Clover–Turfgrass Lawns, Nitrogen Fertilization, and Pollinator Conservation: Comparison
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Please note this is a comparison between Version 2 by Vicky Zhou and Version 1 by Daniel A. Potter.

The runoff or leaching of nitrogen fertilizers from monoculture turfgrass lawns contributes to water pollution, and such lawns are susceptible to insect pests and provide few resources for pollinators. One approach to creating more sustainable lawns is to incorporate white clover (Trifolium repens L.), a nitrogen-fixing legume, into grass seed mixtures or existing turfgrass swards.  Besides augmenting nitrogen in lawns, clover also supports bees and other pollinators.

  • Trifolium repens
  • Schedonorus arundinaceus
  • pollinators
  • Popillia japonica
  • low-input lawns

1. Introduction

Lawns are a pervasive feature of urban and suburban landscapes worldwide [1,2][1][2]. Traditionally defined as “ground covered with closely mowed vegetation, usually grasses” [3], lawns dominate residential lots, institutional and commercial landscapes, parks, cemeteries, playgrounds, street verges, and medians, occupying an estimated 39–54% of the total area devoted to urban development in the continental United States [4]. Traditional turfgrass lawns provide aesthetic, recreational, and human psychological and physical health benefits while providing ecosystem services; e.g., reducing soil erosion and surface runoff, dissipating heat through evaporative cooling, and reducing noise and glare compared with impervious surfaces [2,5,6,7][2][5][6][7]. Turfgrass lawns also sequester atmospheric carbon [4[4][8][9][10],8,9,10], although those benefits may be more than offset by the amounts of fossil fuels, water, nitrogen, and pesticides used in lawn maintenance [4,10,11,12][4][10][11][12]. Thus, there is growing interest in alternative lawns needing fewer resource inputs and providing greater ecosystem services than traditional all-turfgrass lawns [1,2][1][2].
Nitrogen (N) fertilization is a common cultural practice for establishing and maintaining turfgrass lawns. Nitrogen enhances sward density, color, and vigor, and is the nutrient required in the highest amount by the grass plant [13]. However, lawn fertilizers that are applied off-target to impervious surfaces, such as driveways and sidewalks, or to dormant, dying, or dead turfgrass can be moved by irrigation or rainfall into bodies of water, contributing to non-point source pollution [14,15][14][15]. In addition, N applied in soluble forms (e.g., urea) can result in nitrates leaching into groundwater [16].
One way to reduce the need for N inputs is to incorporate white clover (Trifolium repens L.), a nitrogen-fixing legume, into grass seed mixtures or existing turfgrass swards. Productive mixtures of white clover and cool-season turfgrasses can often fix between 100 and 200 kg N ha−1 year−1 with positive effects on turfgrass growth, color, and visual appeal [17,18][17][18]. The typical recommended N fertilization rates for moderately maintained cool-season turfgrass lawns range from 98–195 kg N ha−1 year−1. Consequently, if white clover can supply just a portion of a lawn’s annual N needs, the resulting reductions in fertilizer use could be substantial [19].
Trifolium repens, a perennial forb of Eurasian origin [20], was introduced into North America during the early colonial period, and has also been exported to other temperate regions of the world [20]. It grows naturally in pastures and along roadsides where climatic and soil conditions are suitable, and is intentionally seeded in mixed grass/clover pastures. “Dutch” white clover (DWC) is the vernacular term for a ubiquitous landrace found in many landscapes. Like all T. repens, it grows and spreads via stolons that root at the nodes [20]. A common component of lawn seed mixes before World War II, DWC fell out of favor with the introduction and marketing of broadleaf weed lawn herbicides in the 1950s. Furthermore, DWC tends to form taller, dark green clover-only patches that reduce lawn uniformity, and its flowers attract bees [21[21][22][23],22,23], which some people, e.g., persons with sting allergies or families with small children, may consider to be a hazard.
Microclover® (DLF-Pickseed, Tangent, OR) is a dwarf variety of white clover (T. repens var. “Pipolina”) selected for its small leaf size and low growth habit to allow it to tolerate low mowing heights and to blend in with cool-season turfgrasses without clumping or outcompeting the grass [19,24,25,26][19][24][25][26]. Used at a low rate in grass seed mixtures, or seeded into an existing turf stand, it supplies N to the grass and helps to prevent the establishment of weeds by increasing the canopy density [26]. Microclover® was bred to have fewer and smaller flowers than DWC, although it will flower more if mowed less frequently (L. Brilman, DLF-Pickseed, pers. comm.). Trifolium repens var. “Barbian”, marketed as Turf Clover (Barenbrug USA, Tangent, OR), is another dwarf clover variety developed to blend more easily with turfgrasses than DWC.
In the United States where there are normative social pressures to maintain lawns as low-mowed turfgrass monocultures [27[27][28],28], spontaneous flowering forbs such as clover, the common dandelion (Taraxacum officinale F.H. Wigg.), and violets (Viola spp.) are traditionally viewed as undesirable weeds to be eliminated. Indeed, the US home and garden sector spent USD 450 million on lawn herbicides and plant growth regulators in 2012 [29]. That mindset seems to be changing, however, as the public becomes more aware of the threats to honey bee health [30], as well as steep declines in the populations of wild bees and other pollinators [31,32,33,34][31][32][33][34]. A reduction in floral resources (nectar and pollen) due to urbanization and other land-use changes is thought to be a major factor contributing to wild pollinator decline, especially in North America and Europe [32,35,36][32][35][36]. Urban ecologists are increasingly encouraging the conversion of traditional monoculture turfgrass lawns to more diverse habitats (e.g., [2,37][2][37]). Millions of citizens engage in ecological gardening and landscaping to support pollinators [38[38][39],39], and there is a growing public acceptance of flowering lawns in community parks [40] and residential yards [41,42,43][41][42][43].
Trifolium repens produces copious amounts of pollen and nectar [36[36][44][45],44,45], and is highly attractive to wild bees and honey bees that forage in urban landscapes [21,22,23,46,47,48,49][21][22][23][46][47][48][49]. It is the most important nectar source in Britain [45], providing the highest portion of total nectar production by flowering plants in parks, cemeteries, and road verges, and more than half of the total nectar resources in lawns and other mowed amenity grasslands [36]. Due to its prolonged bloom period, T. repens can help to sustain pollinators during seasonal gaps in floral resource availability from other plants [46,49,50][46][49][50]. Clover lawns can serve as stepping stones for the movement of bees and other pollinators between gardens, natural-area remnants, and other urban green spaces [21,37,51,52,53][21][37][51][52][53]. Such connectedness of floral resources is thought to be important for supporting species richness and an abundance of urban bees [54,55,56,57][54][55][56][57]. Clearly, if most homeowners, schools, and parks dedicated just a portion of their lawns to clover, the benefits to urban bee conservation could be substantial. Besides planting it, encouraging spontaneous T. repens by reducing lawn herbicide use and mowing less frequently are other ways to augment the food supply for urban bees [21,58,59][21][58][59].
White clover monostands are intolerant of heavy foot traffic, and are thus unsuitable for competitive sports fields or busy playgrounds [20,60][20][60]. Mixed clover–turfgrass swards, however, are relatively more resilient to foot traffic [18[18][20],20], and therefore suitable for use in home lawns, park lawns, medians, and other low-to-medium traffic sites [24,61][24][61]. White clover is relatively intolerant of shade, high heat and drought, and winter cold, which can cause canopy dieback and leave bare areas that can become muddy and require reseeding [20,61][20][61]. Attaining and maintaining a consistent percentage of clover to grass is difficult [20] as clover often disappears from mixed swards over several years [62], however it can be reseeded back in. It is also intolerant of many broadleaf herbicides, which can limit options for weed control, although some herbicides provide selective control of some broadleaf weeds in mixed clover–turfgrass swards [63,64][63][64]. Breeding for dwarf clover genotypes that are more resistant to environmental stress may improve their persistence [24].

2. Dwarf White Clover Supports Pollinators, Augments Nitrogen in Clover–Turfgrass Lawns, and Suppresses Root-Feeding Grubs in Monoculture but Not in Mixed Swards

Reconciliation ecology, “the science of inventing, establishing, and maintaining new habitats to conserve species diversity in places where people live, work, and play” [73][65], seeks ways to modify urban landscapes to support native biota without compromising societal utilization. Even in communities where there are normative social pressures to maintain residential and commercial front lawns as turfgrass monocultures, other sites, including backyards, institutional grounds, community parks, cemeteries, and street verges and medians, offer opportunities to integrate low-growing flowers into mowed turfgrass without compromising aesthetics or use for informal games and sports, picnicking, dog-walking, or other recreational activities [2,40][2][40]. However, movement to clover lawns to support pollinators and reduce the need for nitrogen fertilizer and other lawn chemicals will require changes in the aesthetic expectations of homeowners, turfgrass managers, and the general public. Using dwarf varieties of T. repens that blend into the turfgrass sward may encourage a greater acceptance of clover lawns [19].
This study indicates that dwarf white clover supplies nitrogen to tall fescue to a similar degree as Dutch white clover. The elevation in foliar N for grass in association with clover ranged from 17–23%, depending on the clover type, which is comparable to increases that were obtained in tall fescue turf by applying 150–200 kg N/ha per year [74][66]. All three clover types bloomed profusely in the monocultures and produced similar numbers of flowers when incorporated into existing turf-type tall fescue. Despite their generally smaller bloom size, both dwarf clover varieties were visited by similar types of bees as those foraging on DWC. The monocultures of all the clover types had very few white grubs compared with the plots of tall fescue alone, although a similar reduction in grub numbers did not occur in the clover–tall fescue dicultures. Except for somewhat higher populations of leafhoppers and froghoppers, which caused no obvious aesthetic damage, the invertebrate communities in all three clover–tall fescue dicultures were similar to those found in plots of tall fescue alone.
Nitrogen is a limiting element in the diet of most plant-feeding insects [75][67], with a higher nitrogen content commonly associated with increased performance [74,75][66][67]. Black cutworms grew more quickly on all clover types than on tall fescue, likely due to the clovers’ higher foliar nitrogen and lower fiber content. While it is also possible that by augmenting the foliar nitrogen in the associated turfgrasses, the clover in the mixed stands could confer associational susceptibility [76][68], causing the grass to become nutritionally more suitable for graminivorous pests, the same caveat applies when grasses are fertilized with other nitrogen sources [74][66]. In this study, however, none of the field plots sustained noticeable damage from cutworms or other grass-feeding caterpillars (e.g., sod webworms (Crambidae) or armyworms (Spodoptera, Mythimna spp.)) that feed on the same suite of cool-season lawn grasses [71][69].
Herein collected 17 different bee species foraging on T. repens in our monoculture clover plots, compared with 31 and 56 species of bees found in previous surveys of bees visiting spontaneous white clover in urban and suburban park lawns and other lawn sites in Kentucky and Minnesota, respectively [21,43][21][43]. The lower species richness in this study doubtless reflects the fact that the bees were collected from replicated plots at a single site, a turfgrass research facility; whereas in the previous surveys, bees foraging on T. repens were sampled across 16–18 established lawn sites surrounded by relatively more diverse vegetation. In the Minnesota study, the number of bee species collected on clover at a given site ranged from 5–17 in a given year, suggesting that dwarf clovers, too, would recruit additional bee species if planted more widely. Notably, in each of the three aforementioned studies, ≥90% of the non-Apis bee species that were sampled from T. repens are native to the United States, underscoring that this non-native flowering legume hosts a range of polylectic wild urban bees. Although A. mellifera and Bombus spp. (especially B. impatiens) were numerically dominant in all three studies, we found Bombus spp., all native, were even more abundant than A. mellifera in late summer. White clover blooms and provides floral resources for much of the growing season, a characteristic that is particularly valuable to pollinators such as A. mellifera and Bombus spp. that have season-long foraging activity [77][70]. A caveat to our study is that we did not measure the quantity of nectar and pollen provided by the dwarf clovers versus DWC. It is possible that the dwarf clovers’ value to bees could be somewhat reduced if foraging workers must visit more blooms to obtain equivalent floral rewards.
Low-maintenance turfgrass lawns typically harbor numerous species of predatory invertebrates, mostly generalist feeders, that collectively help to regulate populations of herbivorous invertebrates, including pests [78,79][71][72]. The epigeal and arboreal predators we found inhabiting the dwarf and DWC monocultures and clover–tall fescue dicultures, including spiders (mostly Linyphiidae, Erigonidae, and Lycosidae), ants (mostly Lasius and Solenopsis spp.), ground beetles, rove beetles, lady beetles, and predatory Hemiptera, were typical of those found in turf and pasture grass in the eastern United States [80,81,82][73][74][75]. Our samples also contained small parasitic wasps, which were not identified because their trophic relationships in cool-season turfgrasses are largely unknown [71,78][69][71]. Herbivores and earthworms in the clover plots also were similar to those inhabiting turf-type tall fescue lawns in Kentucky [74,82][66][75]. Overall, we saw no indication that incorporating dwarf clovers into tall fescue would significantly change the invertebrate community of lawns.
Herein found very low numbers of P. japonica grubs in all clover monocultures compared with the tall fescue and mixed clover–tall fescue plots. Larval P. japonica feed on the roots of a wide range of plants, including all cool-season turfgrasses [83][76], but according to Fleming [84][77], they “do not thrive” in plantings of T. repens or other Trifolium species. Studies with another generalist root herbivore, Melolontha melolontha (L.) (Scarabaeidae), suggested that T. repens resistance to its grubs is more likely due to a high root lignin content resulting in reduced feeding, as opposed to chemical repellence [85][78]. Many T. repens genotypes contain foliar cyanogenic glucosides [86][79], which deter the feeding and oviposition of some non-adapted insect herbivores [87][80]. However, adult P. japonica feed on the foliage of many cyanogenic plants, including T. repens [84][77], therefore it seems unlikely that those chemicals account for T. repens resistance to the grubs. White clover roots also contain flavonoids [88][81], but whether they play any role in resistance to root-feeding scarab grubs is unknown.
Importantly, P. japonica grub populations were not reduced in any of our clover–tall fescue dicultures compared with tall fescue alone, suggesting that incorporating dwarf or DWC into turf-type tall fescue will not provide associational resistance [76][68] to the grass itself, at least not at the clover density we attained by over-seeding it into existing tall fescue (e.g., Figure 1C,D). It is possible that by starting with a seed mix containing a relatively high proportion of clover, or by reseeding additional clover into a mixed sward, one could establish mixed clover–tall fescue lawns with enhanced resistance to P. japonica. Further research on how clover monocultures and various ratios of clover to turfgrass affect white grub species other than P. japonica, and other lawn insect pests, is warranted. Regardless of the agronomic challenges of obtaining and maintaining a consistent, optimal percentage of clover to grass to match the aesthetic, recreational, and other use requirements for particular sites, the benefits of dwarf clover for supporting pollinators and reducing the need for inorganic or synthetic organic fertilizers justify their more widespread use in low-input lawns.
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Figure 1. (A) Seeding the field plots, June 2015. (B) Cover monoculture plot in bloom, June 2016. (C) Interface of monoculture clover and clover–tall fescue diculture plot. (D,E) Differences in “Dutch” white clover and Microclover leaf and flower sizes.

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