1. Introduction
In some European countries, concerns over pesticide misuse have led to strict regulations, particularly in regards to herbicides used on amenity areas [
1]. In Denmark, metabolites of pesticides were detected in 40% of groundwater wells that supply drinking water to communities [
2]. In the Netherlands, where 40% of all drinking water is derived from surface water, problems with pesticide contamination led to voluntary agreements with municipalities to reduce herbicidal use in amenity areas [
1]. Such voluntary agreements are also in place for athletic fields, as exemplified by the ‘Green Deal’ in The Netherlands [
3]. The ‘Green Deal’ was initiated to restrict the use of pesticides on amenity areas entirely; however an exemption period was granted until 2022 which allows the use of selected pesticides under strict conditions [
3]. In turf settings, herbicides are the most used pesticide in terms of product use [
4]. Herbicides are mainly applied to control broadleaf dicotyledonous weeds, such as white clover (
Trifolium repens L., WC) or dandelion (
Taraxacum officinale L.), which are the most problematic weeds in athletic fields [
5]. In these areas, the main objective is to maintain a mono stand of turfgrass species that produce aesthetically pleasing surfaces with a certain standard of playing quality [
6,
7].
Weeds in athletic fields or golf course fairways can be defined as unwanted species that interfere with the visual appearance and playing quality of these surfaces [
7]. Weeds often establish in sparse areas as a result of abiotic/biotic stress or management failures such as improper water management or fertilization [
8,
9]. Once established, weeds compete with desirable turfgrass species for resources (namely water), light and nutrients, as well as space, such as in below ground root competition [
6].
The requirement for desirable turfgrass species has shifted towards low-input species to reduce the ecological footprint of amenity turfgrass areas [
10,
11]. Consequently, an ecological approach to weed management should focus on establishing sustainable turfgrass species that require low inputs of valuable resources such as water and fertilizer, while providing a dense, healthy turf canopy that competes well against weeds [
6,
9,
12,
13].
Fine fescues (
Festuca L. spp.) can establish a dense turf canopy with minimal inputs of water, fertilizer and pesticides [
12]. These perennial fine-leaved turf species thrive in dry conditions but are also adapted to shade and low pH growing conditions [
12]. Fine fescues can be grouped into two complexes, the red fescues (
Festuca rubra L.) and the sheep fescues (
Festuca ovina L.) [
10,
14]. Within the red fescue species, strong creeping red fescue (
F. rubra L. ssp.
rubra Gaudin) and slender creeping red fescues (
F. rubra L. ssp.
littoralis (G. Mey.) Auquier) produce rhizomes [
14], while Chewings fescue (
F. rubra L. ssp.
commutata Gaudin) exhibits a bunch type growth. Chewings fescue was identified as a species that provides acceptable quality on golf fairways under low- nitrogen inputs, defined as 4.9 g of nitrogen per square meter, and is a superior species under reduced irrigation and low pesticide inputs compared to other fine fescue species and colonial bentgrass (
Agrostis capillaries L., CL) mixtures [
15].
Hard fescue (
F. brevipila Tracey) belongs to the sheep fescue complex, establishes slowly, and has a bunch type rooting system [
16]. Hard fescue has slower establishment vigor compared to creeping red fescue and Chewings fescue, and demonstrated less natural weed suppression capacity [
10,
17].
Tall fescue (
Schedonorus arundinaceus (Schreb.) Dumort.), is a stand-alone fescue complex, establishes quickly from seed [
16] and can be easily separated from other turf species because of the wide leaf width of 4–18 mm. In comparison,
Festuca rubra species have leaf widths of less than 2 mm [
10]. Additionally, tall fescues have a deep rooting system, making them one of the most drought resistant cool-season turfgrass species [
18]. Drought resistance combined with wear tolerance makes tall fescue one of the most used grass species for athletic fields in European transition zones [
19].
Apart from being a sustainable turfgrass species, fine fescues and tall fescue have also demonstrated allelopathic potential [
17,
20,
21,
22]. Fine fescues and tall fescue produce compounds which interfere with the growth of some neighboring plants, imparting them with natural weed suppression capabilities [
20,
22]. A series of field studies of 78 fine-leaf fescue cultivars showed that three Chewings fescue cultivars and one hard fescue cultivar showed ‘good’ natural weed suppression capabilities (defined as more than 70% of weeds suppressed compared to a control) [
17].
Information is lacking on the prevalence of weed suppression capabilities among certain fescue complexes or species. Moreover, it is unknown if fine fescues and tall fescues interfere differently with the growth of different weed species. In this study, we investigated the extent of growth interference of white clover (Trifolium repens L., WC), daisy (Bellis perennis L.) and yarrow (Achillea millefolium L.), three common European broadleaf weeds, during the establishment phase of six fescue cultivars from five species. One tall fescue variety was added to the study to explore if a more rapidly developing grass species, such as Schedonorus arundinaceus (Schreb.) Dumort., outcompetes neighboring weed species differently than fine fescues. It was also investigated if final grass vigor was negatively correlated to weed establishment, if better visual scores could be attributed to low weed cover or high vigor and, lastly, if quantitative vegetation cover estimates gave results that were similar to visual vigor scores.
4. Discussion
The prime objective of the present study was to assess the capacity of fine and tall fescues to interfere with the growth of several common turf weeds. The percent of weed cover was generally higher in 2018 than in 2019, which could be attributed to higher overall temperatures in 2018. Maximum daily temperatures were higher in 2018 (25.3 °C) compared to 2019 (18.4 °C), most likely contributing to the more vigorous weed growth observed in 2018. Optimal temperatures for clover seed germination have been determined to range between 10.9 °C and 17.2 °C [
39]. This might explain the difference in weed cover between the two years, as average daily temperatures were within this optimal range on 74 days in 2018 compared to only 57 days in 2019. Maximum germination rates (82%) for yarrow under frequent irrigation rates have been reported between 22 °C to 29 °C [
40], and 25 °C was reported to be an optimal temperature for germination of daisy [
41]. Therefore, and regardless of weed treatment, germination of weed seed was favored in 2018 due to higher average temperatures, resulting in greater weed cover. However, it was not possible to determine if any of the weed species tested were more or less susceptible to growth interference by fine or tall fescues.
Fescue cultivars did not differ in their ability to interfere with growth of weeds during establishment. Results in this study do not support the few existing studies that suggest that both Chewings fescue (=Musica) and strong creeping red fescue (=Barisse) naturally interfere with growth of mature broadleaf weed species [
10,
15,
17] in part because both species produce detectable amounts of the allelopathic non-protein amino acid m-tyrosine, which contributes to growth interference of other receiver plants [
21].
To examine potential drivers of growth interference, vigor data were collected, based on the assumption that a more rapidly developing sward is likely to outcompete neighboring weed species [
6]. Shortly after germination (14 DAS), differences in vigor between cultivars and year were minor, except for the tall fescue cultivar Melyane, which showed higher vigor (greater than three) compared to other cultivars (vigor scores of two or lower for most cultivar × weed combinations) in 2019. However, the competitive advantages conferred by early vigor in Melyane were short lived, with several cultivar × weed combinations reaching similar vigor at 39 DAS in both years. Some authors have reported that Chewings- and strong creeping red fescue develop vigor more rapidly than hard fescue during establishment [
10,
17], which could not be confirmed based on the data. On the final day of the experiment in 2018, all cultivars established with the same degree of vigor, whereas in 2019 we observed treatment differences and Barpearl and Musica placed in the group with the highest vigor scores for all weed treatments. Tall fescue varieties are known to lose turfgrass cover under low mowing regimes [
24,
42], which was observed in both years after mowing was initiated. Tall fescue varieties are used for athletic fields in European transition zones [
19], however, results showed that Melyane is an unsuitable cultivar for natural weed suppression in turf mowed at 15 mm or less twice a week in cool-season climates such as the Netherlands. This conclusion was based on visual quality data, not on weed cover data.
Even though visual quality in 2018 did not vary among cultivars within the different weeds, Melyane performed poorly in 2019, with scores never exceeding 3.3. Most other cultivars scored greater than six in 2019, with Barpearl and Musica consistently scoring above six in all weeds. Nonetheless, upon examination of visual scores of fescues subjected to different weed treatments, it was unclear if any weed treatment resulted in particularly good or bad scores, as all weed control plots scored lower than the cultivar × weed plots.
In general, annual differences observed between data collected from both years of this study can be attributed to different weather conditions. In 2018, 19 days out of 84 reached a maximum temperature of greater than 30 °C, whereas the next year only seven days of the experimental period reached temperatures above 30 °C. Even though maximum daily temperatures exceeding 30 °C were first recorded late into both years of the study (day 59 in 2018 and day 79 in 2019), the higher mean temperatures in 2018 probably promoted better grass establishment and consequently higher grass vigor. In 2018, irrigating regularly was necessary because of low precipitation rates in November, whereas in 2019 irrigation was only required in the first two weeks, as precipitation was sufficient to ensure good establishment.
Annual differences were also reflected in the correlation analysis. Across both years, data showed that visual quality decreased with an increase in weed cover. This was expected, as weed cover is a factor that influences visual quality [
33]. Also grass vigor was significantly correlated with vegetation cover, strongly in 2019 and to a lesser degree in 2018. Vegetation cover describes the surface area covered by all green vegetation, with no distinction between weeds and grasses. The stronger association between vigor and cover in 2019 is the result of greater differences in the extent of establishment among grasses, reflected by data points distributed over a wider range of cover, from low to high. In 2018, all grasses established equally well, and data points were concentrated at the high cover area (
Figure 1).
Visual quality was positively and significantly associated with vegetation cover and grass vigor in 2019, but not in 2018. Similar to the observed association between grass vigor and vegetation cover, differences in vigor among varieties were greater in 2019 because grasses established differently, which resulted in a wider spread of data points and a stronger association. In this study, a more vigorous growing turf resulted in less weeds. However, the experimental set-up was not designed to examine the underlying mechanisms of growth interference in detail.