan, KS. The soil at both sites was a Chase Silt Loam (fine, smectitic, mesic, Aquertic Argiudoll) with a pH of 6.9. Soil tests indicated adequate levels of phosphorus and potassium. The 1.2 · 1.8-m plots were mown at 7.6 cm. The 2010 site contained turf-type tall fescue (Festuca arundinacea Schreb. Syn Schedonorous arundinaceus Schreb.) with an existing dandelion stand and was irrigated

Springtime Dandelion (Taraxacum officinale) Control with Seven Postemergence Herbicides Applied at Three Anthesis Stages