By: Cale Bigelow1*, Mike Fidanza2, Erik Ervin3, and Xunzhong Zhang4
1Purdue University-Department of Horticulture and Landscape Architecture, 2 The Pennsylvania State University, 3 University of Delaware-Department of Plant and Soil Sciences, 4 Virginia Tech-School of Plant and Environmental Sciences


Overview: Unlike traditional production or specialty crop agricultural systems which tend to focus on a measurable crop yield at the end of the growing season which is ideally more than the previous growing season, successful cultivation of turf systems (e.g. lawns, sod, athletic fields and golf turf) have much different and sometimes vague measures of success. I cannot think of anyone who manages turf that outwardly discusses their intent to reach maximum clipping yields. For example, maximum lawn clipping yield as a goal would likely result in substantially increased mowing needs, high labor and equipment costs and likely an overall low-quality turf. By contrast, the management focus of a high-quality turf used for sport like golf or soccer is surface firmness. Additionally, management goals include persistence under close mowing (e.g. < 15 mm and for golf putting greens < 3 mm) fast ball roll speeds and optimal rooting to promote safe footing and ensure player safety. Compared to shoot production it would not be uncommon for a turf manager to aim for maximum seasonal rooting. Most turf managers are keenly aware and interested in any cultural practice or product that may enable them to maintain, promote and optimize a deep, “healthy”, persistent root system. Lastly, the ability of the plant to tolerate and rebound from stressful environmental (e.g. supraoptimal heat, cold, or acute/chronic drought), biotic (e.g. turf pests) and abiotic (e.g. intense traffic/compaction) conditions is also always of interest.

What is also unique about turf systems is the fact that turfgrass is an amenity crop with a wide spectrum of intended use, cultural management intensities and owner/user expectations. While the large majority of biostimulant and plant health products have normally been consumed by the athletic field/golf market turf industry segments, there are also opportunities in lawn turf which represents the largest acreage of turf. Generally, a lawn system would be deemed “acceptable” if it persists annually and maintains a relatively consistent high level of seasonal green color and shoot density. This is, however, often easier said than done. First, many lawn turf species are planted on the heavily disturbed, impoverished urban soils and many contractors plant poorly adapted turfgrass species/cultivars. These factors create significant management challenges especially when the plants are subject to extreme environmental conditions. For example, the vast majority of lawns in the United States are unirrigated and the US Drought monitor ( for 24 August 2021 listed that > 55% of the USA is experiencing abnormal drought conditions, while > 25% is suffering from extreme drought. If these conditions continue and the plants are not able to be resilient when precipitation returns there will be large scale catastrophic turf loss which will require replanting.
So, why does this matter? One of the biggest benefits of cultivating a perennial turf like a lawn is the fact that compared to other cropping systems or even native prairie plants, lawn turf functions as a tremendous soil stabilizer and environmental filter (Steinke et al. 2007). This fact is important because population growth will continue as urban/suburban areas expand and the ground-cover of choice in these lawns, parks, and recreational areas to protect the soil will be turfgrass. To remain healthy and vigorous turf plants must receive at least a minimum level of nutrition. The problem, however, is that there are increasing public and regulatory pressures around supplemental water inputs and lawns that require fewer fertilizer and pesticide inputs. Although it seems relatively simple to just eliminate fertilizer and water inputs, that would be a naïve approach. In a very short period of time these areas would lose significant density and be more subject to weed invasion and likely cause significant urban soil erosion. While superior turf breeding efforts can help to meet these low input goals, we are still often managing plants on impoverished soils. To be successful with low inputs, alternative nutritional and careful implementation of plant health products will be important. Thus, this appears to be an opportunity for current and future plant biostimulant materials and products.

What are Biostimulants? In the purest sense the word, a biostimulant can be defined as any material when applied to a plant that stimulates “life” (e.g. bio). Numerous materials fall this simple definition and it could even be argued that applying water to a plant is one example of a biostimulant since it “promotes life”. Research and use of plant biostimulants in turf systems is nothing new and work has been ongoing for over three decades. An early pioneer in biostimulant research was Dr. Richard E. Schmidt from Virginia Tech. His program pioneered many basic and applied aspects of biostimulants for many plants. A great deal of that research focused on exogenous applications of various natural and synthetic plant hormones with the intent of helping to optimize rooting (Liu et al. 1998), superior tolerance to soil-borne pests like nematodes (Sun et al., 1997), mitigate the effects of stresses like intense ultra-violet light (Schmidt and Zhang, 2001), heat-drought (Wang et al., 2012, 2013, 2014; Zhang and Schmidt, 1999,2000; Zhang et al., 2012,2013), salinity (Yan, 1993; Nabati et al. 1994), improving winter survival (Schmidt and Chalmers. 1993; Zhang et al., 2013), optimizing nutritional efficiency (Schmidt et al., 1999; Wang et al., 2011), or enhancing recovery from routine stressful mechanical cultural management practices (Bigelow et al., 2010).
Dr. Schmidt and with Dr. Xunzhong Zhang coined an initial operating definition for describing these various plant growth substances they were exploring that promoted plant growth without being nutrients, soil improvers or pesticides. They defined biostimulants as “materials that, in minute quantities, promote plant growth”. The use of the word minute in this definition was important and intended to differentiate the fact that these substances compared to traditional nutrients and/or soil amendments elicited a measurable and beneficial response at much lower application rates. They explained the plant biostimulation by way of hormonal effects and often plant protection against abiotic stress through various antioxidant production. Later, the term “metabolic enhancers” was also used but the important aspect was that something positive was happening to the plant beyond what mineral nutrition supplied.
More recently, the definition of biostimulants has been continually refined. In 2015, Patrick duJardin published the paper, “Plant biostimulants: Definition, concept, main categories and regulation”. In this article a plant biostimulant is defined as “any substance or microorganism applied to plants with the aim to enhance nutritional efficiency, abiotic stress tolerance and/or crop quality traits, regardless of its nutrient content.” The main categories of biostimulants proposed were: Humic and fulvic acids, protein hydrolysates and other N-containing compounds, seaweed extracts and botanicals, chitosan and other biopolymers, inorganic compounds, beneficial fungi and bacteria. These categories were important to define not only traditional biostimulants but also including the beneficial organisms that may elicit a positive plant health response while also helping to guide policymakers who might want to regulate these materials.
In the turf market segment, particularly in the USA, research into the biostimulant was continued at a high level by Drs. Ervin and Zhang who explored numerous aspects of biostimulant materials and their effect on turf physiology from the 2000’s to present. These projects included previously explored materials while including an expanded view of how various synthetic, non-mineral nutritional materials might also promote growth. It was postulated that these materials when applied in conjunction to the naturally biostimulants may produce additive and/or synergistic effect on turf plant growth. In addition to the categories named previously, Ervin (2013) described and suggested mechanisms for a wider range of additional products and chemistries commonly being applied to turf. For example, secondary plant hormones (e.g. salicylic acid) which can induce systemic acquired resistance (SAR) in response to plant pathogens (e.g. fungi, bacteria, insects, nematodes), similarly compounds like phosphites (PO3) that may stimulate phytoalexins (stress induced antimicrobial compounds) for health/diseases suppression. Additional products that could protect tissues from ultra-violet light injury like green pigments or compounds containing titanium dioxide and zinc oxide. Acibenzolar, which mimics salicylic acid effects to induce SAR. Synthetic fungicides like propiconazole had been previously explored by Dr. Schmidt with positive plant responses, but newer chemistries like pyraclostrobin was shown in a number of University research trials to slow down plant respiration and boost antioxidants under heat and drought stress. One interesting find was that pyraclostrobin naturally degrades to the amino acid tryptophan which is a precursor to the plant rooting hormone auxin. Amino acids like proline and glycine-betaine are often suggested for use as osmoregulators or dehydration avoidance compounds. The practice of supplementing amino acids via foliar applications in summer months is a long-standing suggested approach to improve plant health that might be experiencing energy depletion during stressful environmental conditions (e.g. the aim of mitigating seasonal summer decline for cool-season grasses). Lastly, the long standing well researched biostimulants like humic substances, auxin, seaweed and cytokinins are listed. A large fraction of biostimulant products marketed to the turf industry contain seaweed extract (SWE), or seaplant/kelp extract. SWE is naturally high in cytokinin (Crouch and VanStaden, 1993) and research by Schmidt, Ervin and Zhang has shown that SWE was similar to a synthetic cytokinin applications and that monthly application of SWE boosted antioxidant levels, less loss of root viability and improved heat/drought tolerance. Further, Ervin and Zhang showed that combining HA + SWE can provide better plant health during stressful conditions than using either alone. The turf industry contains a vast array of products that claim positive plant or soil health benefits, particularly under stress conditions. Choosing the right material for the intended specific plant benefit is truly important.

Summary: Managed correctly turf systems can be a tremendous environmental asset. A dense, healthy stand of turf offers numerous aesthetic, recreational and environmental benefits, most notably holding urban soils in place and filtering contaminants from urban water runoff. They are, however, perennial plant systems that require a certain level of nutritional and water inputs to maintain vigor and persist. If the goal is to manage them more sustainably with fewer traditional inputs, and make them more resilient to environmental stresses then biostimulants and other plant health products are tools to meet this goal. Researching biostimulant effects on turfgrasses allows for advances in more sustainable approaches to turf nutrition. The organic aspects of grass physiology are as important as mineral aspects in proper plant nutrition. The difference in mineral content between plants of the same cultivar with different growth responses is relatively small. Further, mineral nutrition is poorly correlated with tolerance to stresses. However, knowing the impact that biologically active materials have on turfgrass metabolism allows turf managers to condition turfgrass better to tolerate environmental stress. Gaining an understanding of both the mineral functions and metabolic effects will allow for enhancement of nutritional best management practices to mitigate numerous turf stresses. Using this knowledge provides an additional tool for producing high quality, resilient turf under when grown under adverse environments.”
With the focus on and ever-increasing scrutiny over nutrient and water inputs for managing coupled with sometimes severe turf stress like prolonged drought due to increasingly unpredictable climate conditions applying biostimulants to areas that have traditionally received less attention like lawns may become a more main-stream. Regardless of the biostimulant/plant health product or program it is important to point out that biostimulants are not a substitute for a sound plant mineral nutrition program. Further, if the aim is to incorporate biostimulants as part of a holistic plant health program research in turf systems has demonstrated that they must be applied in advance of environmental stresses, etc. to optimize their benefits. There are exciting innovations looming in this space and evidence-based efforts will lead the way in a better understanding of how these materials will help maintain and improve plant health.


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