Benefits of turf are well documented and include: recreational health, erosion control, increased water infiltration, reduced nutrient leaching, aesthetics, carbon sequestration, and mediation of the “heat-island” effect (Beard and Green, 1994; Qian and Follett, 2002), yet the ecological impact of turf is often questioned, attributable in part to nutrient and water requirements (Milesi et al., 2005; Robbins et al., 2001; Robbins and Birkenholtz, 2003) as well as often-unsustainable monoculture cultivation, which contributes to insect habitat loss and fragmentation (Gels et al., 2002). For these reasons, the turfgrass industry is experiencing new demands for ecologically and economically sustainable maintenance options.
Inclusion of leguminous species, which biologically fixes N and provides a pollinator habitat (Abraham et al., 2010; Held and Potter, 2012; Rogers and Potter, 2004), is a proposed means of increasing the sustainability of certain low-maintenance turfgrass scenarios. However, little is known about inclusion of legumes in maintained turfgrass. Since the advent of herbicides, efforts in the turfgrass industry have often focused on maintaining monocultures for aesthetics and increased playability. Thus, a biologically diverse turfgrass sward with mixed species of grasses and broadleaf plants is sometimes classified as weedy and therefore undesirable for scenarios such as golf courses and sports pitches. However, for many scenarios such as home lawns, roadsides, or other “unimproved” turfgrass areas, the environmental benefits of biodiversity may outweigh those of a monoculture.
White clover inclusion within maintained turfgrass has mainly been limited to cool-season turfgrass scenarios. Research by Sincik and Acikgoz (2007) reported increased color ratings in three cool-season turfgrass–white clover (T. repens L.) mixtures and that white clover fixed greater than 25 g N/m2/year and contributed between 4.2% and 13.7% of that total N to the associated turfgrass. Additional information concerning white clover inclusion within maintained turf is absent from peer-reviewed literature. However, information about the benefits of white clover inclusion within pasture systems is fairly abundant but mainly focused on perennial ryegrass (Lolium perenne L.)–white clover pastures. These mixed systems supply high-quality grazing for animals while simultaneously improving soil fertility (Lampkin, 2002). Estimates of N fixation for grass–white clover pastures range from nil to 40 g N/m2/year, although most are roughly 10 to 25 g N/m2/year (Ledgard and Steele, 1992; McNeill and Wood, 1990).
White clover is well suited for use within warm-season turfgrasses and is already a common feature within bermudagrass pastures of the southeastern United States (Brink and Fairbrother, 1991). Proper white clover establishment is key to maximizing stand uniformity as well as N contribution to associated grasses (Frame and Newbould, 1986). However, there are currently no guidelines for establishment within warm-season turfgrass scenarios common to the southeastern United States. Furthermore, unlike pasture systems, managed turfgrass scenarios may offer unique opportunities to manipulate turfgrass height and density as well as soil characteristics in favor of white clover establishment.
We hypothesized that white clover establishment is comparable to overseeding dormant warm-season turfgrass with cool-season grasses such as perennial ryegrass. However, unlike perennial ryegrass establishment rates (≈500 to 700 kg perennial ryegrass seed/ha), white clover establishment rates are much lower [Frame and Newbould (1986) recommend 3 to 5 kg white clover seed/ha].
There are several agronomic practices used to improve overseeded grass establishment within maintained turf scenarios. Scalping is among the most common techniques and refers to the excessive removal of living tissue at any one mowing occurrence (Turgeon, 2002). Although scalping often results in turfgrass injury, it is a means of exposing bare soil and eliminating turfgrass competition, which is essential to overseeded grass establishment. Verticutting, or vertical mowing, is another mechanical method often used to remove accumulated thatch or to elevate decumbent turfgrass before overseeding. Verticutting is performed by passing a rapidly rotating horizontal shaft with vertically oriented knives over affected turfgrass (Turgeon, 2002). Verticutting is often used in addition to scalping to prepare warm-season turfgrass for overseeding. Hollow tine aerification is less commonly used for fall overseeding but is an agronomic practice used to improve soil characteristics by removing cores of soil from turfgrass. Core sizes may vary, but the desired result is much the same. That is, the cores are removed to alleviate compaction by decreasing soil bulk density, accelerate drying, and increase infiltration of water and gasses. Once performed, cores are often collected or scattered, and the remaining holes are either filled with sand or left open (Turgeon, 2002).
Hypothetically, scalping alone or scalping in combination with verticutting and aerification may be a means of improving seeded white clover establishment through improved seed-to-soil contact and by limiting competition effects from associated turfgrasses. Soil aerification may also alleviate competition but has the added benefit of providing holes in which white clover may find more adequate soil conditions for initial establishment. It is therefore reasonable that it too should be tested as a means of improving white clover establishment.
Other variables that affect white clover establishment are establishment timing and seeding rate. Recommended establishment dates for white clover in the southeastern United States are largely anecdotal. For instance, establishment timing is often recommend from 2 to 6 weeks before historical first frost (1 Nov. in Auburn, AL). Previous research in Florida recommends September planting dates (Dudeck and Peacock, 1983), whereas others have recommended spring seeding to avoid hard freeze in more northern climates (Frame and Newbould, 1986). These dates are highly variable and dependent on locations and climate. Furthermore, they may not account for nuances of a maintained turf sward, which may insulate young white clover seedlings from effects of frost or hard freeze. Anecdotal to our own research, proper stand density is highly dependent on seeding rate, yet it does not appear to be a linear response, perhaps as a result of intraspecies competition.
White clover establishment within cool-season grass swards has largely been dictated by seed mixtures of cool-season grass blends containing creeping bentgrass (Agrostis stolonifera L.), kentucky bluegrass (Poa pratensis L.), or perennial ryegrass plus roughly 3% to 10% white clover by weight (Sincik and Acikgoz, 2007), yet these rates have not been evaluated in existing warm-season turf swards. Likewise, information about interaction effects of white clover and companion grass species is absent from the scientific literature. Alternative, grass–white clover mixtures for turfgrass are commercially available in much of Europe and the United States; however, they have not been evaluated for winter overseeding of dormant warm-season grasses.
Our objectives were to test the effects of pre-seeding mechanical surface disruption, establishment timing, seeding rate, and companion grass species on establishment of two commercially available white clover populations within a dormant hybrid bermudagrass lawn. White clover was chosen as a model species for a variety of reasons, but specifically because turf-compatible white clover varieties are commercially available and as a result of white clover prevalence in maintained turfgrass as a weed species (Watschke et al., 1995). We present results that may influence future scientific studies and the use of white clover inclusion within warm-season turfgrass scenarios.
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