Materials and methods

Experiments were conducted at two central Florida nurseries in 2017 (Dover and Ruskin, FL), at a central Florida nursery (Dover), and at University of Florida Research and Education Centers in Apopka and Balm in 2018. Experiments were initiated at both nurseries on 25 Apr. Mulch treatments evaluated at both locations included PB mini-nuggets (Old Castle Lawn & Garden, Atlanta, GA), PB + a tackifier (Granite Seed, North Lehi, UT), a shredded hardwood mulch {HW [derived from melaleuca trees (Melaleuca quinquenervia) (Forestry Resources, Fort Myers, FL)]}, HW + tackifier, pine sawdust + tackifier, a recycled waste-paper mulch slurry (PM), PM + PB, and a 1.25-mil white polyethylene plastic film (Fig. 1). The tackifier was applied after mulch placement at a rate of 100 lb/acre using a hand-shaker. The tackifier was applied as a dry powder. Once the tackifier was moistened with irrigation and redried, it bound mulch materials together (Fig. 2). Waste-paper mulch was based on a mulch previously described by Cline et al. (2011) but was modified based on available equipment. Waste paper was first shredded and mixed with equal parts water by volume in 5-gal buckets and allowed to sit overnight. The tackifier mentioned previously was added to this paper mixture at 30 g/gal to achieve the desired consistency. A mixing paddle attachment was then used to mix the material into a slurry using a cordless drill. The paper slurry was then added to each pot by hand, completely covering the surface of the container. The plastic mulch was applied after plant liners had been planted by cutting the plastic into a sheet and securing it around the edge of the container using weatherproof tape. Plastic mulch was applied after pots were topdressed with fertilizer and was applied over the top of irrigation spray stakes. At all locations, the plastic mulch was applied by one person and it took ≈15 s per pot to place the plastic and secure it with tape. In all cases, mulch materials were initially applied to containers so that the container surface was completely covered. PB and HW were applied at 2-inch depth based on general recommendations for loose-fill organic materials (Chalker-Scott, 2007). Sawdust was applied at a depth of 1 inch based on grower practices. Sawdust is not a commonly used mulch material in container plant production in Florida, and different depths have not been evaluated. However, several growers have implemented sawdust as a mulch at depths of ≈1 inch with the goal of causing nitrogen immobilization as the sawdust degrades in the top portion of the pot where weed seeds are germinating (Foo et al., 2010). The paper slurry treatment has not been evaluated previously in container plant production, but we found through preliminary trialing that a depth of 1 inch would create the desired hard “crust” after drying, thus 1-inch depth was used for the paper slurry treatment. In addition to mulch treatments, a herbicide treatment of oxyfluorfen + pendimethalin (OH2; Everris NA, Dublin, OH) was included for comparison and applied at the manufacturer recommended rate of 2 + 1 lb/acre active ingredient using a hand-shaker. A nontreated, nonmulched control was also included for comparison. In Dover, treatments were applied to 15-gal podocarpus, and treatments were applied to 25-gal Chinese elms in Ruskin. At both locations, standard PB:peat substrate mixes were used and fertilizers were topdressed according to the growers’ standard practices. All regular maintenance and production activities were performed by nursery cooperators except hand weeding or use of any weed control measure other than spot treatment with postemergence herbicides as needed in the adjacent production areas. The trial was designed as a completely randomized block with four single pot replications per treatment at each location. Pots were weeded by hand on a monthly basis recording the time needed to weed each pot. Weeds were then shaken to remove loose soil from roots and weighed in the field using a portable digital scale. Pots were monitored for 6 months in Dover, at which time the trial was considered concluded. In Ruskin, pots were inadvertently weeded by employees in one instance and trees were laid down to prevent wind damage from Hurricane Irma in early September, resulting in mulch being removed from pots. Thus, weed control data collection was not analyzed from this location. In Ruskin, tree caliper and height were recorded at trial initiation and in October after the trial concluded, and initial and final growth index [(plant height + plant width at widest point + plant width perpendicular to widest point)/3] was recorded on podocarpus in Dover.

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Example of pine bark (PB), hardwood (HW), sawdust, paper slurry (PM), PB + PM, and plastic mulch treatments at potting. PB, HW, and sawdust treatments were also included with the addition of a guar gum tackifier (not shown).

Citation: HortTechnology hortte 29, 6; 10.21273/HORTTECH04437-19

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Hardwood mulch alone (left) and with the addition of a guar gum tackifier (right). Photo was taken after the tackifier had been watered in and subsequently dried.

Citation: HortTechnology hortte 29, 6; 10.21273/HORTTECH04437-19

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In 2018, the experiment was repeated at the nursery in Dover and at University of Florida research centers in Apopka and Balm. The trials were initiated on 19 Apr. in Dover and Balm and 26 Apr. in Apopka using the same mulch materials and methods described previously. ‘Brodie’ eastern red cedar [Juniperus virginiana (15 gal)] were evaluated in Dover using standard grower practices described previously. At research centers in Apopka and Balm, 1-gal ligustrum liners were obtained from local growers and potted into 7-gal containers. In Balm, the plants were potted using a 75:25 (v:v) PB:peat substrate and fertilized via subdressing with an 18N–2.6P–6.6K controlled-release fertilizer with micronutrients (Nutricote; Chisso-Asahi Fertilizer Co., Tokyo, Japan) at a rate of 161 g per container. In Apopka, a 80:20:10 PB, peat, sand (v:v:v) substrate was used with 17N–2.2P–9.1K controlled-release fertilizer (Osmocote Pro; ICL Specialty Fertilizers, Dublin, OH) applied via incorporation at a rate of 12 lb/yard³. Pots were irrigated daily using spray stakes at all locations.

Data collection included monthly weeding times and weed fresh weights at all locations for 6 months after treatments (MAT) were originally applied. Plant growth indices were collected on a subset of plants at trial initiation and at trial conclusion on ligustrum. Ligustrum liners were uniform at the time of potting, so six plants were randomly selected in Apopka and Balm to determine average initial growth index. Growth index was then recorded on all plants at trial conclusion at 6 MAT. Growth index was not recorded on eastern red cedar, as plants were pruned during the trial.

All data were subjected to a mixed model analysis of variance (ANOVA) after ensuring the assumptions of ANOVA were met. Weed control treatment was treated as a fixed effect and location, year, and replication (block) as random effects using JMP Pro software (version 14; SAS Institute, Cary, NC). Weeding time and weed weight data were analyzed by month and cumulatively over the entire experiment to determine which mulch material provided the most effective and consistent weed control under a variety of different environments. Ligustrum growth index was analyzed similarly. As growth evaluations for podocarpus and Chinese elm were not repeated in time or space, the data were analyzed as described previously but are only briefly discussed.

Results and discussion

At 1 MAT, PB, HW, paper slurry + PB, and plastic resulted in less weeding time than sawdust, but few other treatment effects were noted and all treatments reduced hand weeding time relative to the nontreated control (Table 1). All treatments reduced weed weight compared with the nontreated control at 1 MAT but no other treatment differences were observed at that time (Table 2). By 2 MAT, all treatments with the exception of both sawdust treatments and the herbicide treatment reduced total weeding time compared with the nontreated control, whereas all treatments with the exception of sawdust + tackifier reduced weed weights compared with the nontreated control. At 3 MAT, all treatments required similar hand weeding times but reduced weed weight relative to the nontreated control. At 4 MAT, plastic and PB + tackifier were the only treatments that significantly reduced both hand weeding time and weed weight relative to the nontreated control. Weeding time in PB + tackifier, plastic, PB, HW, paper slurry + PB, and sawdust treatments were similar at 5 MAT and all reduced weeding time relative to the nontreated control. These treatments also resulted in lower weed weights with the exception of sawdust. Pine bark + tackifier, paper slurry + PB, and plastic were the only treatments to reduce both weeding time and weed weight at 6 MAT.

Table 1.

Effects of mulch treatments on monthly and cumulative hand weeding times at nursery and research center locations in Florida.

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Table 2.

Effects of mulch treatments on monthly and cumulative weed weight at nursery and research center locations in Florida.

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Across all evaluation dates and trial locations, plastic mulch resulted in the greatest cumulative reduction in weeding time (93%), with PB, paper slurry + PB, and the HW treatment resulting in similar reductions (64% to 77% reductions). Treatments including HW + tackifier, sawdust, and the herbicide resulted in no significant reduction in weeding time compared with nontreated pots. All treatments reduced cumulative weed weight relative to the nontreated control. Plastic reduced weed weights greater than any other treatment (97%) with the exception of all PB treatments, HW, and the paper slurry, which were similar (70% to 91% reductions). When considering both weeding time and weed weight, plastic resulted in the greatest overall weed control, whereas PB, paper slurry + PB, and HW were the only treatments similar to plastic in both regards. Plastic mulch is used extensively in vegetable and fruit production (Sander, 2019) for weed control and has shown promise in preliminary container trials. Steed et al. (2018) reported a cumulative decrease in weeding time of 86% when 1-gal containers planted with ligustrum were covered in 1.25-mil plastic compared with a nonmulched control group. Although this method added minor production costs, labor savings resulting from reduced hand weeding far exceeded the added cost of installation, removal, and disposal of plastic mulch.

It is important to note that the herbicide treatment was applied only once over the course of 6 months to determine at which point mulch becomes more effective than a commonly applied herbicide. In this study, weeding time was similar between the herbicide treatment and the nontreated by 2 MAT, but reductions in weed weight were observed up until 4 MAT. In Florida, herbicides are typically applied every 8 to 10 weeks (Stewart et al., 2017). This coincides with our data showing that a reduction in weed weight was observed until 4 MAT, that is, a sequential application would have been required before 4 MAT to achieve acceptable control. Although mulch is more difficult to apply and costlier than herbicides, it does offer advantages in terms of length of control and spectrum of control. In this trial, treatments were inoculated with naturally occurring weed populations at trial locations and thus a broad spectrum of weeds was present. Oxyfluorfen + pendimethalin is a highly efficacious combination on many container weed species (Neal et al., 2017), but mulch will likely control an even broader spectrum of weed species due to its ability to create unfavorable environments for germination and growth (Saha et al., 2019).

The use of tackifier with PB, HW, or sawdust had little effect on weeding time or weed weight on most evaluation dates (Tables 1 and 2), similar to reports observed by Cline et al. (2011) in apple orchards. PB weeding time and weed weight were similar with and without tackifier on all evaluation dates and when considering cumulative weeding time or weight. Although sawdust and HW with and without tackifier were similar on most evaluation dates, in some instances greater weed growth or weed time occurred with the addition of tackifier.

Guar gum–based tackifiers as used in this experiment are commonly incorporated with organic mulch materials for use in hydroseeding and erosion management (Fehmi and Kong, 2012; Grismer and Hogan, 2005). In our study, tackifiers were evaluated primarily to determine if they could prevent mulch from blowing or washing out of the pots, which is a common concern in Florida because of frequent storm events during summer months. In our trials, little mulch was lost over the course of 6 months and the use of tackifiers provided no meaningful benefit in terms of weed control with PB, HW, or sawdust. However, tackifiers are often mixed with mulch materials before use, and tackifiers are primarily used with mulch that has been extensively processed and is of small particle size. With the exception of sawdust, the mulch evaluated here was composed of relatively large particles (Saha et al., 2019), thus tackifier had little influence on mulch characteristics.

Although tackifiers are typically incorporated with mulch before application, they were topdressed in these studies because of the difficulty of mixing at nursery locations, which could have reduced effectiveness. Trials also were conducted in Florida during primarily the summer months with frequent rainfall, which also likely prevented mulch blow out. Although tackifiers had little influence on results in this trial, they should be evaluated for use with other mulches, such as rice hulls, that provide a high level of weed control and dry quickly following irrigation (Altland and Krause, 2014; Altland et al., 2016) but are prone to blowing out of containers or removal by birds.

The paper slurry mulch (used alone) provided an advantage in reducing weeding time over the nontreated control on three evaluation dates (1, 2, and 5 MAT), a reduction in weed weight compared with the control on the first four evaluation dates. It also resulted in less cumulative weeding time and weed weight compared with the nontreated control. This paper slurry is applied wet, but soon dries to form a hard covering over the container surface. We did not notice it becoming hydrophobic at any time throughout the trials and was readily wettable with irrigation or rainfall. Higher levels of control were achieved at earlier evaluation dates, because over time, this material became prone to shrinking away from the container edge at later evaluation dates (Fig. 3). By 6 MAT, the material had begun to break down following high levels of rainfall, but still provided a cumulative advantage over the nonmulched control due to higher levels of control achieved during the first 3 months. Additional research is needed with this material and other spray-on mulches as described in Cline et al. (2011), because they could potentially be reapplied later in the production cycle. Addition of other tackifiers or polymers could also possibly extend longevity.

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Example of shrinkage observed with paper slurry mulch at later evaluation dates. Photo was taken at 3 months after trial initiation in Balm, FL.

Citation: HortTechnology hortte 29, 6; 10.21273/HORTTECH04437-19

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Growth evaluations of ligustrum at the two research locations show that plants increased in growth index by 45% to 70% over the 6-month study period and mulch type had no influence on plant growth (Table 3). Results for podocarpus in Dover and Chinese elm in Ruskin were also similar, with no difference in growth index increase in podocarpus and similar increases in tree height and caliper for elms regardless of treatment (data not shown). Although greater growth and establishment is often reported following mulch application to landscape plants (Chalker-Scott, 2007; Cregg and Schutzki, 2009), in many cases this is due to water conservation and preventing wilt after planting. In a nursery environment, water would be supplied each day and, thus, benefits to plant growth are likely to be less significant, as the nursery is an ideal growing environment.

Table 3.

Effects of mulch treatments on growth index and growth increase of ligustrum in Apopka and Balm, FL.

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Overall, data suggest mulch materials including PB, and plastic mulch would provide a high level of weed control in Florida nurseries. These treatments consistently outperformed a nontreated control and a single granular herbicide application on most evaluation dates. HW mulch provided similar control to plastic and PB on many evaluation dates and overall in terms of reductions in weeding time and weed weight. Mulch degradation was not assessed in this study, but mulch durability will have a significant effect on weed control over time (Foo et al., 2010; Ruggeri et al., 2016). Greater durability likely contributed to higher levels of control with the larger particle materials such as PB and HW, in addition to the plastic mulch at later evaluation dates. Sawdust and paper slurry may be more effective on shorter-term crops but did not show needed durability in these 6-month evaluations.

PB has been extensively studied as a mulch, but use of plastic is relatively new. Although some growth reductions to ligustrum have been reported in previous studies with liners in 1-gal containers (Steed et al., 2018), we observed no differences in growth rates of ligustrum or any of the other species evaluated. Differences in growth are likely due to differences in irrigation practices between our study and those used in Steed et al. (2018). In our trials, we used spray stakes that were positioned underneath the plastic, whereas Steed et al. (2018) used overhead irrigation on smaller (1 gal) containers. Steed et al. (2018) suggested that slower growth resulting from use of plastic was likely due to less available water and possibly greater accumulation of salts, as container leaching was minimal. Although leachates were not monitored in our study, placing spray stakes underneath plastic likely eliminated this potential issue, as evidenced by no differences in plant growth.

Use of plastic deserves further investigation in nursery crops because it could potentially mitigate many different production costs, including herbicides, hand weeding, fertilizer, and labor costs overall (Steed et al., 2018). Although plastic films could potentially provide advantages, there also would be disadvantages such as increasing the difficulty of monitoring irrigation (if irrigation emitters are placed under plastic) and significantly reducing the benefits of supplemental rainfall. Use of plastic sheets and tape would be cumbersome to apply on a commercial scale, but designing a plastic mulch product specifically for container nursery production, such as including drawstrings for securing the plastic on the pot, could make application more feasible. It is also important to note that use of polyethylene film is becoming more scrutinized due to negative environmental consequences resulting from plastic pollution (Brodhagen et al., 2017). When used in agricultural fields as a plastic mulch, removal is labor intensive, disposal is costly, and recycling may not be an option because soil particles can adhere tightly to the films. Another issue is that once the plastics age in the field, they are easily torn and small plastic particles contaminate soils and water bodies via runoff (Jambeck et al., 2015). Use of biodegradable plastics could be explored as a possible solution to alleviate environmental issues (Sintim and Flury, 2017), as could designing plastic mulch materials specifically for use in container nurseries to allow for reuse in multiple seasons and crops.

Costs are difficult to quantify because of many variables that affect mulch costs, but most mulches regardless of composition will be more costly than preemergence herbicide applications. However, in longer-term nursery crops in large spaced containers, the long-term control achieved with mulch and resulting reductions in hand weeding and herbicide over this longer period is likely to result in overall cost savings, as discussed in Bartley et al. (2017).

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