Introduction
Producers in temperate regions (plant hardiness zones 5 to 7) (USDA-ARS 2023) have been trying to grow June-bearing strawberry cultivars in multiyear plasticulture systems since at least the early 2000s (Demchak et al. Reference Demchak, Harper, Kime and Lantz2005). Plasticulture entails laying plastic mulch on raised rows, planting plug plants in late summer of Year 1, harvesting in the spring of Year 2, renovating in the summer of Year 2 by mowing plants to crown level after harvest and allowing them to regrow, and harvesting again in the spring of Year 3 (Arana et al. Reference Arana, Meyers and Guan2025). Following the spring harvests, long photoperiods and high temperatures in summer promote the growth of stolons (also known as runners) (Hartmann Reference Hartmann1947; Heide Reference Heide1977), leading to vegetative propagation. Runners are specialized horizontal stems that generate daughter plants at each node, which are harvested before they can develop roots in nursery systems or allowed to root and bear fruit in perennial matted‑row systems (Darrow Reference Darrow1929; Heide et al. Reference Heide, Stavang and Sønsteby2013). However, runners are undesirable in plasticulture. Runners should be removed in plasticulture fields because they become physiological sinks, competing directly with the mother plant (Fiola et al. Reference Fiola, O’Dell and Williams1997; Poling and Durner Reference Poling and Durner1986; Poling Reference Poling2016). Additionally, once daughter plants reach the bare ground area between plastic mulch–covered rows, (i.e., row middles), their rooting and subsequent growth may complicate field maintenance tasks and future harvests. This is especially true for the multiyear plasticulture system.
Manual removal of runners is labor-intensive and has become increasingly difficult and expensive due to widespread agricultural labor shortages and the rising costs of manual intervention (Christiaensen et al. Reference Christiaensen, Rutledge and Taylor2020). The task becomes even more demanding once runner establishment occurs; that is, when daughter plants root in the row middles, because removing runners at this stage requires considerably more time and physical effort. Consequently, researchers have evaluated alternative strategies for managing runners. One runner suppression strategy is to use plant growth regulators (PGRs) to limit runner production and growth. Previous studies have shown that paclobutrazol can reduce runner number (Braun and Garth Reference Braun and Garth1986), while prohexadione-calcium reduces runner length (Duval and Golden Reference Duval and Golden2005; Hytönen et al. Reference Hytönen, Mouhu, Koivu and Junttila2008; Kim et al. Reference Kim, Lee, Kang, Hwang, Kim, Lee, Kang and Hwang2019). However, runner suppression from these PGRs often reduced mother plant vegetative growth or yield. Additionally, the high cost of developing PGRs, along with the lengthy evaluation process required for their use in strawberry runner management, is likely to limit their approval for this specific application in the near future.
Another potential strategy for managing runners is to use herbicides. Among the herbicides currently registered for use on strawberries, napropamide and pendimethalin are promising options for preventing runner establishment because they are soil-residual compounds that inhibit root development. Soil-residual herbicides provide extended weed control by persisting in the soil, and their longevity is influenced by interactions between herbicide chemistry, soil properties, moisture, temperature, and management practices (Helling Reference Helling and Van Acker2005). The site of action of napropamide remains inconclusive and is currently classified as Group 0 by the Herbicide Resistance Action Committee (HRAC) and Weed Science Society of America (WSSA). However, several specific physiological effects have been identified, including root growth inhibition, shoot growth inhibition to a lesser extent than root growth inhibition, and oxidative stress (Shitiz et al. Reference Shitiz, Mishra, Raithatha, Patel, Tater, Deshpande and Date2025; Di Tomaso et al. Reference Di Tomaso, Ashton and Rost1988a; Zhang et al. Reference Zhang, Cui, Zhu and Yang2010). Pendimethalin is a HRAC/WSSA Group 3 herbicide that inhibits microtubule assembly (Morrissette et al. Reference Morrissette, Mitra, Sept and Sibley2004). By impeding microtubulin synthesis, pendimethalin interferes with mitosis and the formation of cell wall microfibrils, leading to the cessation of cell enlargement and elongation, particularly in the roots and shoots of emerging seedlings (Appleby and Valverde Reference Appleby and Valverde1989). Both herbicides are registered for preemergence control of annual grasses and broadleaf weeds in strawberry fields. According to labels, they can be applied after transplanting to row middles. When applying the herbicide after transplant, shielded equipment or properly spaced nozzles must be used to ensure the herbicide does not contact strawberry foliage.
Because strawberry daughter plants form an adventitious root system composed of shoot-borne roots without a dominant primary root (Papp et al. Reference Papp, Gracza and Simon2004; Poling Reference Poling2012), this shallow root system may be especially vulnerable to soil-residual herbicides that inhibit root growth. If these herbicides are applied to the row middles following summer renovation (Meyers and Arana Reference Meyers and Arana2023), they may prevent runner establishment by suppressing daughter plants from rooting, thereby eliminating the need to exert force to remove well‑anchored daughter plants. Additionally, some growers rely on winter conditions to reduce runner survival; however, mortality is more likely when runners are not successfully established. Once daughter plants root, they are more likely to survive the winter.
Although napropamide and pendimethalin are registered for weed management in strawberry production, using them for runner suppression represents an alternative use pattern that has not been evaluated. Given their soil‑residual activity and root‑growth inhibition mechanisms, we hypothesized that summer applications of these herbicides could prevent daughter plants from rooting, thereby facilitating manual runner removal and providing growers with greater flexibility in runner‑management timing. To test this hypothesis, we conducted greenhouse studies to evaluate the effects of napropamide and pendimethalin on daughter plant rooting before field validation.
Materials and Methods
In 2025, two greenhouse studies with repeated trials were conducted at the Purdue University horticulture greenhouses, in West Lafayette, Indiana (40.4208°N, 86.9147°W). Greenhouse conditions for the napropamide trials consisted of an average air temperature of 25.5 C with 71.8% relative humidity and 14 h to 15 h of daylight. Greenhouse conditions for the pendimethalin trials consisted of an average air temperature of 26.2 C with 78.1% relative humidity and 14 h of daylight. Each experimental unit consisted of three 15-cm-diam polyethylene pots filled with a 2:1:1 blend of commercial bark mix media (Berger BM7 35% Bark High Porosity; Hummert International; Earth City, MO), loamy sand topsoil, and sand, respectively. The resultant substrate had a sandy texture (88% sand, 6% silt, and 6% clay), pH 6.8, with 2.5% organic matter.
On March 4, the bareroot plants of the ‘Galletta’ strawberry cultivar (Indiana Berry & Plant Co.; Plymouth, IN) were planted into pots with drip irrigation and grown for 9 wk in Trial 1 and 10 wk in Trial 2 for the napropamide study, and for 18 wk in Trial 1 and 19 wk in Trial 2 for the pendimethalin study. These initial bareroot plants served as the mother plants. For each mother plant pot, two adjacent pots were filled with substrate but left unplanted and without drip emitters to serve as side pots in a three-pot experimental unit (Figure 1). Pendimethalin trials were initiated after napropamide trials as both experiments required extra drip emitters system to irrigate the side pots after herbicide application. Although mother plants were irrigated after planting, there were not enough emitters to water all side pots simultaneously. All flowers and runners were removed from each mother plant on a weekly basis until 1 wk before the start of each trial. Flowers were removed to maintain vegetative growth, and runners were removed to prevent tangling and to maintain organization of experimental units. Runners were removed using hand clippers, cutting as close as possible to the crown.
Simulated herbicide application methods used in greenhouse trials at Purdue University to pre-screen herbicides for suppressing strawberry runner establishment by reducing the rooting of runner-borne daughter plants. Four application treatments were evaluated using a three‑pot experimental unit consisting of a central pot containing the mother plant and two attached runners, each bearing an unrooted daughter plant, and two side pots that held the daughter plants after the herbicides were applied. The Nontreated panel shows that no herbicide was applied. The Broadcast panel shows the mother plant and its attached runners with runner-borne daughter plants being sprayed. The In‑row panel shows only the mother plant and its attached runners with runner-borne daughter plants being sprayed, while the side pots were excluded. The Row‑middle panel shows that only the side pots were sprayed, while the mother plant and its attached runners with runner-borne daughter plants were excluded.
Irrigation water quality parameters were determined by a commercial laboratory (A&L Great Lakes Laboratories, Fort Wayne, IN). Plants were initially watered with acidified clear water (pH 7.3; EC 0.8 mmho cm−1), which contained 96 mg L−1 Ca and 38 mg L−1 Mg, with an alkalinity of 195 mg L−1 as CaCO3. Once plants were established, irrigation alternated between acidified water and fertilizer solution (pH 7.1; EC 1.53 mmho cm−1). The fertilizer solution was made using a water-soluble fertilizer (20 N-1.3 P-15.8 K; ICL Specialty Fertilizers, Dublin, OH), which supplied the following elements (in mg L−1): 150 N, 9.8 P, 119 K, 12 Mg, 1.5 Fe, 0.4 Mn, 0.4 Zn, 0.2 Cu, 0.2 B, and 0.1 Mo. Nitrate and ammoniacal sources of nitrogen were provided as 61% and 39% total nitrogen, respectively.
On the day of herbicide application (napropamide, May 5 for Trial 1 and May 12 for Trial 2; pendimethalin, July 8 for Trial 1 and July 14 for Trial 2), two runners were selected on each mother plant, each bearing one unrooted daughter plant with one or two open trifoliate leaves (Figure 2); all additional runners were removed. For clarity, these two runners are referred to as the first runner and the second runner. Following runner selection, four treatments were evaluated: 1) a nontreated control, in which no herbicide was applied; 2) a simulated broadcast application, in which the entire three-pot system was sprayed; 3) a simulated in-row application, in which only the mother plant and runners were sprayed while the side pots were excluded; and 4) a simulated row-middle application, in which only the side pots were sprayed (Figure 1). The simulated broadcast and in‑row treatments included foliar applications to the mother plant, runners, and attached daughter plants, with or without spraying the side pots, respectively, whereas the row‑middle treatment was a soil‑applied residual application to the side pots only. Following the herbicide application, each daughter plant was secured to the substrate in the side pots using a 10-cm garden staple (Pro Duty Landscape Garden Staples; Menards, Eau Claire, WI) and a drip emitter was placed in each side pot to provide uniform irrigation across the three‑pot system. The experiments were arranged in a randomized complete block design with four replications.
Developmental stage of unrooted daughter plants used for herbicide applications. Top (napropamide): Daughter plants with a recently opened trifoliate leaf and minimal apical runner extension. Bottom (pendimethalin): Daughter plants with one to two open trifoliate leaves and more advanced apical runner extension. Photos were taken on the day of herbicide application.
Napropamide (Devrinol DF-XT; United Phosphorus, Inc; King of Prussia, PA) was applied at 4.5 kg ai ha−1, consistent with the labeled rate for strawberries grown with plastic mulch. Pendimethalin (Prowl H2O; BASF Corporation; Research Triangle Park, NC) was applied at 0.8 kg ai ha−1, consistent with label recommendations for strawberries grown on coarse soils with less than 3% organic matter. Herbicides were applied using a compressed-air spray booth (Generation III track sprayer; DeVries Manufacturing, Inc.; Hollandale, MN) fitted with a single 8002 EVS nozzle tip (TeeJet Technologies, Glendale Heights, IL). The sprayer was calibrated to deliver 187 L ha–1 (napropamide) or 140 L ha−1 (pendimethalin) both at 207 kPa. Within 24 h of the herbicide application, all pots were hand‑watered with a hose to activate the herbicide without causing leaching.
For the napropamide study, visible injury of the mother plant was rated on a scale of 0% (no injury) to 100% (crop death), and the number of new runners was recorded at 2, 4, 6, and 8 wk after treatment (WAT). At 8 WAT, mother plants were removed from the pot, after which aboveground biomass was cut at the substrate surface and weighed to determine fresh biomass. Roots were shaken free of excess substrate and rinsed with water before collecting the belowground fresh biomass. On each runner fixed to the side pots, data collection included visual injury of the daughter plant, measuring length of new runners (in centimeters) extending beyond the daughter plant, and counting additional daughter plants produced along that runner. Measurements were collected at 2 and 4 WAT for the first runner and at 2, 4, 6, and 8 WAT for the second runner. The first runner was destructively sampled at 4 WAT to measure daughter plant pull force in newton units using a digital force gauge (model VDFG-300; VPOER Co., Ltd., Industrial Zone Yueqing, China) and plastic-beaded ties, followed by collection of aboveground and belowground biomass (in grams) of the runner and its daughter plant. The second runner was destructively sampled at 8 WAT to collect the same measurements.
For the pendimethalin study, data collection of the mother plant included only visual injury ratings and the number of new runners at 2 and 4 WAT. Aboveground and belowground biomass data were not collected on the mother plant because observations indicated no treatment effects, and measurements focused on the response of the runners. Runner measurements at 2 and 4 WAT for both runners included visual injury of daughter plants, new runner growth length, and additional daughter plant counts. Due to excessive daughter plant rooting, both runners were destructively sampled at 4 WAT to measure daughter plant pull force and to collect aboveground and belowground biomass, shortening the trial duration compared with the napropamide trials.
Data were analyzed using RStudio software (Posit team 2025). Model assumptions were evaluated using Q–Q plots and residual plots. Injury percent, runner count, and new daughter plant count data were analyzed using generalized linear mixed models with the glmmTMB package. Injury data were modeled using a beta distribution with a logit link function, while count data were modeled using Poisson distributions. The nontreated control was excluded from injury analysis due to zero variance. To accommodate the bounded nature of the beta distribution, which excludes exact zeros and ones, injury data were converted to proportions and then transformed following the method outlined by Smithson and Verkuilen (Reference Smithson and Verkuilen2006). New growth length, pull force, and biomass data were analyzed using linear mixed models with the lme4 package and log transformed as needed to meet assumptions of normality and homoscedasticity. Due to persistent non-normality after transformation, pull force and belowground biomass data in the napropamide trials were analyzed using Kruskal-Wallis tests.
For all models, treatment and trial were initially included as fixed effects. For runner measurements in the pendimethalin trial, runner identity (first vs. second) was initially included as an additional fixed effect. Analysis of variance was used to test main effects and interactions. Trial, runner identity, and their interactions with treatment were not significant (P > 0.05) and were removed from final models, which included only application treatment as a fixed effect. Mean separation for parametric analyses was conducted using a Tukey HSD test at α = 0.05, whereas a Dunn test with Holm adjustment (α = 0.05) was used for nonparametric analyses. Back‑transformed means are presented to facilitate interpretation.
Results and Discussion
Napropamide
Mother Plant
Napropamide injury on mother plants consisted of vein‑localized white discoloration (Figure 3). Due to minimal herbicide effects, parameter data will be discussed but not shown. Broadcast and in‑row applications caused an average of 12% injury at 2 and 4 WAT, which declined to 7% by 6 WAT. This decline reflected the expansion of new, uninjured leaves. Symptoms on previously affected leaves neither worsened nor disappeared. The row‑middle application caused no injury. By 8 WAT, injury was minimal (≤5%) across treatments. Despite these visual symptoms, mother plant growth was not affected by napropamide application methods. Treated plants produced a similar number of runners and accumulated similar aboveground and belowground biomass as nontreated plants. Averaged across treatments, mother plants produced two new runners at 2 WAT and five new runners at 8 WAT, and 263 g of fresh aboveground biomass and 171 g of belowground biomass.
Strawberry mother plant injury at 2 wk after treatment (WAT) following simulated napropamide applications. Panels show representative leaves from each application method. Visible injury consisted of vein‑localized white discoloration.
First Runner
Injury and new daughter plant number indicated no treatment differentiation, whereas all other parameters exhibited a treatment effect (Table 1). Injury to the daughter plant consisted of necrosis, leaf distortion, stunting, and chlorosis. At 2 WAT, daughter plant injury ranged from 15% to 33%, and at 4 WAT, from 14% to 21% across application treatments. New growth length was reduced by the broadcast and in-row application treatments at 2 and 4 WAT. New growth after broadcast and in-row applications fell by approximately 68% at 2 WAT and by 59% at 4 WAT compared with the nontreated control. Mean new growth length in the row-middle application treatment (25 cm) was similar to that of the nontreated control (34 cm) at 2 WAT; however, by 4 WAT, it was similar (57 cm) to both the other application treatments (29 cm and 31 cm) and the nontreated control (72 cm). Despite the reductions in new growth length, at 4 WAT, herbicide treatments had no effect on the total number of daughter plants initiated from that new growth (one to three).
Injury, new growth length, number of new daughter plants, pull force, and fresh aboveground and belowground biomass responses of strawberry daughter plants from the first runner at 2 and 4 wk after treatment with napropamide under simulated application methods.a–e
a Abbreviation: WAT, weeks after treatment.
b The first strawberry runner, bearing one daughter plant at the time of treatment, was destructively sampled at 4 WAT to obtain pull force and biomass measurements. Each experimental unit consisted of a three-pot system with a central pot containing a strawberry mother plant and two side pots that held the daughter plants following napropamide application.
c Data are pooled across two trials.
d Napropamide rate: 4.5 kg ai ha−1.
e Means followed by different letters within a column differ according to the Tukey HSD test (α = 0.05) for parametric data, or a Dunn test with Holm adjustment (α = 0.05) for nonparametric data.
Pull force averaged 10 newton in the nontreated control, whereas all herbicide treatments required no force to remove the daughter plants. Likewise, fresh belowground biomass was greater in the nontreated control than all plants that received herbicide treatments (Figure 4). The nontreated plants averaged 0.921 g of fresh belowground biomass, whereas plants treated with napropamide, regardless of application method, showed more than a 95% reduction in belowground biomass. The fresh aboveground biomass of nontreated plants averaged 4.9 g, whereas the broadcast treatment resulted in a 49% reduction compared with the nontreated control. The in-row and row-middle application treatments demonstrated intermediate responses, not differing from either the nontreated control or the broadcast treatment.
Terminated daughter plants at 4 wk after treatment (WAT) across simulated napropamide application methods. Specimens shown were destructively sampled to obtain pull force and aboveground and belowground biomass measurements.
Second Runner
Injury at 4, 6 and 8 WAT, and the number of new daughter plants at 4 and 6 WAT indicated no treatment differentiation, whereas all other second runner parameters indicated a treatment effect (Table 2). Injury symptoms from napropamide were the same observed in the first runner. At 2 WAT, injury was 20% following the broadcast treatment, which was equivalent to the 19% observed following the in-row treatment and greater than the row-middle treatment (10%). From 4 to 8 WAT, injury was similar across treatments.
Injury, new growth length, number of new daughter plants, pull force, and fresh aboveground and belowground biomass responses of strawberry daughter plants from the second runner at 2, 4, 6 and 8 wk after treatment with napropamide under simulated application methods.a–e
Table 2. Long description
The table presents data on the effects of different napropamide application methods on strawberry daughter plants. It includes measurements of injury, new growth length, number of new daughter plants, pull force, and biomass. The table has 4 rows and 11 columns. Column headers are Injury, New growth length, New daughter plants, Biomass, and units are percent, centimeters, number, Newton, and grams respectively. Row labels are Nontreated, Broadcast, In-row, and Row middle. Row 1: Nontreated, 2, 4, 6, 8 WAT, 31 a, 65 a, 100 a, 127 a, 2, 4, 34 a, 61.4 a, 30.7 a. Row 2: Broadcast, 20 a, 22 a, 25 a, 21 a, 6 c, 15 c, 31 c, 40 c, 1, 1, 1 b, 12.2 b, 3.9 b. Row 3: In-row, 19 ab, 18 ab, 18 ab, 16 ab, 14 bc, 31 bc, 53 bc, 70 bc, 2, 3, 3 ab, 18.3 b, 9.8 ab. Row 4: Row middle, 10 b, 12 b, 13 b, 10 b, 23 ab, 53 ab, 81 ab, 106 ab, 2, 3, 4 ab, 31.6 ab, 6 ab.
a Abbreviation: WAT, weeks after treatment.
b The second strawberry runner, bearing one daughter plant at the time of treatment, was destructively sampled at 8 WAT to obtain pull force and biomass measurements. Each experimental unit consisted of a three-pot system with a central pot containing a strawberry mother plant and two side pots that held the daughter plants following napropamide application.
c Data are pooled across two trials.
d Napropamide rate: 4.5 kg ai ha−1.
e Means followed by different letters within a column differ according to the Tukey HSD test (α = 0.05) for parametric data, or a Dunn test with Holm adjustment (α = 0.05) for nonparametric data.
New growth after broadcast and in-row treatments was reduced by more than 55%, 52%, 47%, and 45% at 2, 4, 6, and 8 WAT, respectively, compared with new growth on the nontreated control plants. New growth following the row-middle treatment was similar to that of nontreated plants and those that received the in-row treatment but greater than the broadcast application. At 8 WAT, both daughter plant production and pull force after the broadcast treatment were smaller than those of nontreated plants (two vs. four daughter plants; 97% reduction in pull force from 34 newton). The results from in-row and row-middle napropamide treatments were similar to those of both the broadcast and the nontreated control treatments for these parameters.
Broadcast and in-row napropamide treatments resulted in the greatest aboveground biomass reductions (80% and 70%, respectively) compared with the nontreated control (61.4 g), while the aboveground biomass from plants that received the row-middle application was similar to that of both the nontreated plants and the other treatments. In contrast, the greatest reductions (87% and 81%) in belowground biomass occurred with the broadcast and row-middle applications, respectively, compared with nontreated control plants (30.7 g), while the in-row belowground biomass was similar to that of both nontreated plants and other treatments.
Napropamide exposure caused vein‑localized white discoloration in the strawberry mother plant leaves, consistent with oxidative stress–related pigment loss reported in Brassica napus (Cui et al. Reference Cui, Zhang, Wu, Zhu and Yang2010). The mother plant’s tolerance to napropamide, evidenced by the lack of reductions in runner production or biomass despite foliar injury, is consistent with reports by other researchers who studied strawberry and other perennial species. For instance, Putnam and Hancock (Reference Putnam and Hancock1982) showed that applying napropamide (4.4 kg ha−1) 2 h after transplanting strawberries into bare ground did not compromise plant survival or development. Similarly, Derr (Reference Derr1994) reported that a broadcast application of napropamide (4.5 kg ha−1) applied immediately after seedlings of five perennial daisy species were transplanted into 4-L containers caused less than 16% injury when assessed at 4 WAT and did not affect plant stand. Those findings suggest that napropamide poses little risk when applied to established plants, which is consistent with our observations. However, the napropamide product label specifies that only row‑middle applications are permitted in plasticulture strawberries to minimize the herbicide’s contact with the foliage of mother plants; thus, the mother plant injury we observed under direct napropamide exposure does not reflect typical labeled use.
Daughter plant rooting was suppressed in all napropamide treatments, as evidenced by the near-total reduction in pull force and lack of belowground biomass at 4 WAT. This indicates that napropamide successfully targeted active root initiation and meristematic growth, likely by inhibiting DNA synthesis and cell division (Di Tomaso et al. Reference Di Tomaso, Rost and Ashton1988b). Reduced new growth length from the daughter plants may reflect translocation of napropamide from the roots to vegetative tissues through apoplastic movement, as described by Barrett and Ashton (Reference Barrett and Ashton1981) or it may result from oxidative stress (Cui et al. Reference Cui, Zhang, Wu, Zhu and Yang2010) caused by direct herbicide contact with vegetative tissues, as inferred from the vein-localized white discoloration observed in mother plant leaves. By 8 WAT, only the broadcast application of napropamide consistently resulted in shorter new growth, fewer new daughter plants, less pull force, and reduced biomass. The in-row and row-middle treatments were more variable, resulting in values that were similar to those of both nontreated control plants and those that received the broadcast application.
This reduction in effectiveness at 8 WAT may be attributed to the degradation of napropamide. Photolysis is a primary degradation pathway for this herbicide. For example, Donaldson and Miller (Reference Donaldson and Miller1996) reported up to 70% loss in loamy sandy soil after 14 d of sunlight exposure. If residue concentrations decline substantially within weeks of application, new or unaffected daughter plant tissue may initiate rooting later in the season. In support of this idea, Putnam and Hancock (Reference Putnam and Hancock1982) found that by 103 d after applying napropamide, daughter plant counts and rooting were comparable to those of a nontreated control. This lack of long-term rooting inhibition aligns with the transient suppression observed in the current study.
Our study successfully captured the interval between peak daughter plant rooting suppression and the subsequent recovery due to herbicide degradation, demonstrating that the effective window for applying napropamide as an aid for runner removal is narrow. These findings suggest that suppressing daughter plant rooting throughout the summer likely requires sequential herbicide applications, a strategy that should be further evaluated under field conditions.
Pendimethalin
Mother Plant
All mother plant data were not affected by herbicide treatment (data not shown). Injury from pendimethalin consisted of leaf distortion. Observed injury at 2 and 4 WAT was less than 6% across all application treatments. Similarly, the number of runners produced by the mother plant ranged from three to four at 2 WAT and from six to seven at 4 WAT across all treatments.
First and Second Runners
Injury at 4 WAT was affected by treatment, whereas all other data indicated no treatment differentiation (Table 3). Injury symptoms on the daughter plants consisted of leaf distortion and necrosis (Figure 5). At 2 WAT, injury from pendimethalin was similar across treatments, ranging from 10% to 14%. At 4 WAT, injury was 7% following the broadcast treatment, which was equivalent to the 5% observed following the row-middle treatment and greater than the in-row treatment (3%). New growth length was similar across treatments at 2 and 4 WAT. At 4 WAT, the number of new daughter plants initiated per runner from the new growth was three and the pull force required to remove the daughter plants ranged from 30 to 41 newton across treatments. Aboveground and belowground biomass were also similar across treatments, ranging from 16.6 g to 20.4 g and from 7.7 g to 11.7 g, respectively (Figure 6).
Injury, new growth length, number of new daughter plants, pull force, and fresh aboveground and belowground biomass responses of strawberry daughter plants from the first and second runners at 2 and 4 wk after treatment with pendimethalin under simulated application methods.a–d
a Abbreviation: WAT, weeks after treatment.
b The first and second strawberry runners, each bearing a single daughter plant at the time of treatment, were originally designated for destructive sampling at 4 and 8 WAT, respectively. Due to excessive rooting, both runners and their associated daughter plants were destructively sampled at 4 WAT to obtain pull force and biomass measurements. Each experimental unit consisted of a three-pot system with a central pot containing the strawberry mother plant and two side pots that held the daughter plants following pendimethalin application.
c Data are pooled across the first and second runners and across two trials.
d Pendimethalin rate: 0.8 kg ai ha−1.
e Means followed by different letters within a column differ according to the Tukey HSD test (α = 0.05).
Strawberry daughter plant injury at 2 wk after treatment (WAT) following simulated pendimethalin applications. Panels show representative leaves from each application method. Visible injury consisted of leaf distortion and necrosis.
Terminated daughter plants at 4 wk after treatment (WAT) across simulated pendimethalin application methods. Specimens shown were destructively sampled to obtain pull force and aboveground and belowground biomass measurements.
Pendimethalin had no effect on any measured parameter on the mother plant regardless of application method, which aligns with previous findings. Clay (Reference Clay1980) reported that pendimethalin applied over the top of strawberries and directly to the roots at 1.5 kg ha−1, a rate higher than that used in the present study, was safe. Similarly, Miller et al. (Reference Miller, Libbey and Maupin2013) found no reduction in runners or daughter plants after 2.5 mo, even when pendimethalin was applied at 2.24 kg ha−1 to established plants, which is a rate exceeding the current labeled range of 0.8 to 1.6 kg ha−1. The uniform runner production across all treatments further confirms that runner initiation and development in established mother plants is not disrupted by pendimethalin. Despite the lack of pendimethalin effect on the mother plant, the product label requires that it be applied in ways that avoid contact with the foliage of mother plants.
Pendimethalin failed to inhibit daughter plant rooting, which was the primary objective of this study. Daughter plants across all treatments rooted, established, and grew with no differences in new growth length, daughter plant number, pull force, or aboveground and belowground biomass. Only daughter plant injury at 4 WAT differed, with broadcast applications causing slightly more visible injury (7%) than in-row applications (3%), yet this did not translate into any meaningful suppression of rooting, establishment, or further growth.
The daughter plants in this study lacked roots at the time of herbicide exposure but they had one to two expanded leaves and were still attached to the mother plant via the runner stem. Yamazaki et al. (Reference Yamazaki, Hamamoto and Ikeda2008) demonstrated that daughter plants that remain connected to the mother plant exhibit higher photosynthetic rates and greater root development during early establishment compared with detached cuttings, and that daughter plants are strongly dependent on the mother plant for metabolite supply during this period. This sustained connection likely provided sufficient resources for the daughter plants to tolerate pendimethalin exposure and establish normally. In addition, the potting media remained near saturation because drip emitters were placed on the side pots after herbicide application and irrigation was applied frequently, and these high‑moisture conditions, together with warm greenhouse temperatures, likely favored microbial and hydrolytic processes known to accelerate pendimethalin degradation (Strandberg and Scott-Fordsmand Reference Strandberg and Scott-Fordsmand2004). As a result, pendimethalin likely dissipated rapidly, shortening the herbicide exposure period and reducing the potential for injury to daughter plants. Together, these factors help explain why pendimethalin failed to inhibit rooting under the conditions of this study.
The results of this study demonstrate that napropamide can effectively suppress daughter plant rooting when applied to row middles at a labeled rate (4.5 kg ai ha−1) for at least a month, showing its potential as a tool to aid runner removal in strawberry production. However, suppression was transient, with rooting recovery observed by 8 WAT, likely resulting from napropamide photodegradation over time. This suggests that a single napropamide application may not provide season‑long suppression. Because the maximum labeled rate per crop cycle was applied in this study, maintaining runner suppression throughout summer would likely require integration with other herbicides rather than repeated napropamide application to maintain suppression. In contrast, pendimethalin failed to inhibit daughter plant rooting under any application method, and its use as a stand-alone treatment for this purpose cannot be recommended. The combination of daughter plants remaining attached to the mother plant during herbicide exposure and the rapid degradation of pendimethalin under high soil moisture and warm temperatures likely limited herbicide uptake and prevented rooting inhibition. Nevertheless, Vinson et al. (Reference Vinson, Price, Kessler, Coneva, Mwanza and Price2021) found that pendimethalin tank-mixed with napropamide caused greater strawberry mother plant growth suppression than pendimethalin alone or other herbicide combinations. Although this response is unfavorable for mother plant growth, it may be advantageous in scenarios in which herbicides are directed to row middles for summer weed control, providing the added benefit of suppressing daughter plant rooting and therefore warrants further evaluation.
The methodology employed in this study provides a novel framework for screening herbicides as an aid in runner management and is particularly relevant for plasticulture strawberry production in the Midwest, where runners produced during the summer represent a management challenge for growers who intend to maintain their strawberry plants into a second production season. A key limitation of this framework is that the simulated row middles (side pots) received drip irrigation, creating wetter conditions than would occur in a field setting. Future greenhouse studies should refine this approach by withholding drip irrigation from the side pots and applying intermittent overhead irrigation to better approximate summer rainfall patterns. Future research should also build on this framework by screening additional herbicides, alone or in tank mixes, in the greenhouse, before progressing to field evaluation. Given the promising results observed with napropamide, summer field evaluations of this herbicide, including sequential applications of other herbicides within the limits listed on the product labels, should be prioritized as a next step toward developing a practical and effective runner management strategy for strawberry production.
Practical Implications
Growers who apply napropamide to the row middles for weed management in established plasticulture strawberry systems during the summer may benefit from the suppression of daughter plant establishment, which is expected to ease manual runner removal. Until field studies confirm these results, the effect of napropamide on daughter plant rooting should be considered an incidental benefit of weed control rather than a deliberate management strategy. Pendimethalin showed no utility for this purpose when applied alone and cannot be recommended until further research evaluates it under drier, field-realistic conditions or in combination with other herbicides.
Acknowledgments
We thank Celia Corado, Nicolle Salamanca, Lian Durón, Helen Nocito, Nathan Deppe, and Alexandra Jewel for assisting with this research.
Funding
This work was supported by the U.S. Department of Agriculture (USDA) Agricultural Marketing Service in collaboration with the Indiana State Department of Agriculture under Specialty Crop Block Grant A337-23-SCBG-22-204, and by USDA–National Institute of Food and Agriculture Hatch project 7000862. The contents of this article are solely the responsibility of the authors and do not necessarily represent the official views of the USDA.
Competing Interests
The authors declare they have no competing interests.
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