Abstract
The western portion of the Pacific Northwest is known for being dry in the summer and cool and humid in the other months. Tall fescue is valued for its drought and heat tolerance, making it a desirable choice in regions where water is scarce and often restricted by legislation during periods of drought in the summer. However, cool and humid climates make it challenging to manage tall fescue in the winter because unacceptable quality is often observed due to low-temperature diseases and thinning in turf. A field trial was initiated in Autumn 2020 in Corvallis, OR, USA to assess the effects of mowing height as well as fertility timing and rate on tall fescue performance. Two mowing heights of 5.1 and 7.6 cm, four seasonal fertility timings, and three levels of annual N rates of 98, 196, and 294 kg·ha−1·yr−1 were evaluated using a 2 × 4 × 3 factorial experiment in a strip-plot design. Quantitative data of percent green cover and normalized difference vegetation index (NDVI) suggest that autumn fertilization is needed in cool, humid areas where tall fescue is actively growing in the winter months. The annual fertilization rate of 294 kg·ha−1·yr−1 N produced higher green turf cover and NDVI, compared with 98 or 196 kg·ha−1·yr−1 N. Furthermore, divergent effects of mowing heights were observed during winter compared with other months, suggesting that tall fescue could be mowed lower at 5.1 cm during cool, humid winter months and higher at 7.6 cm in other seasons for better overall turfgrass growth and less winter disease and thinning. Our research provides practical cultural practices for managing tall fescue turf in the Pacific Northwest or similar climates.
Tall fescue (Festuca arundinacea) is a cool-season grass that possesses wide adaptability to the cool-season and transition zones in the United States (Meyer and Watkins 2003). Tall fescue is commonly marketed and used as a low-input, environmentally friendly lawn and is also promoted as having low fertility and water-use requirements. With recent water restrictions in arid areas and the need to reduce water inputs, tall fescue is also a desirable turfgrass species for golf course rough and native landscapes.
The western portion of the Pacific Northwest of the United States is known for being dry in the summer and cool and humid in the other months. Tall fescue is a desirable choice in these regions where water is scarce and is often restricted by legislation during periods of drought. Although tall fescue performs well in the summer in these areas, a common issue with tall fescue planted in cool, humid climates like western Oregon is overall discoloration and thinning due to fungal diseases in the winter months. The thinning increases vulnerability to weed invasion, such as annual bluegrass (Poa annua). Low-temperature diseases caused by Microdochium nivale have been described by Tronsmo et al. (2001) for a wide range of symptoms from water-soaked dark margins on leaves to pinkish-white patches. Microdochium patch is a common disease in the Pacific Northwest, but the majority of the research is on golf turf (Mattox et al. 2023; Vargas 2005).
Nitrogen fertilization on tall fescue has been shown to decrease smooth crabgrass (Digitaria ischaemum) infestation and improve winter turfgrass quality when nitrogen rates increased from 98 to 196 kg·ha−1·yr−1 N in Maryland, USA (Dernoeden et al. 1993). In western Washington, perennial ryegrass (Lolium perenne) quality was improved and red thread (Laetisaria fuciformis) was suppressed after late autumn N fertilization (Miltner et al. 2004). In western Oregon, Microdochium patch was unaffected when 4.9 kg·ha−1 of N was applied every 2 weeks to annual bluegrass, whereas 9.8 kg·ha−1 N increased Microdochium patch (Mattox et al. 2017). It was unclear how different N application rates and timings would affect Microdochium patch >severity and turfgrass quality of tall fescue. The objective of this study was to determine the effects of mowing height and fertility regime (fertility timing and rate) on tall fescue green cover, color, and disease in a cool, humid region.
Materials and Methods
A field experiment was conducted at the Lewis Brown Horticultural Research Farm in Corvallis, OR, USA on a Chehalis silty clay loam soil. ‘Thor’ tall fescue was sown on 14 Jul 2020 at the rate of ∼29 g·m−2. The experimental design was a 2 × 4 × 3 factorial arranged in a strip-plot design with two mowing heights, four fertilization timings, and three fertilization rates. The treatments were replicated four times. Two 75-cm-wide mowing strips at heights of 5.1 and 7.6 cm were applied across 12 1.5 × 1.5 m plots. The entire field was initially mowed weekly at 5.1 cm after establishment; mowing treatments were then initiated on 20 Nov 2020. Research plots were mowed every other week from December until February and weekly from March to November using a rotary mower (HRX217VKA; Honda, Tokyo, Japan) with clippings removed. Tall fescue plots were fertilized five times throughout the year with four seasonal timings (Table 1) and three rates. Fertilizer treatments were applied using a 28N–3P–10K fertilizer plus micros (The Andersons Professional Turf™ Products; Maumee, OH, USA), which contains 14% slow-release polymer-coated urea (50% of the total N). The three levels of annual N rates evaluated in this study were 98, 196, and 294 kg·ha−1·yr−1 N. Irrigation at ∼0.6 cm was applied to the entire trial immediately after a fertilizer application except for when natural precipitation occurred.
Seasonal fertilizer application timings.
The entire study area was irrigated at the 80% reference evapotranspiration replacement rate during the summer months. Glyphosate was applied at 0.287 kg a.i./ha on 16 Apr 2021 to control annual bluegrass. This herbicide application resulted in noticeable reduction of turfgrass quality (e.g., percent green cover; Fig. 1) and yellowing (Fig. 2), thus the May 2021 data were removed from further analyses. Additionally, mesotrione was applied at 0.140 kg a.i./ha on 20 Oct, 5 Nov, 19 Nov, and 3 Dec 2021 to selectively control annual bluegrass and bentgrass (Agrostis spp.).
Tall fescue percent green cover for each observation date from Nov 2020 to Mar 2023 presented in a box-and-whisker plot. The low percent green cover values in May 2021 were the result of a glyphosate application on 16 Apr 2021 to control annual bluegrass.
Citation: HortScience 59, 9; 10.21273/HORTSCI17871-24
Tall fescue normalized difference vegetation index (NDVI) for each observation date from Nov 2020 to Mar 2023 presented in a box-and-whisker plot. The low NDVI values in May 2021 were the result of a glyphosate application on 16 Apr 2021 to control annual bluegrass.
Citation: HortScience 59, 9; 10.21273/HORTSCI17871-24
Turfgrass quality and color were assessed using percent green cover and normalized difference vegetation index (NDVI), respectively. Monthly data were collected from 21 Nov 2020 to 21 Mar 2023. Digital images were collected with a digital camera attached to a light box as described by Kowalewski et al. (2023). High-quality digital images were saved in JPEG format with 1600 × 1200 pixels as reported by Wang et al. (2021). Digital images were analyzed using a Java program (TurfAnalyzer; Green Research Services, LLC, Fayetteville, AR, USA) based on previous research (Karcher and Richardson 2005; Richardson et al. 2001). Percent green cover was calculated using threshold settings of hue 75 to 360, saturation 35 to 100, and brightness 0 to 90. Five random NDVI measurements were taken per plot using a FieldScout CM1000 NDVI meter (Spectrum Technologies, Inc., Aurora, IL, USA) held at ∼90 cm above the turf canopy, and the averages were used for statistical analyses.
Data were subjected to analysis of variance (ANOVA) using SAS 9.4 Proc Mixed (SAS Institute Inc., Cary, NC, USA). Fisher’s protected least significant difference (LSD) at the 0.05 P level was used to determine treatment differences. Percent green cover data were divided into two groups to reflect turfgrass quality without disease (median percent green cover above 70%) and turfgrass quality affected by disease in the winter (median percent green cover below 70%), as indicated in Fig. 1. Similarly, NDVI data were divided into two groups to reflect turfgrass color without disease (median NDVI ≥0.85) and turfgrass color affected by disease in the winter (median NDVI <0.85), as indicated in Fig. 2. Correlation analysis between percent green cover and NDVI was performed in SAS using data from all observation dates except for May 2021.
Results and Discussion
In this study, percent green cover and NDVI were used to quantitatively assess turf quality and color, respectively, and were highly correlated (Pearson correlation coefficient of 0.85514, P < 0.001). Tall fescue turf performed well most of the year in the Pacific Northwest, as indicated by high median values of percent green cover above 70% (Fig. 1) and NDVI ≥0.85 (Fig. 2). However, in the winter months (Dec 2021, Jan 2022, Feb 2022, Mar 2022, Nov 2022, Dec 2022, Jan 2023, and Feb 2023), tall fescue turf exhibited substantial decreases in percent green cover with median values below 70% (Fig. 1). Similarly, the median NDVI values were lower than 0.85 in Dec 2021, Jan 2022, Feb 2022, Dec 2022, Jan 2023, and Feb 2023 (Fig. 2). The reduction in percent green cover and NDVI was associated with Microdochium patch disease based on the symptoms of tan or brown turf with water-soaked outer edges (Fig. 3) as described by Tredway et al. (2023) and further confirmed by morphological identification of the fungal pathogen using microscopy.
A representative light box image of tall fescue turf exhibiting symptoms of Microdochium patch on 20 Jan 2023.
Citation: HortScience 59, 9; 10.21273/HORTSCI17871-24
The main effects of fertility timing and rate had significant influence on the percent green cover and NDVI regardless of disease, whereas the mowing height effect differed in the presence and absence of disease (Tables 2 and 3). The fertility program without autumn application resulted in significantly lower percent green cover and NDVI than any other fertility program regardless of the presence or absence of disease (Tables 2 and 3). These results suggest that in areas similar to the Pacific Northwest where tall fescue is actively growing in the winter, fertilizer application in autumn is beneficial. In a similar climate, research has shown that late autumn and winter N fertilization can improve perennial ryegrass and Kentucky bluegrass (P. pratensis) turf quality (Miltner et al. 2004). As expected, the percent green cover and NDVI increased significantly as the annual N rate increased from 98 to 294 kg·ha−1·yr−1 N (Tables 2 and 3). Clippings were collected and removed during the study to avoid contamination across plots from different nutrient levels in the clippings. If environmental impacts and sustainability are concerns when managing tall fescue, clippings can be returned to reduce N fertilization needs (Kopp and Guillard 2002; Liu and Hull 2006). Interestingly, the higher mowing height of 7.6 cm resulted in slightly higher percent green cover in the absence of disease, although this effect was considered not significant at the 0.05 P level, whereas the lower mowing height of 5.1 cm produced significantly higher percent green cover than the height of 7.6 cm (55.6% vs. 48.0%, respectively) in the presence of disease (Table 2). The higher mowing height of 7.6 cm also resulted in statistically higher NDVI in the absence of disease but lower NDVI in the presence of disease (Table 3).
Analysis of variance and means table for percent green cover in tall fescue analyzed using digital image analysis as affected by mowing height, fertility timing, and fertility rate in Corvallis, OR, USA.
Analysis of variance and means table for normalized difference vegetation index (NDVI) in tall fescue as affected by mowing height, fertility timing, and fertility rate in Corvallis, OR, USA.
There were no two- or three-way interactions in percent green cover (Table 2). In contrast, a significant mowing height by fertility rate interaction was identified with NDVI in the presence and absence of disease (Table 3). In the absence of disease, the highest N rate of 294 kg·ha−1·yr−1 produced a greater difference in NDVI between 5.1 and 7.6 cm mowing heights compared with the lowest N rate of 98 kg·ha−1·yr−1, suggesting that the 7.6 cm mowing height and the 294 kg·ha−1·yr−1 N rate could have an additive effect in improving turf color (Fig. 4A). During Microdochium patch season, a greater difference was observed between the 5.1 and 7.6 cm mowing heights at the fertility rate of 98 compared with 294 kg·ha−1·yr−1 N, suggesting that when N was applied at a high rate, the mowing height could have a lesser effect on turf color (Fig. 4B). Nevertheless, the aforementioned interactions did not alter the conclusion from their respective main effects that increasing N rates resulted in higher NDVI and that better turf color as indicated by higher NDVI values was associated with mowing at 5.1 cm when disease was present and mowing at 7.6 cm when disease was absent (Table 3 and Fig. 4).
Tall fescue normalized difference vegetation index (NDVI) response to the interaction of mowing height and fertility rate. (A) NDVI data were averaged across 22 observation dates (Nov 2020, Dec 2020, Jan 2021, Feb 2021, Mar 2021, Apr 2021, Jun 2021, Jul 2021, Aug 2021, Sep 2021, Oct 2021, Nov 2021, Mar 2022, Apr 2022, May 2022, Jun 2022, Jul 2022, Aug 2022, Sep 2022, Oct 2022, Nov 2022, and Mar 2023) when disease was absent. (B) NDVI data were averaged across six observation dates (Dec 2021, Jan 2022, Feb 2022, Dec 2022, Jan 2023, and Feb 2023) when disease was present. Means followed by the same lowercase letter in each graph were not significantly different at the 0.05 P level.
Citation: HortScience 59, 9; 10.21273/HORTSCI17871-24
Although, an N rate of 9.76 kg·ha−1 applied every 2 weeks can lead to increase in Microdochium patch severity on annual bluegrass putting green turf compared with 0 or 4.88 kg·ha−1 N (Mattox et al. 2017). In tall fescue turf, we observed that the highest N rate of 294 kg·ha−1·yr−1 evaluated in this current study resulted in the highest turf green cover (Table 2). It is possible that the improvement in the turfgrass health and growth outweighs the effects of fertilization on increasing Microdochium patch disease in tall fescue. Digital image analysis was able to assess the percent green cover quantitatively in tall fescue in this study; however, there are limitations associated with quantifying disease (Kowalewski et al. 2023). The lack of green turf cover in the winter months (Fig. 1) was likely associated with disease symptoms as well as general yellowing and thinning of the tall fescue turf. Unfortunately, the digital image analysis used in this study was not able to distinguish disease symptoms vs. abiotic stresses. There is growing interest in advanced artificial intelligence technology such as deep learning, which has great potential to assist in identifying plant diseases (Li et al. 2021; Saleem et al. 2019).
Tall fescue is considered one the most drought- and heat-tolerant cool-season turfgrasses (Beard 1973). Despite the desired traits and ongoing breeding efforts for summer quality and abiotic stress tolerance (Carrow and Duncan 2003; Karcher et al. 2008; Tate et al. 2023), improving tall fescue winter quality in a cool and humid climate has rarely been researched. Using digital image analysis, we demonstrated that tall fescue had acceptable green turf cover after establishment in the first winter but exhibited low percent green cover in the second and third winters (Fig. 1). Our research also suggests that autumn fertilizer application is needed to increase percent green turf cover and NDVI. Both mowing heights evaluated in this study are within the optimal mowing height range for turf-type tall fescue. Indeed, research has shown that turf-type tall fescue can adapt to a wide range of mowing heights from 1.3 to 10 cm (Braithwaite et al. 2022; Burns 1976; Dernoeden et al. 1993; Richie et al. 2002; Straw et al. 2020). The lower mowing height of 5.1 cm enhanced winter turfgrass quality compared with the 7.6 cm height of cut. These results suggested that mowing at varying heights in the summer and in the winter could be an option for managing tall fescue in the Pacific Northwest. Similarly, a study conducted in Maryland, USA indicated that the higher mowing height of 8.8 cm in tall fescue produced best summer turf quality but exhibited lowest winter turf quality, compared with mowing at 3.2 or 5.5 cm (Dernoeden et al. 1993). A field study has shown that mowing Kentucky bluegrass at a height of 7.6 cm resulted in higher canopy photosynthetic rates and better turf quality compared with mowing at 3.8 cm (Song et al. 2015). Higher mowing heights are also associated with greater root systems (Beard 1973; Christians 2011), which, especially in the deeper root zone, are desirable for drought tolerance in the summer (Carrow 1996). Therefore, the higher mowing height in the absence of winter disease could have benefited tall fescue with improved above- and below-ground growth to withstand abiotic stresses. On the other hand, the reduction in disease and enhanced green cover in the winter demonstrated in this study by mowing lower was likely because it created less conducive canopy conditions (such as reduced leaf wetness) for disease development. However, this hypothesis will need to be tested in future studies.
Our research provides practical cultural practice considerations for managing tall fescue turf in the Pacific Northwest or similar climates. Autumn fertilization is needed in cool, humid areas where tall fescue is actively growing in the winter months. The higher annual fertilization rate of 294 kg·ha−1·yr−1 N produced higher green turf cover and NDVI when clippings were removed, compared with 98 or 196 kg·ha−1·yr−1 N. Tall fescue could be mowed lower at 5.1 cm during cool, humid winter months and higher at 7.6 cm in other seasons for better overall turf growth and less winter disease and thinning.
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