Rooting

Two node terminal and subterminal stem cuttings of three plant species [‘Nanho Blue’ butterfly bush (Buddleja davidii), ‘Catawba’ crape myrtle (Lagerstroemia indica), and ‘Phantom’ hydrangea (Hydrangea paniculata)] were collected 19 May 2020 from container-grown stock plants. Standard procedures were used to prepare cuttings based on Davies et al. (2018). All cuttings were dipped in rooting hormone (1000 ppm indole-3-butyric acid + 500 ppm napthaleneacetic acid; Dip’N Grow, Clackamas, OR) for 3 s and a single cutting was inserted into each container (2.5-inch diameter, SVD250; T.O. Plastics, Clearwater, MN) filled with a 100% pine bark substrate amended with 6 lb/yard³ 18N–2.6P–6.6K controlled-release fertilizer (Nutricote 18–6–8 Total Type 180; Florikan Corp., Sarasota, FL), 1 lb/yard³ micronutrient fertilizer (Micromax; ICL Specialty Fertilizers, Dublin, OH), and 0.5 lb/yard³ wetting agent (AquaGro 2000G; Aquatrols, Paulsboro, NJ). Mulches included coarse vermiculite (#2A; Thermo-O-Rock, New Eagle, PA), rice hulls (Riceland Foods, Stuttgart, AR), paper pellets (MPP, Wolcott, NY), and pine pellets (Tractor Supply Co., Brentwood, IN). A non-treated control (nonmulched containers) was also included and consisted of containers filled with pine bark substrate. Coarse vermiculite and rice hulls were selected due to current widespread use as a substrate component (vermiculite) or mulch in container-grown nursery crop production (rice hulls), and paper and pine pellets were selected due to uniform small particle size and widespread availability. Mulches were applied (before sticking cuttings) at two depths (0.5 and 1 inch) level with the top of the container. Pine and paper pellets expand after exposure to water, thus pellets were applied at a sufficient thickness to achieve a final depth of 0.5 or 1 inch to the top of the container following saturation. After containers and mulches had been saturated, cuttings were stuck (single cutting per container; 25 cuttings per treatment), completely randomized (within plant species), and placed in a shade house (50% shade) under intermittent mist (10 s every 8 min from 6:00 am to 9:00 pm). The study was ended when sufficient rooting had occurred for the nontreated control of each crop species. Rooted cuttings were harvested on 16 July (‘Phantom’ hydrangea), 29 July (‘Nanho Blue’ butterfly bush), and 8 Sept. (‘Catawba’ crape myrtle), roots were washed and rooting percentage, root dry weight, and shoot dry weight (‘Catawba’ crape myrtle and ‘Nanho Blue’ butterfly bush only) were measured. Digital root analysis (total root length and root volume) of ‘Catawba’ crape myrtle and ‘Phantom’ hydrangea was completed using WinRHIZO software (Reagent Instruments, Quebec, Canada).

Weed control efficacy

In a separate study, four weed species were evaluated and included creeping woodsorrel (Oxalis corniculata), hairy bittercress (Cardamine hirsuta), large crabgrass (Digitaria sanguinalis), and mulberry weed (Fatoua villosa). Container substrate and mulches were prepared as described for Rooting and weed seed was applied to the mulch surface. Weed seeds were surface-applied to simulate infestation from neighboring containers or weeds in surrounding areas. Although container substrates can become infested with weed seeds before use if not stored properly, substrates used for propagation are typically weed free. A separate set of containers was used for each weed species and 20 (creeping woodsorrel, hairy bittercress, mulberry weed) or 30 (large crabgrass) seeds were sown (28 Feb.) per container (eight replicates per treatment). Containers were completely randomized (within species) and placed in a greenhouse (covered with polyethylene plastic) under intermittent mist (as described earlier). Weed seed germination (percent) was recorded at 2, 4, and 6 weeks after sowing (WAS). At 7 WAS, shoot fresh weight was collected for all weed species.

Mulch germination bioassay

Each mulch was mixed with deionized water (500 mL mulch:200 mL water) and allowed to saturate for 24 h before filtering the solution through a standard coffee filter (white, 8–12 c; Great Value, Bentonville, AR). Two filter paper circles (90 mm, Whatman 598; GE Healthcare, Amersham, UK) were saturated in the filtrate then placed in a petri dish (100 × 15 mm; VWR International, Radnor, PA). Seeds (20 per dish) of two bioindicator species [lettuce (Lactuca sativa) and tomato (Solanum lycopersicum)] were sown in separate dishes (three replicates/mulch/species), lids were replaced, and dishes were sealed with parafilm. All dishes were placed in a growth chamber (26/20 °C day/night temperature, 14 h photoperiod) for 2 weeks then seed germination was recorded.

Rooting percentage and weed seed germination data were analyzed with generalized linear models using the binary distribution and a logit link function using the GLIMMIX procedure of SAS (ver. 9.3; SAS Institute, Cary, NC). All other data were analyzed with linear models using the GLIMMIX procedure of SAS and differences between treatment means were determined using the Shaffer simulated method (P < 0.05).

Rooting

Neither mulch type nor mulch depth affected rooting percentage of the three species. Rooting percentage for the non-treated control was 75% (‘Nanho Blue’ butterfly bush), 100% (‘Catawba’ crape myrtle), and 100% (‘Phantom’ hydrangea). Average rooting percentage for 0.5- and 1-inch mulch depth was, respectively, 58% and 54% (‘Nanho Blue’ butterfly bush), 98% and 100% (‘Catawba’ crape myrtle), and 100% and 96% (‘Phantom’ hydrangea). Although ‘Nanho Blue’ butterfly bush rooting percentage was 15% to 35% lower in the mulched treatments compared with the non-treated control, a high amount of variability within each treatment likely resulted in no statistical differences for ‘Nanho Blue’ butterfly bush. Other rooting parameters, however, were affected by mulch type/depth but response varied by crop species. Root dry weight of ‘Nanho Blue’ butterfly bush was similar among treatments (Table 1). ‘Nanho Blue’ butterfly bush shoot dry weight was reduced by paper pellets at both mulch depths but was similar for all other treatments compared with the non-treated control (Table 2).

Table 1.

Root dry weight of three ornamental crop species treated with four mulches at two depths [0.5 and 1 inch (1.27 and 2.54 cm)].

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Root dry weight of ‘Catawba’ crape myrtle was similar to the nontreated control for all treatments except paper pellets where root dry weight was lower at both mulch depths (0.5 and 1 inch) (Table 1). Root length and volume of ‘Catawba’ crape myrtle were similar to the nontreated control for vermiculite at both mulch depths and for pine pellets at 0.5-inch depth. Root growth (length and volume) of ‘Catawba’ crape myrtle was lower for all other treatments compared with the non-treated control and both root length and volume were more than 50% lower in paper pellets (Table 3). ‘Catawba’ crape myrtle shoot dry weight was similar to the non-treated control for vermiculite at both mulch depths along with rice hulls and pine pellets at 0.5-inch depth (Table 2). All other treatments had reduced shoot dry weight compared with non-treated control, and it was more than 50% lower for paper pellets at both mulch depths.

Table 2.

Shoot dry weight of two ornamental crop species treated with four mulches at two depths [0.5 and 1 inch (1.27 and 2.54 cm)].

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Table 3.

Total root length and root volume of two ornamental crop species treated with four mulches at two depths [0.5 and 1 inch (1.27 and 2.54 cm)].

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Table 4.

Percent germination [2, 4, and 6 weeks after sowing (WAS)] of creeping woodsorrel and hairy bittercress seeds sown to four mulches at two depths [0.5 and 1 inch (1.27 and 2.54 cm)].

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Root dry weight of ‘Phantom’ hydrangea was similar among all the treatments (Table 1). Root length and volume of ‘Phantom’ hydrangea was lower in paper pellets at the 1-inch depth compared with the nontreated control but was similar to the non-treated control for all the other treatments (Table 3).

Among the mulches used in our study, vermiculite had the least effect on rooting of all three crop species evaluated. Rice hulls did not affect root dry weight but reduced root length and volume of ‘Catawba’ crape myrtle. Vermiculite is widely used in container substrates for greenhouse-grown crops and improves substrate water and nutrient retention (Robbins, 2018). Parboiled rice hulls have been identified as an alternative substrate component and are an agricultural by-product (Buck and Evans, 2010; Currey et al., 2010; Evans, 2008). Because vermiculite and rice hulls can be used as a substrate component without any negative effect on plant growth, the potential negative impact rice hulls may have on rooting cuttings must be considered when applied as a mulch during propagation.

Pine pellets are composed of compressed sawdust, whereas paper pellets are made of compressed recycled paper. No additional ingredients were listed on the packaging, but other chemicals may be used in the manufacturing process and could have phytotoxic properties. Results from the seed germination bioassay showed pine pellets exhibited phytotoxic properties and inhibited lettuce and tomato germination (data not shown). Lettuce germination was 1.6% (pine pellets), 78.3% (paper pellets), and 100% for all other treatments, whereas tomato germination was also lowest for pine pellets (28.3%) but more than 93% for all other treatments. The phytotoxic compounds may have contributed to reduced root development in cuttings with pine pellets but does not explain reduced root growth of ‘Catawba’ crape myrtle in rice hulls.

Other factors that could affect rooting of cuttings and subsequent shoot growth include mulch depth and physical properties of the mulch. Certain mulches may retain more water and potentially limit water movement or moisture content in the rooting zone. Rice hulls had greater porosity (air space) compared with the other mulches and the pine bark substrate (data not shown), thus varying moisture content along the portion of the stem inserted in the container could have resulted in reduced root initiation and growth. The cuttings were inserted ≈2 inches into the container resulting in 25% to 50% of the inserted portion of the stem in contact with mulch depending on mulch depth. Mulch depth also offsets the volume of pine bark substrate by up to 30% (1-inch mulch depth), which could have reduced the amount of nutrients available for root and shoot growth, but substrate nutrient content was not measured.

Weed control efficacy

Germination percentage for creeping woodsorrel at 2 WAS was lower for all mulch treatments compared with the non-treated control (Table 4). At 6 WAS, rice hulls at both depths and pine pellets and paper pellets at 0.5-inch depth reduced germination of creeping woodsorrel by more than 46% whereas pine pellets at 1-inch depth reduced germination by 74%. Creeping woodsorrel shoot fresh weight was lowest in rice hulls, pine pellets, and paper pellets at both depths (Table 5). Rice hulls at both mulch depths lowered the shoot fresh weight of woodsorrel by more than 90%, whereas no measurable shoot fresh weight was recorded for pine pellets or paper pellets (both depths).

Table 5.

Shoot fresh weight (7 weeks after sowing) of four weed species (creeping woodsorrel, hairy bittercress, large crabgrass, mulberry weed) from seeds sown to four mulches at two depths [0.5 and 1 inch (1.27 and 2.54 cm)].

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Germination percentage for hairy bittercress at 2 WAS lowest for pine pellets followed by, paper pellets, followed by rice hulls compared with the non-treated control at both mulch depths (Table 4). Rice hulls reduced hairy bittercress germination by more than 25%, whereas pine pellets and paper pellets reduced germination by more than 59% at both mulch depths compared with the non-treated control. By 4 WAS, pine pellets (0.5 and 1 inch) were the most effective treatments and reduced hairy bittercress germination by more than 41% in comparison with the non-treated control. At 6 WAS, hairy bittercress germination was lowest for pine pellets at the 0.5-inch (3.8%) and 1-inch (0.0%) depths. Hairy bittercress shoot fresh weight was lowest in pine pellets and paper pellets at both mulch depths (Table 5).

Germination percentage of large crabgrass at 2 WAS was lowest for rice hulls at 0.5 inch, pine pellets at both depths, and paper pellets at 1 inch (Table 6). By 4 WAS, only pine pellets (1 inch) reduced percent germination of large crabgrass (by 22%) compared with the non-treated control. At 6 WAS, germination was similar among all the treatments. Large crabgrass shoot fresh weight was lower for all the treatments compared with the non-treated control except vermiculite (0.5 inch) (Table 5). Pine pellets at both mulch depths lowered the shoot fresh weight of large crabgrass by more than 99%. Paper pellets at 0.5- and 1-inch depth lowered the shoot fresh weight by 99% and 100%, respectively.

Table 6.

Percent germination [2, 4, and 6 weeks after sowing (WAS)] of large crabgrass and mulberry weed seeds sown to four mulches at two depths [0.5 and 1 inch (1.27 and 2.54 cm)].

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At 2 WAS, mulberry weed germination percentage was similar or greater for all mulches compared with the non-treated control (Table 6). At 4 WAS, mulberry weed germination was lower for rice hulls (0.5 and 1 inch) and paper pellets compared with the non-treated control. At 6 WAS, rice hulls at both mulch depths and paper pellets at 1-inch depth lowered the germination by more than 24%. Mulberry weed shoot fresh weight was lowest in pine pellets and paper pellets at both mulch depths and was reduced more than 92% compared with the non-treated control (Table 5).

Weed seed germination in the non-treated control varied by species and ranged 58% to 98%. A screened pine bark was used as the rooting substrate, but many growers use peat-based substrates for propagation which have smaller particles size and would likely result in similar or greater weed seed germination. Weed control efficacy can vary depending on the type of mulch used and application depth. In agreement with our results, weed control efficacy of loose-fill mulches generally increased with an increase in the application depth (Cochran et al., 2009; Knox et al., 2015; Penny and Neal, 2003; Smith et al., 1997). Mulch applied at adequate depths, generally 1 inch or greater, has provided better control of common nursery and landscape weed species compared with preemergence herbicides (Bartley et al., 2017; Burrows, 2017; Marble et al., 2017; Saha et al., 2019). In our study, pine pellets and paper pellets at 0.5- and 1-inch depth resulted in minimal growth of four weed species (creeping woodsorrel, hairy bittercress, large crabgrass, mulberry weed). Although weed seeds germinated on the surface of pine and paper pellets and were still alive at 6 WAS, seedlings only developed two true leaves and did not accumulate a measurable amount of shoot biomass.

Pine pellets and paper pellets provided a solid compact barrier which may have inhibited the weed seedling roots from penetrating the substrate surface and accessing nutrients within, thus limiting shoot growth. In a study by Smith et al. (1998), recycled wastepaper pellets applied to a depth of 1 inch completely suppressed prostrate spurge germination in container production. In a study by Altland (2019), rice hulls at 0.5- or 1-inch depth provided 100% control of bittercress and liverwort growth in containers. Similarly, Altland et al. (2016) reported parboiled rice hulls provided effective control of creeping woodsorrel and flexuous bittercress when applied at 0.5 to 1-inch depth over the container substrate surface. In our study, rice hulls reduced shoot weight of creeping woodsorrel and large crabgrass but were not effective on hairy bittercress or mulberry weed. Although rice hulls work well in controlling weeds in container production, the frequent irrigation and high moisture environment of propagation negated the hydrophobic properties of rice hulls and provided adequate conditions for weed seed germination and shoot fresh weight. Vermiculite was also not effective at suppressing weed shoot weight likely due to increased moisture content of the mulch layer and ineffective surface barrier.

Low fertility, large particle size, and hydrophobic nature are desirable properties of mulches for container-grown crops but may not have the same impact or be practical in the propagation environment. We demonstrated mulch products with small particle size (paper and pine pellets) provided excellent weed control efficacy (percent control of the non-treated control) for creeping woodsorrel (100%), hairy bittercress (98% to 100%), large crabgrass (96% to 100%), and mulberry weed (100%) during cutting propagation. Nevertheless, paper and pine pellets may reduce cutting root growth of certain crop species when applied at a depth greater than 0.5 inch. Paper and pine pellets applied at 0.5-inch depth would be a viable option for crops propagated in greenhouses and on crop species that are known to be herbicide sensitive such as ‘Phantom’ hydrangea. Longevity of weed control efficacy has not been widely studied, but preliminary trials conducted by the authors (data not shown) indicate mulches would be effective on hairy bittercress and large crabgrass for 8 to 10 weeks. These products are readily available, but nursery propagators would need to determine the cost-effectiveness of using mulches compared with the labor and time associated with hand weeding. Growers would also need to alter practices such as filling flats/containers to accommodate the top mulch layer. Some plant species may be more sensitive to differences in chemical and physical properties in the rooting zone, therefore growers should conduct small trials with individual crop species before large scale adoption.

Units

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