United States 

Department of 

Agriculture

Forest Service

Pacific Northwest 

Research Station

Research Paper

PNW-RP-587

April 2011

Quantifying Competitive 

Ability of Perennial Grasses 

to Inhibit Scotch Broom

Timothy B. Harrington
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Author

Timothy B. Harrington is a research forester, Forestry Sciences Laboratory,

3625 93rd Avenue SW, Olympia, WA 98512-9193.

Cover photos: Upper and lower photos, by Tim Harrington, were taken 72 and 113 

days after sowing seeds of Scotch broom and grasses.
Abstract

Harrington, Timothy, B. 2011. Quantifying competitive ability of perennial 

grasses to inhibit Scotch broom. Res. Pap. PNW-RP-587. Portland, OR: U.S. 

Department of Agriculture, Forest Service, Pacific Northwest Research Station. 

15 p.

Greenhouse pot studies were conducted to quantify the competitive abilities of 

three native perennial grass species to inhibit development of Scotch broom

(Cytisus scoparius (L.) Link) seedlings: spike bentgrass (Agrostis exarata Trin.), 

blue wildrye (Elymus glaucus Buckley), and western fescue (Festuca occidentalis

Hook.). In single-species stands (1) soil water content decreased with increasing 

grass density, (2) soil water depletion per plant differed among species as ratios of 

2.4:1.3:1 for bentgrass, fescue, and wildrye, respectively, and (3) average percentage 

of ground cover per plant was ranked by species as bentgrass (14 percent), wildrye 

(8 percent), broom (8 percent), and fescue (5 percent). Regression models predicted 

90, 85, and 72 percent reductions in average biomass per plant of broom when 

grown with approximately 250 plants/m2

 of bentgrass, wildrye, and fescue, respectively. Bentgrass and wildrye were more competitive than fescue because of their 

early-season depletion of soil water and rapid development of cover.

Keywords: Addition series, induced competition, soil water content, biomass.
Summary

Scotch broom (Cytisus scoparius (L.) Link) is a nonnative shrub that has invaded 

savannah and woodland sites throughout the United States, Australia, and New 

Zealand. Greenhouse pot studies were conducted to evaluate the competitive 

abilities of three native perennial grass species to inhibit development of broom 

seedlings: spike bentgrass (Agrostis exarata Trin.), blue wildrye (Elymus glaucus

Buckley), and western fescue (Festuca occidentalis Hook.). In separate addition 

series studies, four densities of broom seedlings were grown for 140 days with and 

without four densities of each grass species. In single-species stands (1) soil water 

content decreased with increasing grass density, (2) soil water depletion per plant 

differed among species as ratios of 2.4:1.3:1 for bentgrass, fescue, and wildrye, 

respectively, and (3) average percentage of ground cover per plant was ranked 

by species as bentgrass (14 percent), wildrye (8 percent), broom (8 percent), and 

fescue (5 percent). Regression models predicted 90, 85, and 72 percent reductions in 

average biomass per plant of broom when grown with 250 plants/m2

 of bentgrass, 

wildrye, and fescue, respectively. Bentgrass and wildrye were more competitive 

than fescue because of their early-season depletion of soil water and development 

of cover. The addition series approach provided a relatively simple method for 

quantifying competitive abilities of plant species being considered for grassland 

community restoration.

One potential approach for restoring existing stands of broom to native grass 

communities uses a low-intensity disturbance to eliminate broom (e.g., manual cutting or spot treatment of herbicides) and stimulate abundance of associated competing vegetation to prevent broom regeneration. Grass seedlings are subsequently 

planted into natural or created small gaps within the plant canopy. A second potential approach uses prescribed fire to simultaneously (1) control existing broom, (2) 

reduce the broom seed bank, (3) reduce soil nitrogen content to favor perennial 

grasses, and (4) create a favorable seedbed for grass sowing.

Perennial grasses 

suppressed cover and 

biomass development 

of Scotch broom 

seedlings by 72 percent or more.
1

Quantifying Competitive Ability of Perennial Grasses to Inhibit Scotch Broom

Introduction

Scotch broom (Cytisus scoparius (L.) Link) is a large (1 to 3 m height) leguminous 

shrub that has become widely distributed in Australia, New Zealand, and 19 eastern 

and 8 western U.S. states (USDA NRCS 2010). A native to Western Europe, the 

species was first introduced in California in the 1850s as an ornamental (Gilkey 

1957). Scotch broom’s abundant production and prolonged viability of seeds enable 

it to occupy sites for decades (Bossard and Rejmánek 1994, Turner 1933). Seedlings 

grow vigorously to form dense stands that exclude native plants and alter community structure of savannahs, woodlands, and young forests (Bossard and Rejmánek 

1994, Shaben and Myers 2009, Wearne and Morgan 2004). Effective herbicide 

treatments exist for controlling Scotch broom (Blair and Zedaker 2005, Ketchum 

and Rose 2003); however, restoration of native species must follow soon after treatment to prevent reestablishment of broom and other invasive plant species.

Induced competition has been tested as an approach to inhibit development of 

invasive species while restoring native species (Endress and others 2008, Middleton 

and others 2010, Vasquez and others 2008). The approach relies on concepts of 

integrated weed management (Holt 2004) and species’ dominance (Denslow and 

Hughes 2004). Integrated weed management combines two or more strategies for 

preventing, suppressing, or controlling unwanted plants, and, thus, it overcomes 

the dominance potential of an invasive species. Restoration of native plant species 

limits subsequent invasion of nonnative species by mechanisms of competition and 

establishment limitation (Blumenthal and others 2005).

In this research, competitive abilities of three perennial grass species to inhibit 

development of Scotch broom seedlings were quantified in six addition series 

studies conducted in a greenhouse. The research tested the hypothesis that the grass 

species would differ in their ability to inhibit development of broom because of 

differences in timing and intensity of interspecific competition. Soil water depletion 

and plant growth were measured periodically in single-species stands to identify 

species’ attributes of competitive ability. Periodic development of foliar cover 

and final biomass were measured in two-species stands to quantify competitive 

outcomes.

Methods

Experimental Design and Treatments

Greenhouse studies were conducted at the Olympia Forestry Sciences Laboratory 

from March through August 2007 to quantify the competitive abilities of spike 

bentgrass (Agrostis exarata Trin.), blue wildrye (Elymus glaucus Buckley), and 

western fescue (Festuca occidentalis Hook.) to inhibit development of Scotch 

Scotch broom has 

extensively invaded 

areas within 19 eastern 

and 8 western U.S. 

states.
2

Research paper PNW-RP-587

broom seedlings. These perennial grass species were chosen for study because 

they are native to extensive areas of the Western United States and differ in growth 

rate and size (Hitchcock 1971). Six addition series studies were conducted: three in 

which Scotch broom was grown at four densities with and without four densities of 

each grass species, and one for each grass species grown at four densities with and 

without four densities of broom (a total of 68 density combinations) (table 1). An 

addition series is a systematic study design that compares the competitive abilities 

of two or more plant species grown in mixture by varying the density and proportion of each species independently (Radosevich 1987, Spitters 1983).

Grass seeds were purchased from a local vendor (Silver Falls Seed Company, 

Silverton, Oregon1), and broom seeds were collected in summer 2006 from roadside 

plants near Olympia, Washington. To ensure maximum rates of germination, broom 

seeds were soaked in deionized water for 24 hours at room temperature (21 °C) and 

then subjected to cold stratification for 30 days at 5 °C (Harrington 2009). Standard 

germination tests were conducted for each species to estimate sowing rates needed 

to create the desired range of seedling densities. Seeds of each species were sown 

in 6.1-L plastic pots (0.22-m top diameter) on 26 March. The growth medium 

consisted of potting soil (Sunshine mix #2, Sun-Gro Horticulture Distribution Inc., 

Bellevue, Washington) supplemented with additional perlite at 30 percent of the 

total volume. A fixed weight of growth medium was added to each pot. On 23 May, 

58 days after sowing seeds, seedlings were removed from or transplanted into each 

pot as required to create a set of systematic densities for each addition series study. 

At the time of transplanting, seedlings had achieved limited development; broom

seedlings still had cotyledons present and had begun to form their first set of true 

leaves, and the grass species were at the two- to three-leaf stage.

Plant densities in each addition series study were replicated three times in a 

randomized complete block design with blocking based on location within the 

greenhouse. Pots were watered every 3 to 7 days with an automated drip-irrigation 

system (model CO32, DIG Corporation, Vista, California). Fertilizer (Sunshine 

Technigro water soluble fertilizer, 20N 18P 18K with micronutrients, Sun-Gro 

Horticulture Distribution Inc., Bellevue, Washington) dissolved in water to create

a nitrogen concentration of 100 ppm was applied every 2 weeks at the rate of 100 

ml per pot. The objective of the irrigation and fertilizer treatments was to provide 

each pot the same amount of water and nutrients at a rate likely to sustain normal 

1 The use of trade or firm names in this publication is for reader information and 

does not imply endorsement by the U.S. Department of Agriculture of any product 

or service.
3

Quantifying Competitive Ability of Perennial Grasses to Inhibit Scotch Broom

plant development. To quantify species’ differences in soil water depletion, weight 

of each pot containing single-species stands was measured immediately prior to 

scheduled irrigation on 7 June, 5 July, and 10 August, corresponding to 73, 101, and 

137 days after sowing, respectively. Volumetric water content was calculated after 

accounting for the weights of dry soil and the container; weight of plant biomass 

(<5 percent of total) was not included in these calculations.

Projected foliar cover (nearest 5 percent) of each species was estimated visually 

within the perimeter of each pot at 73, 101, and 137 days after sowing. Average 

canopy height (nearest centimeter) of each species also was measured on the same 

three dates. Broom height was calculated as the mean of individual seedling heights 

in each pot. On 13 August, 140 days after sowing seeds, seedlings in each pot were 

counted by species and their aboveground biomass was collected, dried to a constant weight at 65 °C, and weighed (nearest 0.01 g).

Statistical Analysis

All statistical analyses were conducted by using SAS (SAS Institute 2005) with 

a significance criterion of α = 0.05. Average values per plant of foliar cover and 

aboveground biomass were calculated by species for each pot. For single-species 

stands having three plants per pot (four plants per pot for spike bentgrass), the three 

Table 1—Plant densities for addition series studies of Scotch broom grown 

with each of three perennial grass species

Species Grass density Scotch broom density

– – – – – – – – – Plants per pot – – – – – – – – – 

0 1 2 3 4 5

Scotch broom 0 x x x x x

Spike bentgrass 1 x

4 x x x x x

8 x x x x x

12 x x x x x

16 x x x x x

Blue wildrye 1 x 

3 x x x x x

5 x x x x x

7 x x x x x

9 x x x x x

Western fescue 1 x

3 x x x x x

6 x x x x x

9 x x x x x

12 x x x x x
4

Research paper PNW-RP-587

periodic measurements of cover and height were pooled and each variable was 

subjected to a repeated-measures analysis of variance (ANOVA) in the MIXED

procedure to test for potential differences among species, measurement dates, and 

their interaction. If ANOVA indicated significant (p < 0.05) effects for a given 

factor or factor interaction, multiple comparisons of species’ least-squares means 

within each measurement date were conducted with Tukey’s Honestly Significant 

Difference test with probabilities adjusted to control the type I error rate (Quinn 

and Keough 2002). Prior to ANOVA, an angular transformation (i.e., arc-sine, 

square-root) was applied to the cover values (expressed as proportions) and a 

logarithmic transformation was applied to the height values to homogenize their 

residual variances (Sokal and Rohlf 1981).

To compare rates of soil water depletion among species, relations of soil water 

content to density of single-species stands were tested with linear regression in the 

REG procedure. An angular transformation was applied to soil water content and a 

logarithmic transformation was applied to density to linearize the relations prior to 

regression. The slope of the regression can be interpreted as a relative measure of 

soil water depletion by a single plant of a given species. Relative ability to deplete 

soil water was compared among species by calculating ratios of the regression 

slopes to the slope of lowest value.

To quantify effects of intra- and interspecific competition on cover and biomass 

of a given species, data from each addition series study were subjected to stepwise 

linear regression in the REG procedure to fit the following logarithmic model (Kira 

and others 1953):

Y = A0 + A1

(lnG) + A2(lnB) 

Where Y = average cover or biomass per plant (g) of the focal species, 

A0-2 = regression coefficients to be estimated,

G = density of the associated grass species (plants per pot), and 

B = density of Scotch broom (plants per pot).

An angular transformation was applied to cover and a logarithmic transformation was applied to biomass to linearize the relations prior to regression. To include 

treatments in the analysis for which B or G equaled zero, the value 1 was added to 

each density variable prior to converting it to natural logarithms. Separate regressions of cover were conducted for each measurement date to quantify species’ 

differences in timing and intensity of competition. The regression intercept, A0

,

can be interpreted as the average cover or biomass of a single, isolated plant of the 

focal species. The regression slopes, A1 and A2, can be interpreted as the competitive effects of a single grass plant and a single broom plant, respectively. Relative 
5

Quantifying Competitive Ability of Perennial Grasses to Inhibit Scotch Broom

competitive ability was compared among species by calculating ratios of the regression slopes to the slope of lowest value. Biomass predictions from the regression 

equations were adjusted with a log-bias correction (Sprugel 1983). Residuals from 

each ANOVA and regression were plotted against predicted values to check for 

nonhomogeneous variances.

Results

Species’ Differences in Soil Water Depletion

At 73 days after sowing, soil water content was negatively related to density of 

single-species stands of spike bentgrass and blue wildrye (table 2, fig. 1). Negative 

relations also were observed for bentgrass and western fescue on day 101, but the 

relations were not significant for any species on day 137 because of increased variation in soil water content. The regression of soil water content versus Scotch broom 

density was not significant for any measurement date (F[1,13] < 0.96; p > 0.345). 

Species differed in their ability to deplete soil water as ratios of the regression 

slopes (A1) of 2.4:1.3:1 for bentgrass, fescue, and wildrye, respectively.

Species’ Differences in Growth

Average foliar cover per plant in single-species stands of three plants per pot (four 

plants per pot for spike bentgrass) differed according to the main effects of species 

(F[3,22] = 3.10; p = 0.047) and measurement date (F[2,22] = 53.14; p < 0.001), but the 

interaction of species by date was not significant (F[6,22] = 0.78; p = 0.59). When 

Table 2—Regression models for predicting soil water content in 

single-species stands of spike bentgrass, blue wildrye, and western 

fescue at 73 and 101 days after sowing

	 	 Regression coefficients

Species Days A0 A1 R2

s

y.x

n

Spike bentgrass 73 0.861 -0.098 0.51 0.10 15

101 0.740 -0.156 0.48 0.17 15

Blue wildrye 73 0.867 -0.065 0.28 0.09 15

Western fescue 101 0.774 -0.086 0.29 0.13 14

Note: Each regression coefficient is significant (p < 0.05). Model predictions are 

illustrated graphically in figure 1. The regression model tested for each species was: Y = 

A0 + A1(lnD), where Y is volumetric soil water content (proportionate value transformed to 

arc-sine, square-root) and D is density of grass (plants per pot). Sy.x is the standard error of 

the estimate.

Per-plant depletion 

of soil water from 

bentgrass was about 

twice that from fescue 

or wildrye.
6

Research paper PNW-RP-587

averaged across measurement dates, cover was ranked by species as spike bentgrass 

(14 percent), blue wildrye (8 percent), Scotch broom (8 percent), and western fescue 

(5 percent); however, only values for bentgrass and fescue differed significantly 

(p = 0.029). When averaged across species, cover differed at 73 (4 percent), 101 (9 

percent), and 137 (15 percent) days after sowing (p < 0.001).

Canopy height varied according to the interaction of species by measurement 

date (fig. 2; F[6,22] = 7.33; p < 0.001). On day 73, wildrye height (27 cm) was greater 

than that of fescue (12 cm) (p = 0.002), and on day 137, broom height (45 cm) was 

Figure 1—Average values (± SE) and 

regression model predictions of soil 

water content as affected by plant density in single-species stands of Scotch 

broom, spike bentgrass, blue wildrye, 

and western fescue at 73, 101, and 137 

days after sowing. Regression statistics 

are given in table 2.
7

Quantifying Competitive Ability of Perennial Grasses to Inhibit Scotch Broom

Figure 2—Average canopy height (±SE) of single-species stands of 

Scotch broom, spike bentgrass, blue wildrye, and western fescue at 73, 

101, and 137 days after sowing. Difference in letters indicates means 

that differed significantly for each measurement date (p < 0.05).

greater than that of fescue (20 cm) (p = 0.002). Although broom started as the 

shortest species, it was the tallest species at completion of the research. Height of 

the grass species declined 2 to 7 cm from day 101 to day 137 as flowering and seed 

maturity occurred, causing the culms to bend.

Species’ Differences in Competitive Ability

Average cover per plant of Scotch broom had negative relations with density of 

spike bentgrass and blue wildrye (table 3). These relations were first detected on 

day 73 and continued for the duration of the research. Broom cover also had negative relations with density of fescue on days 101 and 137. In general, the regression 

slopes (A1) became increasingly negative over time with concomitant increases in 

the coefficient of determination (R2), indicating increasing intensity of interspecific 

competition from the grass species. On day 137, grass species differed in their 

ability to competitively inhibit cover of broom as ratios of the regression slopes (A1) 

of 1.3:1.2:1 for bentgrass, wildrye, and fescue, respectively. Effect of intraspecific 

competition from broom was detected on day 137 in the addition series studies with 

bentgrass and wildrye, but for fescue, the effect was only marginally significant 

(F[1,60] = 3.97; p < 0.051); the effect from broom was greater when grown with 

wildrye (A2 = -0.148 ± 0.05 SE) than with bentgrass (A2

 = -0.131 ± 0.05 SE).
8

Research paper PNW-RP-587

Average cover per plant of each grass species had a negative relation with its 

own density (i.e., intraspecific competition effect, A1) for each of the measurement 

dates (table 3). On day 137, intraspecific competition effects differed among grass 

species as ratios of the regression slopes (A1) of 1.4:1.3:1 for bentgrass, fescue, and 

wildrye, respectively. Interspecific competition effect of broom on wildrye was 

detected on days 101 and 137; however, the magnitude of the effect (A2 = -0.024 and 

-0.032) was only 21 to 29 percent of that from intraspecific competition (A1 = -0.115 

and -0.110). Interspecific competition effects of broom on bentgrass and fescue were 

not significant (p > 0.103 and p > 0.072, respectively).

Biomass values for single, isolated plants of wildrye, bentgrass, fescue,

and broom averaged 20, 13, 8, and 6 g, respectively (calculations included backtransforming the regression intercept, A0

 in table 4, and applying the log-bias

correction). Interspecific competition effect of grass on broom biomass differed 

among species as ratios of the regression slopes (A1) of 1.8:1.5:1 for bentgrass, wildrye, and fescue, respectively; however, intraspecific competition effect of broom 

Table 3—Regression models for predicting average cover per plant of Scotch broom, spike 

bentgrass, blue wildrye, and western fescue at 73, 101, and 137 days after sowing

	 Regression coefficients

Focal species Associated species Days A0 A1 A2 R2s

y.x

n

Scotch broom Spike bentgrass 73 0.188 -0.029 — 0.16 0.07 63

101 0.265 -0.067 — 0.51 0.07 63

137 0.555 -0.098 -0.131 0.53 0.11 63

Scotch broom Blue wildrye 73 0.246 -0.064 — 0.07 0.07 63

101 0.272 -0.059 — 0.26 0.09 63

137 0.593 -0.092 -0.148 0.45 0.11 62

Scotch broom Western fescue 73 — — — — — 63

101 0.278 -0.048 — 0.20 0.09 63

137 0.391 -0.074 — 0.24 0.13 63

Spike bentgrass Scotch broom 73 0.413 -0.072 — 0.54 0.04 63

101 0.592 -0.125 — 0.73 0.04 63

137 0.680 -0.151 — 0.73 0.05 63

Blue wildrye Scotch broom 73 0.229 -0.035 — 0.10 0.04 63

101 0.458 -0.115 -0.024 0.57 0.05 63

137 0.526 -0.110 -0.032 0.48 0.06 63

Western fescue Scotch broom 73 0.181 -0.029 — 0.12 0.04 63

101 0.339 -0.078 — 0.26 0.07 63

137 0.527 -0.138 — 0.35 0.10 63

— = Regression coefficient not significantly different from zero (p > 0.05).

Note: Each regression coefficient is significant (p < 0.05). The regression model tested for each addition series study was: 

Y = A0 + A1(lnG) + A2(lnB), where Y is average cover per plant (proportionate value transformed with arc-sine, square-root) 

of the focal species and G and B are densities of grass and Scotch broom (plants per pot), respectively. The value 1 was 

added to each density variable before transforming to natural logarithms. Sy.x is the standard error of the estimate.
9

Quantifying Competitive Ability of Perennial Grasses to Inhibit Scotch Broom

was not significant (F[1,60] > 0.33; p > 0.565). The regression models predicted 90 

percent, 85 percent, and 72 percent reductions in broom biomass when grown with 

9 plants per pot (equivalent to approximately 250 plants/m2) of bentgrass, wildrye, 

and fescue, respectively, relative to broom biomass in the absence of grass competition (fig. 3A).

Intraspecific competition effect on grass biomass differed among species as 

ratios of the regression slopes (A1) of 1.5:1.3:1 for wildrye, bentgrass, and fescue, 

respectively (table 4, fig. 3B). However, interspecific competition effect of broom 

on grass biomass was not significant (F[1,60] > 3.22; p > 0.078).

Discussion

The addition series studies provided a common experimental approach for quantifying and comparing the competitive abilities of several grass species to inhibit development of Scotch broom seedlings. Despite periodic irrigation every 3 to 7 days, 

soil water content developed a negative relation with density of each grass species. 

During the first 101 days of the experiment, soil water varied consistently with 

grass density. In the remaining 39 days of the experiment (July and early August), 

the frequency of irrigation was increased to meet evapotranspiration demands.

Near the end of the experiment (day 137), soil water content was not significantly 

related to plant density. Two possible explanations may account for this declining 

correlation with time. First, increases in irrigation frequency may have exceeded 

evapotranspiration demands, resulting in similar soil water content among plant 

Table 4—Regression models for predicting average biomass per plant of 

Scotch broom, spike bentgrass, blue wildrye, and western fescue at 140

days after sowing

Regression

	 coefficients

Focal species Associated species A0 A1 R2s

y.x

n

Scotch broom Spike bentgrass 0.539 -0.984 0.31 1.58 61

Scotch broom Blue wildrye 0.779 -0.806 0.20 1.41 63

Scotch broom Western fescue 0.809 -0.545 0.13 1.39 63

Spike bentgrass Scotch broom 2.487 -0.844 0.67 0.31 63

Blue wildrye Scotch broom 2.846 -0.954 0.37 0.53 63

Western fescue Scotch broom 1.840 -0.651 0.18 0.73 63

Note: Each regression coefficient is significant (p < 0.05). Model predictions are illustrated 

graphically in figure 3. The regression model tested for each addition series study was: Y = 

A0 + A1(lnG) + A2(lnB), where Y is average-plant aboveground biomass (grams transformed 

to natural logarithms) of the focal species, and G and B are densities of grass and Scotch 

broom (plants per pot), respectively. The value 1 was added to each density variable before 

transforming to natural logarithms. Sy.x is the standard error of the estimate.
10

Research paper PNW-RP-587

densities. Second, with maturity and flowering of the grass species, plant demand 

probably decreased, resulting in an accumulation of soil water for all plant densities.

For a given measurement date, the per-plant rate of soil water depletion from 

bentgrass (i.e., regression slope, A1

) was about twice that observed from wildrye or 

fescue. Average cover per plant of bentgrass (14 percent) was almost twice that of 

wildrye (8 percent) and almost three times that of fescue (5 percent). In addition, 

the absence of a significant interaction of species by measurement date for cover 

indicates that species’ rankings in cover did not change during the study. In combination, these results suggest that differences in soil water depletion among species 

can be partially explained by their intrinsic rates of cover development.

Another attribute that explained differences in competitive ability among 

the grass species was the timing at which competitive effects were first detected. 

Figure 3—Average values (± SE) 

and regression model predictions 

of aboveground biomass per plant 

of (a) Scotch broom as affected 

by interspecific competition 

from three grass species and (b) 

spike bentgrass, blue wildrye, 

and western fescue as affected by 

intraspecific competition at 140 

days after sowing. Regression 

statistics are given in table 4.

The most competitive 

grass species, 

bentgrass, had the 

greatest rates of cover 

development and soil 

water depletion.
11

Quantifying Competitive Ability of Perennial Grasses to Inhibit Scotch Broom

Significant relations between soil water content and plant density were detected for 

bentgrass and wildrye on day 73, whereas a significant relation for fescue was not 

detected until day 101. Similarly, the regression analyses of plant cover indicated 

that competitive effects of bentgrass and wildrye on Scotch broom were detected 

on day 73, whereas the competitive effect for fescue was not detected until day 101. 

Height development of fescue also lagged behind those of the other species for the 

duration of the research. Despite these differences in competitive attributes, each of 

the three grass species competed effectively with Scotch broom to strongly inhibit 

development of seedlings. Reductions in broom biomass did not change appreciably 

as grass density exceeded four plants per pot, equivalent to 109 plants/m2, indicating that a threshold in competition had been reached above which diminishing 

effects of interspecific competition were observed (fig. 3A).

An important question stimulated by the research was whether broom competition would ultimately exclude these grass species. Interspecific competition effect 

of broom on grass cover was only detected for wildrye. For this grass species, 

interspecific competition effect of broom was less than a third of intraspecific competition effect of grass, suggesting that broom is unlikely to competitively exclude 

grass, at least within the period of this research. Interspecific competition effect of 

broom on bentgrass cover was not significant perhaps because average cover per 

plant of bentgrass (14 percent) was almost twice that of broom (8 percent), whereas 

the other grass species had average covers (5 to 8 percent) similar to that of broom.

Caution should be observed when extrapolating the short-term results of this 

research to longer term application in restoration of perennial grasses. Because 

Scotch broom is a nitrogen fixer, it has the potential to alter soil chemical properties 

and thereby potentially shift competitive outcomes. In savannah communities of the 

Pacific Northwest and Australia, researchers have identified increases in soil carbon and nitrogen contents following invasion of broom, attributable to its abundant 

litter production and nitrogen fixation (Caldwell 2006, Fogarty and Facelli 1999, 

Haubensak and Parker 2004). Such soil conditions are believed to favor continued 

recruitment of broom (Fogarty and Facelli 1999) and place perennial grasses at a 

disadvantage in the plant community (Redente and others 1992, Vasquez and others 

2008).

Restoring perennial grasses to broom-invaded sites involves a two-step process that first eliminates existing broom plants and then facilitates establishment 

of native grasses. The best method for removing broom without increasing its 

regeneration appears to be one that stimulates abundance of associated competing vegetation. For example, within its native range in France, broom abundance 

was substantially lower 3 years after cutting mature plants than after cutting was 
12

Research paper PNW-RP-587

combined with soil tillage primarily because abundance of associated competing 

vegetation, relative to the uncut broom stand, was increased by cutting alone and 

decreased by cutting plus tillage (Paynter and others 1998). On broom-invaded sites 

in Australia, an identical comparison of treatments resulted in no differences in 

broom abundance 6 years later because the treatments did not significantly change 

abundance of competing vegetation (Sheppard and others 2002). One approach for 

establishing perennial grasses without reducing abundance of associated vegetation is to plant either bare-root or containerized seedlings within natural or created 

small gaps in the canopy of the intact plant community (Middleton and others 

2010).

Alternatively, repeated application of prescribed fire could be used to control 

existing broom, restore soil nitrogen content of savannah soils to pre-invasion levels 

(Haubensak and others 2004), and create a seedbed in which to sow perennial 

grasses. Although fire may stimulate broom germination somewhat via seed scarification (Parker 2001), a single prescribed fire has been shown to reduce the initial 

flush of broom seedlings by as much as 68 percent (Haubensak and others 2004). 

Because broom is capable of germinating at relatively low temperatures (Bossard 

1993, Harrington 2009), seeding of perennial grasses in autumn, as soon as possible 

after a prescribed fire, is likely to ensure their establishment at the same time or 

earlier than emergence of broom seedlings.

Conclusions

Competition from each of the grass species inhibited biomass development of 

Scotch broom seedlings by 72 to 90 percent. The most competitive species, spike 

bentgrass, was effective at depleting soil water early in the study. Bentgrass also 

had the greatest average cover per plant, which essentially prevented broom from 

developing competitive effects of its own. The least competitive species, western 

fescue, lagged behind the other two grass species in cover and height development 

and in the timing at which its competitive effects were first detected.

The addition series approach used in this research provided a relatively simple 

method for quantifying and comparing the competitive abilities of species being 

considered for restoration of plant communities. In this research, irrigation and 

fertilization were applied to each pot at a constant rate to provide a common test 

environment for quantifying competitive ability of individual species. Alternatively, 

the rate at which belowground resources are applied also could be manipulated to 

determine whether variation in resource availability affects species’ competitive 

ability.

Prescribed fire can be 

used to control existing 

Scotch broom plants, 

reduce the number 

of soil-stored seeds, 

and create a seedbed 

for sowing perennial 

grasses.
13

Quantifying Competitive Ability of Perennial Grasses to Inhibit Scotch Broom

Acknowledgments

Partial financial support for this research was provided by a seed-money grant from 

the Center for Invasive Plant Management, Montana State University, Bozeman. 

Special thanks to J. Dollins, D. Livada, and D. Stephens for technical assistance and 

to P. Gould for review of the manuscript.

English Equivalents

When you know:	 Multiply by:	 To find:

Centimeters (cm) 0.394 Inches

Meters (m) 3.28 Feet

Square meter (m2

) 10.76 Square feet

Liters (L) 0.265 Gallons

Milliliters (ml) 0.0338 Ounces

Grams (g) 0.0022 Pounds

Degrees Celsius (°C) 1.8 °C + 32 Degrees Fahrenheit

Plants per square meter 0.0929 Plants per square foot

Literature Cited

Blair, M.P.; Zedaker, S.M. 2005. Evaluation of clopyralid, fluroxypyr, 

metsulfuron methyl, and triclopyr for Scotch broom control. Proceedings of the 

7

th annual meeting of Southeast Exotic Pest Plant Council. Birmingham, AL. 

http://www.se-eppc.org/2005/evaluationposter.cfm. (November 2, 2010).

Blumenthal, D.M.; Jordan, N.R.; Svenson, E.L. 2005. Effects of prairie 

restoration on weed invasions. Agriculture, Ecosystems, and Environment.

107: 221–230.

Bossard, C.C. 1993. Seed germination in the exotic shrub Cytisus scoparius

(Scotch broom) in California. Madroño. 40: 47–61.

Bossard, C.C.; Rejmánek, M. 1994. Herbivory, growth, seed production, 

and resprouting of an exotic invasive shrub, Cytisus scoparius. Biological 

Conservation. 67: 193–200.

Caldwell, B.A. 2006. Effects of invasive scotch broom on soil properties in a 

Pacific coastal prairie soil. Applied Soil Ecology. 32: 149–152.

Denslow, J.S.; Hughes, R.F. 2004. Exotic plants as ecosystem dominants. Weed 

Technology. 18: 1283–1287.

Endress, B.A.; Parks, C.G.; Naylor, B.J.; Radosevich, S.R. 2008. Herbicide 

and native grass seeding effects on sulfur cinquefoil (Potentilla recta)-infested 

grasslands. Invasive Plant Science and Management. 1: 50–58.
14

Research paper PNW-RP-587

Fogarty, G.; Facelli, J.M. 1999. Growth and competition of Cytisus scoparius, an 

invasive shrub, and Australian native shrubs. Plant Ecology. 144: 27–35.

Gilkey, H.M. 1957. Weeds of the Pacific Northwest. Corvallis, OR: Oregon State 

College. 441 p.

Harrington, T.B. 2009. Seed germination and seedling emergence of Scotch 

broom (Cytisus scoparius). Weed Science. 57: 620–626.

Haubensak, K.A.; D’Antonio, C.M.; Alexander, J. 2004. Effects of nitrogenfixing shrubs in Washington and coastal California. Weed Technology.

18: 1475–1479.

Haubensak, K.A.; Parker, I.M. 2004. Soil changes accompanying invasion 

of the exotic shrub Cytisus scoparius in glacial outwash prairies of western 

Washington [USA]. Plant Ecology. 175: 71–79.

Hitchcock, A.S. 1971. Manual of the grasses of the United States. Volume 1.

2

nd ed. (revised by A. Chase). New York: Dover Publications, Inc. 569 p.

Holt, J.S. 2004. Principles of weed management in agroecosystems and wildlands. 

Weed Technology. 18: 1559–1562.

Ketchum, J.S.; Rose R. 2003. Preventing establishment of exotic shrubs (Cytisus 

scoparius (L.) Link. and Cytisus striatus (Hill)) with soil active herbicides 

(hexazinone, sulfometuron, and metsulfuron). New Forests. 25: 83–92.

Kira, T.H.; Ogawa, H.; Sakizaki, N. 1953. Intraspecific competition among 

higher plants. I. Competition-yield-density interrelationship in regularly 

dispersed populations. Journal of the Institute of Polytechnics, Japan: Osaka

City University. 7: 73–94.

Middleton, E.L.; Bever, J.D.; Schultz, P.A. 2010. The effect of restoration 

methods on the quality of the restoration and resistance to invasion by exotics. 

Restoration Ecology. 18: 181–187.

Parker, I.M. 2001. Safe site and seed limitation in Cytisus scoparius (Scotch 

broom): invasibility, disturbance, and the role of cryptogams in a glacial outwash 

prairie. Biological Invasions. 3: 323–332.

Paynter, Q.; Fowler, S.V.; Memmott, J.; Sheppard, A.W. 1998. Factors affecting 

the establishment of Cytisus scoparius in southern France: implications for 

managing both native and exotic populations. Journal of Applied Ecology.

35: 582–595.
15

Quantifying Competitive Ability of Perennial Grasses to Inhibit Scotch Broom

Quinn, G.P.; Keough, M.J. 2002. Experimental design and data analysis for 

biologists. Cambridge, United Kingdom: Cambridge University Press. 537 p.

Radosevich, S.R. 1987. Methods to study interactions among crops and weeds. 

Weed Technology. 1: 190–198.

Redente, E.F.; Friedlander, J.E.; McLendon, T. 1992. Response of early and late 

seral species to nitrogen and phosphorus gradients. Plant and Soil. 140: 127–135.

SAS Institute. 2005. The SAS System for Windows, Version 9.1. Cary, NC.

Shaben, J.; Myers, J.H. 2009. Relationships between Scotch broom (Cytisus 

scoparius), soil nutrients, and plant diversity in the Garry oak savannah 

ecosystem. Plant Ecology. DOI: 10.1007/s11258-009-9655-7.

Sheppard, A.W.; Hodge, P.; Paynter, Q.; Rees, M. 2002. Factors affecting 

invasion and persistence of broom Cytisus scoparius in Australia. Journal of 

Applied Ecology. 39: 721–734.

Sokal, R.R.; Rohlf, J.F. 1981. Biometry. 2nd ed. San Francisco, CA: W.H. Freeman. 

859 p.

Spitters, C.J.T. 1983. An alternative approach to the analysis of mixed cropping 

experiments. 1. Estimation of competition effects. Netherland Journal of 

Agricultural Science. 31: 1–11.

Sprugel, D.G. 1983. Correcting for bias in log-transformed allometric equations. 

Ecology. 64: 209–210.

Turner, J.H. 1933. The viability of seeds. Royal Botanic Gardens, Kew, England, 

Bulletin of Miscellaneous Information. 6: 257–269.

U.S. Department of Agriculture (USDA) Natural Resources Conservation 

Service. 2010. PLANTS database, Scotch broom, Cytisus scoparius (L.) Link. 

http://plants.usda.gov/java/profile?symbol=CYSC4. (November 2, 2010).

Vasquez, E.; Sheley, R.; Svejcar, T. 2008. Nitrogen enhances the competitive 

ability of cheatgrass (Bromus tectorum) relative to native grasses. Invasive Plant 

Science and Management. 1: 287–295.

Wearne, L.J.; Morgan, J.W. 2004. Community-level changes in Australian 

subalpine vegetation following invasion by the non-native shrub Cytisus 

scoparius. Journal of Vegetation Science. 15: 595–604.
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