Effect of Weed Competition on Growth of Container Grown Ornamentals Plants in Four Different Container Sizes

Abstract

The objective of this study was to assess the growth of two woody ornamental plants when subjected to different levels of weed competition in four different container sizes, representing different stages of production. Ligustrum (Ligustrum lucidum W.T.Aiton) and Japanese holly (Ilex crenata Thunb.) liners were potted individually into 3.8 L, 11.4 L, 24.7 L, and 56.8 L containers, respectively. Weed coverage of 0%, 50%, and 100% in each container size was maintained by surface sowing seeds of six common nursery weed species by volume, based on media surface area in each pot. Results showed that the shoot dry weight of ligustrum at 50% and 100% weed levels was reduced by 28% and 35%, 55% and 56%, 41% and 43%, and 12% and 14% in 3.8 L, 11.4 L, 24.7 L, and 56.8 L containers, respectively. The shoot dry weight of Japanese holly at 50% and 100% weed levels was reduced by 18% and 22%, 51% and 52%, 51% and 53%, and 40% and 53% in 3.8 L, 11.4 L, 24.7 L, and 56.8 L containers, respectively. Results indicate that weed competition at 50% and 100% weed level was similar across all four container sizes, and weeds remained competitive even in the larger container sizes.

1. Introduction

Weed management in container-grown nursery crops continues to be one of the most challenging and costly aspects of production because of limited herbicide availability due to phytotoxicity concerns, leading to heavy reliance on hand weeding [1]. Additionally, weed competition combined with a limited volume of containers can significantly affect plant–water relations, nutrient uptake, respiration, root and shoot growth, and overall yield [2]. This restricted root environment can make ornamentals more susceptible to weed competition, with many authors reporting growth reductions of 50–80% over the span of only a few months of production [3,4,5]. Presence of only one plant of common weed species, such as redroot pigweed (Amaranthus retroflexus L.) or large crabgrass (Digitaria sanguinalis (L.) Scop.), in a 2.4 L container decreased the dry weight of Japanese holly by up to 60% in one of the first reports of container weed competition [4]. In another study, bush cinquefoil (Potentilla fruticose Gray. ‘Gold Drop’) dry weight was reduced by as much as 52% in a season due to presence of one weed in the container [5]. The same authors reported that a population density of five weeds per container consisting of barnyardgrass (Echinochloa crus-galli (L.) P. Beauv.), Digitaria sanguinalis, or Chinese foxtail (Setaria faberi R.A.W.Herrm.) and competing for 83 days decreased shoot dry weight of Bailey’s redosier dogwood (Cornus sericea L. ‘Baileyi’) by 72% [6]. Other authors have also documented the detrimental impact of common weed species including coffeeweed (Senna obtusifolia (L.) H.S. Irwin and Barneby), cocklebur (Xanthium strumarium L.), eclipta (Eclipta prostrata L.), prostrate spurge (Euphorbia prostrata Aiton), Echinochloa crus-galli, and Setaria faberi on numerous ornamental species grown in containers [6,7,8,9].

In addition to reductions in plant size, weed competition or the presence of weeds, in general, can reduce the marketability and salability of ornamentals that are grown primarily for aesthetic value [3]. Weed competition can cause an ornamental plant to reduce leaf size, have fewer flowers, and result in low vigor [10,11]. For instance, Walker and Williams [5] reported that flower production of bush cinquefoil ‘Gold Drop’ was reduced by 99% due to the presence of Digitaria sanguinalis. Even when excluding quantifiable aesthetic attributes such as flower number, leaf size, or overall growth, many end customers will reject container plant material if they are infested with weeds for fear of creating future weed issues, either in their own nurseries or in the landscape after transplanting.

While research has previously focused on the competitive effects of weeds in containers, many questions remain, especially when considering how well competition is understood in many fields and/or agronomic cropping systems. For instance, in many agronomic crops, economic thresholds have been developed, aiding producers with knowledge of when during the production cycle the cost of weed control equals the increased return in yield [12,13,14,15,16]. Additionally, it is difficult to derive a significant amount of information with individual models due to the differences between container-grown ornamentals and in-ground-grown plants. Thousands of ornamental taxa are produced in the nursery industry and the number of common weed species that can infest ornamentals present unique challenges in fully understanding competition dynamics inside a container. In contrast to many agronomic crops, ornamentals are not grown for a season and harvested, but are often in production for several years depending on the grower/nursery and their end consumer; therefore, crop scheduling is not feasible for most woody ornamental growers [17]. For instance, some nurseries specialize in propagation through vegetative means, and others produce smaller container plants, such as a 3.8 L or 11.4 L container-sized material.

In the majority of previous competition studies focusing on container-grown ornamentals, one container size was used, and single or multiple weed species were grown at differing densities to determine their effect on the growth, size, flowering, or other aspects that would be important to a consumer [3,4,5,6,7,8]. With this research, we can more clearly understand the role of weeds on crop characteristics, but previous research has almost exclusively focused on one or two container sizes, and it is unclear how weeds compete with crops throughout production. Previous studies have shown that certain weed species may be less competitive with ornamentals in larger container sizes as there is more room for growth and the fact that the ornamental liner is going to be comparatively larger, thus able to be competitive for resources [3,4]. A clear understanding of the timing in a production cycle (i.e., container size) when weeds tend to be most competitive would help growers of multiple container-sized plants better understand at which point weeds are most detrimental. In addition, this information helps provide baseline data for expanding upon previous research focusing on container weed-crop competition. Based on our hypothesis that weeds will be more competitive with ornamentals in smaller containers, we examined the growth of two common woody ornamental plants when subjected to different levels of mixed-weed competition in four different container sizes to determine at which point weeds would be most competitive, and what container size(s) would be ideal candidates for future container weed competition studies.

2. Materials and Methods

Experiments were conducted at the Mid-Florida Research and Education Center in Apopka, FL. On 20 March, four different sized liners of ligustrum (Ligustrum lucidum W.T.Aiton) and Japanese holly (Ilex crenata Thunb.) including 0.2 L, 0.9 L, 3.0 L, and 13.6 L that were originally grown from stem cuttings and purchased from a local nursery were potted individually into 3.8 L (17.8 cm height, 19.7 cm diameter), 11.4 L (24.1 cm height, 27.9 cm diameter), 24.7 L (29.2 cm height, 35.6 cm diameter), and 56.8 L (40.6 cm height, 45.1 cm diameter) sized nursery containers, respectively. All containers were filled with a substrate composed of pine bark: peat: sand mix (80:10:10 v:v:v) amended with dolomitic lime to yield a pH of 5.5 and fertilized with a controlled release fertilizer (CRF) (Osmocote^® Plus micronutrients 21-4-8 N-P-K (8–9 mo), ICL Specialty Fertilizers, Dublin, OH, USA) at 4.7 kg m⁻³. In addition to four sizes of containers, three weed levels comprising 0%, 50%, and 100% coverage of the container surface were maintained with respect to the container size. Five common nursery weed species were seeded by first mixing seeds with sand and sowing by hand on the media surface based on the surface area of each container size and weed competition level (

Table 1. Substrate surface area and weed seeds sown in 3.8, 11.4, 26.5, and 56.8 L nursery pots to maintain 0%, 50%, or 100% weed coverage levels.

		Competition Level a
Pot Size b	Surface Area	0%	50%	100%
L	cm2	Seeds/Pot c
3.8	324.13	0	5	10
11.4	612.84	0	10	20
26.5	992.64	0	15	30
56.8	1507.02	0	25	45

^a Competition level represents the surface of the potting substrate that was occupied by weed species on a 0–100% scale where 0 = 0% weed coverage (hand weeded every 2 wk), 50 = 50% of the surface covered by weeds (50% of the substrate surface maintained free of weeds with hand weeding), and 100 = 100% weed coverage and no hand weeding. ^b Pot size is shown in volume. Top diameters for 3.8, 11.4, 26.5, and 56.8 L pots were 20.3, 27.9, 35.6, and 43.8 cm, respectively. ^c Seeds/pot shows an approximate number of Eclipta prostrata L., Pilea microphylla (L.) Liebm., Phyllanthus tenellus Roxb., Euphorbia hirta L., Ageratum houstonianum Mill., and Bidens alba DC. seeds were sown for each pot size to establish respective weed coverage levels. Seeds of each species were measured by volume and mixed with bagged construction sand prior to sowing, with the exception of Bidens alba DC., which was sown separately and not mixed with sand.

).

The five weed species consisted of Eclipta prostrata, phyllanthus (Phyllanthus tenellus Roxb.), garden spurge (Euphorbia hirta L.), artillery weed (Pilea microphylla (L.) Liebm.), and bluemink (Ageratum houstonianum Mill.). A sixth species, Spanish needles (Bidens alba DC.) was sown separately but not mixed with sand prior to sowing. All six species are classified as warm-season annuals to short-lived perennials and are common troublesome species in Florida nurseries. Seeds from each species were collected from wild populations in central and south Florida during the summer prior to initiating the experiment. Germination was assessed prior to beginning the experiment, and all species reached approximately 50% germination. Weed competition levels of 0% and 50% were maintained by inspecting and hand-weeding pots on a biweekly basis. For the 0% competition level, all weeds were removed, and weeds removed from half of the media surface were considered as the 50% competition level. The 100% competition level was not weeded. All plants were placed on a full sun nursery pad, irrigated 1.3 cm per day via overhead irrigation (Xcel-Wobbler; Senninger Irrigation, Clermont, FL, USA.) via two irrigation cycles (7:00 a.m. and 2:45 p.m.), and evaluated at either 48 or 60 weeks after planting (WAP). Plants in 3.8 L and 11.4 L containers were evaluated for 48 WAP, and plants growing in 24.7 L and 56.8 L containers were evaluated for 60 WAP. Since 24.7 L and 56.8 L containers had larger volumes for the plants to grow, they were grown for an additional 12 WAP for plants to reach a mature salable size. At 24 WAP, all the pots were fertilized with 17N–2.2P–9.1K CRF (Osmocote Blend 17–5–11 (12–14 months); ICL Specialty Fertilizers, Dublin, OH, USA) via topdressing at a rate of 31 g, 94 g, 188 g, and 244 g per container for the 3.8 L, 11.4 L, 24.7 L, and 56.8 L sized containers, respectively.

Data collection included plant growth index [(height + width at widest point + perpendicular width) ÷ 3] measured every 12 weeks after planting. At the conclusion of the study (either 48 or 60 WAP), plant shoots were cut at soil level and roots were washed and dried in a forced-air oven at 60 °C for 7–21 days, reaching a constant weight for shoot and root dry weight collection. Following shoot dry weight collection, a subsample of leaves from ligustrum and Japanese holly were collected from three randomly selected replicates from each treatment and analyzed for total nutrient content by a commercial laboratory. The experiment was a completely randomized design with six single-container replications for each treatment and each ornamental species. Both experimental runs were initiated in March 2020 due to the long duration of the study.

Statistical Analysis

Data were subjected to analysis of variance using statistical software (JMP^® Pro ver. 14, SAS Institute, Cary, NC, USA). Before analysis, all data were tested to ensure that the ANOVA assumptions were met. As only three weed levels were included and the objective was to make all possible comparisons among weed levels on each evaluation date, post hoc multiple comparisons analysis was performed when appropriate using Tukey’s Honest Significant Differences test. Linear or quadratic trends for the percent reduction in plant growth index relative to weeded plants were evaluated over time using regression models. In all cases, significance levels were set as α = 0.05.

3. Results

Due to the variety of weed species utilized and the length of the experiments, the populations of each weed species were not monitored or recorded on each evaluation date. At 3 weeks after sowing, the emergence of all six species was observed in each container with Bidens alba being the predominant species due to a high germination rate and rapid shoot growth. In contrast, Pilea microphylla and Ageratum houstonianum were the species that comprised the lowest coverage in containers. Populations remained relatively constant until late December 2020 when temperatures of –1 °C were reached which resulted in some cold injury to Bidens alba and the death of many of the other warm season annual weeds occupying the containers (Figure 1).

3.1. Effect of Weed Density on Growth and Foliar Nutrient Content of Japanese Holly

At 12 WAP, the growth index of Japanese holly in all four container sizes was reduced when subjected to either 50% or 100% weed coverage levels (Figure 2).

In contrast, a significant growth index reduction was observed in all other container sizes through the end of the experiment, occurring at 48 WAP for 11.4 L containers and at 60 WAP for both 24.7 L, and 56.8 L containers (Figure 2). The growth reduction of Japanese holly grown in larger containers followed either a linear trend, increasing over time (11.4 L and 56.8 L at the 100% competition level) or there was no trend and competition remained relatively constant throughout the experiment (24.7 L containers and 56.8 L container at the 100% competition level (

Table 2. Growth reduction from 50% or 100% weed coverage over time in either Japanese holly or ligustrum grown in four different container sizes.

Ornamental Plant b	Container Size c	Weed Level d	WAP a					Trend e
			12	24	36	48	60
Japanese Holly	L	%	Growth Reduction f
			%
	3.8	50	23	27	25	10	----	Q ***
		100	31	32	28	17	----	Q **
	11.4	50	23	39	43	42	----	L **
		100	27	40	47	45	----	L ***
	24.7	50	12	19	16	17	23	NS
		100	18	19	21	22	24	NS
	56.8	50	13	16	16	15	19	NS
		100	14	17	15	18	25	L **
Ligustrum
	3.8	50	37	43	40	13	----	Q ***
		100	38	47	42	16	----	Q ***
	11.4	50	43	58	51	32	----	Q ***
		100	55	63	58	38	----	Q ***
	24.7	50	9	25	27	15	9	Q ***
		100	9	28	32	21	13	Q ***
	56.8	50	3	14	15	13	7	Q ***
		100	3	16	17	16	1	Q ***

^a WAP = weeks after planting. ^b Ornamental plants consisted of Japanese holly and ligustrum in four different container sizes. ^c Four different container sizes were used to grow either Japanese holly or ligustrum to estimate the effects of weed competition at different stages of production. Liner plants grown in 0.2 L, 0.9 L, 3.0 L, and 13.6 L were transplanted into 3.8 L, 11.4 L, 24.7 L, and 56.8 L containers, respectively. ^d Weed levels were based on a visual estimate of the container media surface and were maintained with biweekly hand weeding. Weed levels were established by surface sowing six common nursery weed species at trial initiation. ^e Linear (L) or quadratic (Q) trends of growth reduction in plants over WAP using model regressions at p < 0.01 (**), or 0.001 (***). NS = nonsignificant. ^f Growth reduction was calculated based on a percent decrease in plant growth index [(plant height + width + perpendicular width)/3] in comparison with the weeded (0% coverage) control group.

).

In contrast to the growth index, shoot dry weight showed a significant effect on Japanese holly biomass in all container sizes and at both weed levels (Figure 3).

Shoot dry weight reductions of 18% and 22%, 51% and 52%, 51% and 53%, and 40% and 53% were observed in the 3.8 L, 11.4 L, 24.7 L, and 56.8 L containers at the 50% and 100% weed levels, respectively. With the exception of the 56.8 L container, shoot weight reductions were similar among both weed levels. In the largest 56.8 L container, the 100% weed level further reduced Japanese holly shoot biomass by 22% compared with the 50% weed level, indicating a greater negative impact on shoot growth compared with the other smaller container sizes. Fewer differences were observed in root dry weight with reductions only observed in 11.4 L and 24.7 L containers, and further, no differences were observed among weed competition levels.

Foliar N concentrations for Japanese holly plants grown at 50% and 100% weed levels were reduced in 3.8 L, 11.4 L, and 56.8 L containers (

Table 3. Foliar nutrient concentration of Japanese holly grown in 3.8 L, 11.4 L, 24.7 L, and 56.8 L containers at 0%, 50%, and 100% weed coverage levels.

Ornamental Plant b	Container Size c	Weed Level d	Foliar Nutrient Concentration a
			%
Japanese Holly	L	%	N	P	K	S	Mg	Ca
	3.8	0	2.5 a f	0.14 a	1.14 a	0.24 a	0.41 b	1.31 a
	3.8	50	2.1 b	0.12 b	0.74 b	0.19 b	0.51 a	1.41 a
	3.8	100	2.0 b	0.10 c	0.67 b	0.18 b	0.54 a	1.58 a
	11.4	0	2.4 a	0.2 a	0.90 a	0.17 a	0.51 b	0.8 b
	11.4	50	2.2 b	0.19 a	0.70 b	0.15 b	0.70 a	1.1 a
	11.4	100	2.1 b	0.17 a	0.63 b	0.14 b	0.71 a	1.1 a
	24.7	0	1.9 a	0.11 a	0.85 a	0.25 a	0.65 b	1.93 a
	24.7	50	1.7 a	0.09 a	0.52 b	0.20 a	0.77 a	2.10 a
	24.7	100	1.7 a	0.08 a	0.51 b	0.22 a	0.78 a	2.10 a
	56.8	0	2.2 a	0.11 a	0.85 a	0.25 a	0.53 b	1.5 b
	56.8	50	1.8 b	0.09 b	0.54 b	0.25 a	0.65 a	2.0 a
	56.8	100	1.7 b	0.08 b	0.47 b	0.22 b	0.73 a	2.1 a
Survey value e			2.18–2.36	0.12–0.13	0.72–1.00	0.23–0.29	0.39–0.43	1.21–1.59
Ligustrum	L	%	N	P	K	S	Mg	Ca
	3.8	0	1.1 a f	0.11 b	0.58 b	0.14 a	0.21 a	1.51 a
	3.8	50	1.3 a	0.15 a	0.82 a	0.15 a	0.21 a	1.28 a
	3.8	100	1.3 a	0.15 a	0.83 a	0.15 a	0.22 a	1.27 a
	11.4	0	1.2 b	0.11 b	0.59 a	0.19 a	0.26 a	1.8 a
	11.4	50	1.4 ab	0.17 a	0.70 a	0.18 a	0.26 a	1.5 b
	11.4	100	1.5 a	0.18 a	0.67 a	0.18 a	0.27 a	1.6 ab
	24.7	0	1.0 a	0.10 b	0.64 a	0.17 a	0.23 a	1.59 a
	24.7	50	1.1 a	0.12 ab	0.68 a	0.16 a	0.23 a	1.41 a
	24.7	100	1.2 a	0.14 a	0.55 a	0.19 a	0.26 a	1.67 a
	56.8	0	1.2 a	0.10 a	0.54 a	0.21 a	0.31 a	1.8 a
	56.8	50	1.1 ab	0.10 a	0.53 a	0.23 a	0.29 a	1.8 a
	56.8	100	1.0 b	0.11 a	0.55 a	0.22 a	0.29 a	1.8 a
Survey value			1.44–2.34	0.16–0.19	1.77–2.09	0.16–0.27	0.28–0.32	1.21–2.22

^a Foliar nutrient content was analyzed on a subsample of leaves from plants at trial conclusion (48 or 60 weeks after planting). Leaves were collected from three randomly selected replicates from each treatment following shoot dry weight collection. ^b Ornamental plants consisted of Japanese holly and ligustrum in four different container sizes. ^c Four different sized container consisting of 3.8 L, 11.4 L, 24.7 L, and 56.8 L were used to grow plants with 0%, 50%, and 100% weed levels. ^d Weed levels consisted of 0% coverage, 50% coverage, and 100% coverage. Six common nursery weed species were seeded on the media surface with 50% and 100% coverage. ^e Survey values approximate the critical values for deficiency or toxicity obtained from Bryson et al, 2014. ^f Means followed by the same letter within a column are not significantly different according to Tukey’s HSD test at p < 0.05.

).

There was no difference in the foliar N concentration for plants growing in 26.5 L between any of the treatments. Foliar P concentration for plants at 50% and 100% weed levels were negatively affected in 3.8 L and 56.8 L containers, while there was no difference in any of the treatments in 11.4 L and 26.5 L containers. Foliar K concentration was reduced in all container sizes at both weed levels, but few consistent differences were observed in S, Mg, or Ca foliar concentrations.

3.2. Effect of Weed Density on Growth of Ligustrum

Similar to results observed with Japanese holly, the growth index of ligustrum plants in 3.8 L containers was lower when subjected to 50% or 100% weed levels through 36 WAP, but growth was similar among all treatments by 48 WAP (Figure 2).

Likewise, ligustrum plants in 11.4 L containers were significantly larger with no weed competition, growing 47% to 61% larger compared with plants grown at 50% or 100% weed levels. In contrast to results observed with Japanese holly, only the 100% weed level reduced ligustrum growth index by the conclusion of the experiment at the 24.7 L size, with no difference in growth index among plants at 0% or 50% weed levels. Similarly, while growth index reductions were observed on most evaluation dates throughout the experiment in 56.8 L ligustrum, no differences in growth were observed at 60 WAP. In all cases, reductions in ligustrum growth followed a quadratic trend where the greatest reductions in growth occurred from 24 to 48 WAP, similar to the trend observed in Japanese holly. This is likely due to an initial minimal effect of weed competition as weeds are becoming established, and then competition effects are potentially increased during the ideal growing season. This could result in weeded plants reaching their maximum or close to their maximum size at earlier dates and slowing their growth, thus limiting observed reductions in growth index.

While differences in growth index at the trial conclusion were only observed in plants grown in 11.4 L and 24.7 L containers, shoot biomass reductions were observed with ligustrum in all container sizes, but biomass reductions were similar among plants produced at 50% and 100% weed levels (Figure 3).

While shoot weight reductions were consistent among all plants subjected to weed competition, root weight reductions were only observed in plants in the 3.8 L or 11.4 L container sizes, indicating that weeds were possibly not as competitive with root growth at the two largest container sizes.

Foliar nutrient analysis for ligustrum revealed few clear and consistent trends with many macronutrient concentrations, and in some cases, N, P, or K levels were higher in plants subjected to weed competition compared with the hand-weeded control plants (Table 3).

4. Discussion

The growth index of both ornamental species revealed a clear and consistent reduction in growth on most evaluation dates as a result of competition similar to reports in other cropping systems [18,19]. This effect was diminished in only the 3.8 L Japanese holly and in the 3.8 L and 56.8 L ligustrum by the time plants were harvested with all plants having similar growth index. In most cases, the growth reduction followed a quadratic trend with the highest growth reductions occurring from 24 to 48 WAP. In these cases, it is likely that competition effects during the early weeks were minimal as weeds were becoming established. During the period of most active growth, weeds had become established in the weedy pots, and growth was slower for plants under competition; however, weeded plants (0% competition) were able to rapidly increase growth. Finally, by the later dates of these experiments where competition effects were reduced, weeded plants had already grown close to their maximum size by ~36 weeks, growth likely slowed due to container size constraints, and thus regardless of competition level, plants continued to grow and reached a similar size, albeit at a slower growth rate. As plants begin to outgrow a container, they can become stressed. Too small of a container volume for a plant can negatively impact the net assimilation rate and leaf area ratio, and potentially lead to water deficit stress or nutrient deficiencies as plants mature and out-grow the container [20]. Thus, results suggest that, in most cases, Japanese holly and ligustrum plants will continue to grow and can reach a marketable size when subjected to weed competition, although production times could be delayed by several months. Moreover, this effect could be different based on ornamental species. For instance, in the slower-growing Japanese holly which had not fully outgrown the 24.7 or 56.8 L container, growth was still reduced or was relatively constant throughout the study period.

While in some cases plant growth index was similar regardless of competition level at the final evaluation, biomass data showed clearly that both weed levels had a marked negative impact on plant growth, regardless of species or container size. As plants were not pruned, growth indices were likely not the best determinant of growth but were utilized as a non-destructive measure to determine differences in competition levels at each evaluation timing. Both species can tend to form thin canopies and long shoots when not sheared, thus resulting in differing results between biomass determination and growth measurements, especially when plants are mature, which has been observed previously with other ornamentals [21,22]. Competition can result in reduced crop growth or yield as a result of creating nutrient deficiencies [23], causing water stress, shading or light reduction [24], and space [25]. Additionally, large plants growing in small containers can also result in roots being ”pot-bound” which can further cause various negative consequences [26].

Nutrients and water were adequately supplied throughout these experiments, but it is unknown if all plants had adequate or the same access to nutrients due to the competition dynamics over the course of the experiment. Additionally, many weed species have been shown to have the ability to utilize these resources more efficiently than many crop plants, and the effect may be exacerbated in a smaller container size as space can become limited [27,28]. Competition for nutrients was possibly the cause of or contributed to the reduced biomass in Japanese holly where foliar N, P, and K concentrations were, in many instances, lower in plants under competition compared with weeded plants. Interestingly, there was an increase in Ca and Mg content in Japanese holly in several container sizes when subjected to weed competition. This phenomenon has been observed in several species with water stress, a weed competition mechanism that has been shown to increase the plant tissue concentrations of N, K, Ca, Mg, Na, and Cl under certain scenarios in different plant species [29,30,31]. In contrast, no clear trend was observed with ligustrum in which competition did not decrease N, P, and K concentrations in shoots. Such differences could be related to their optimal or critical values for nutrients (Table 3), their growth rate, or possibly other factors not captured in this study. Several field studies have shown that the nutrient content of plants depends on crop species, nutritional elements, and climate [32,33]. In addition to nutritional content, other factors such as water, space, and shading can be more detrimental depending on the crop plant and the environment or growing conditions [25], and the same case is likely applicable for ornamental plants due to the various taxa produced. In order to quantify specific competition dynamics, further research is warranted to determine what primary competition mechanisms led to reduced plant biomass in containers, which could be accomplished by potentially collecting shoot tissues for nutritional analysis over time, and measuring water stress, light levels, or root growth over time.

Regardless of container size or species, biomass reductions were observed and in cases where plant growth indexes were similar, weed competition resulted in longer production times in a nursery setting. As these experiments were preliminary in nature, weed populations were maintained only on an estimated visual coverage basis as a whole, and establishment or maintenance of individual populations of each species by count, placement within the container, or coverage was beyond the scope of this work. Thus, this research did not follow a classical weed competition experimental methodology [32], and any effects from competition with a singular weed species or consistent population of a group of weed species cannot be deduced from this work. However, results do show that weed competition, in relation to coverage percent of the container surface, will cause a detrimental effect on ornamental plant growth in many common marketable container sizes, and this effect will most likely be evident by an increase in production time. It is not clear why there was no consistent difference in the two weed levels, as competition effects would be expected to increase as weed density increased [32]. It is possible that competition effects would initially begin low but increase as weed density increased, and finally plateau once plants were exposed to a high enough level of weed pressure where no further effects would be realized. Similar results were observed by Fretz [4] where one Digitaria sanguinalis plant caused a similar reduction in Japanese holly biomass to thirty-two Digitaria sanguinalis plants. As there is limited volume in a container and weeds compete among themselves for space and resources, biomass per weed plant can decrease as weed number increases [34].

5. Conclusions

Overall, results from this experiment agree with previous literature and illustrate the detrimental effect of weed competition on container-grown plants. The shoot dry weight of Japanese holly at 50% and 100% weed levels was reduced by 18% and 22%, 51% and 52%, 51% and 53%, and 40% and 53% were observed in the 3.8 L, 11.4 L, 24.7 L, and 56.8 L containers, respectively. Whereas the shoot dry weight of ligustrum at 50% and 100% weed levels was reduced by 28% and 35%, 55% and 56%, 41% and 43%, and 12% and 14% in 3.8 L, 11.4 L, 24.7 L, and 56.8 L containers, respectively. The results of this study show that weed competition at both 50% and 100% weed levels can be detrimental to the growth of a container-grown plant. However, future research is needed to fully understand weed-plant competition in long-term containerized nursery plants and to develop weed control thresholds for ornamental plants from a growth standpoint. As more knowledge is developed on this research topic, we will be able to generate many of the same prediction and economic tools utilized by farmers in other cropping systems.