Florida’s perennial summer fertilizer ban will begin again this month. Since 2015, no nitrogen or phosphorus applications have been permitted between June 1 to Oct. 1, and other parts of the country are following suit. Fertilizer restrictions exist in at least 11 states, and others are examining the impact of runoff in watersheds and environmentally sensitive areas.

This trend has many superintendents looking at holistic approaches to turfgrass health, including cultural practices and softer inputs, mainly biostimulants.

Harnessing nature’s inherent power

Biostimulants are mostly organic materials that, when applied in small quantities, may positively affect grass growth – especially root development – the health of the grass or its tolerance to biotic and abiotic stresses. The objective is to enhance plant metabolic activity and help it tolerate stresses such as temperature extremes and drought.

There are a number of compounds that are considered biostimulants. Some of the major categories of biostimulants are plant hormones and extracts, microbials, humus and humic acids, fulvic acid, amino acids and extracts of seaweed.

Superintendents at courses around the country have reported positive results after using biostimulants, and some product manufacturers have been quick to take credit. But agronomists say it’s often difficult to determine whether positive results have been caused by the biostimulants or some other factor(s).

“The use of biostimulants is increasing but it’s difficult to say how much,” says David Gardner, associate professor of agronomy at Ohio State University. “But suppliers are certainly pushing these materials.”

It’s also difficult to quantify the positive or negative effects of a given biostimulant product, Gardner says, due to a lack of solid research and the fact that commercial biostimulants are often combined with chemical fertilizers.

“Herbicides, for example, are regulated by the U.S. Environmental Protection Agency], which requires a certain amount of testing to ensure that product can do its job. University researchers are often commissioned to do trials, but you really don’t see that with biostimulants. There has not been a lot of funded biostimulant research by independent scientists.”

As a result, evaluating a biostimulant product may mean relying on research that could be biased or incomplete.

“We are now in the awkward realm of having anecdotal evidence to suggest that many of these products work, but not enough published research to support that,” Gardner says. In addition, just because a product generates positive results in a lab test, that doesn’t mean it will be effective in the real world.

Shawn Emerson, director of agronomy at Desert Mountain Golf Club in Scottsdale, Arizona, has seen some changes in attitudes toward the use of biostimulants in the golf industry.

“Twenty or 25 years ago, people didn’t understand them and didn’t buy into it,” he says. “Today, people understand that if you can assist the plant in using less nutrients or making more nutrients available to plant, the better off it will be.”

Today, a majority of golf courses use some form of biostimulant, Emerson says, particularly humic and fulvic acids.

“We have learned that a healthy plant uses less water,” says Emerson, who applies biostimulants on a monthly basis, in higher amounts during the nongrowing months. “Microbial stimulants in the soil can help a plant that is going through drought or heat stress. A lot of people started with seaweed extract, then moved into amino acids and humates. They are now going toward more specific biostimulants for plant health, in conjunction with fungicides.”

Although a valuable tool, biostimulants are not a complete solution. “Biostimulants are kind of like vitamin supplements: They can’t just be used on their own and expect results,” Emerson says, adding that it’s better to supplement what you are already doing with conventional fertilizers.

Mother Nature makes the rules

Local weather and climate patterns play a major role in determining how a course uses biostimulants. At Copperleaf Golf Club in Bonita Springs, Florida, director of golf course and grounds maintenance David Dore-Smith has seen “significant improvement” in turf quality in the course’s newly renovated greens and the high-sand-content soil in his fairways and tee boxes. Weekly applications of humic acids and fulvic acids – both natural compounds that comprise a fraction of soil organic matter – are part of his regular turf regimen.

Dore-Smith applies the biostimulants year-round at a rate of a 0.5 to 1 gallon per acre. “Every time we aerify the greens, whether it be using solid tines or core removal, we apply a granular organic product so that it has the best chance to get into the soil profile where it can be immediately utilized,” he says.

Dore-Smith also applies worm compost tea once a month at a ratio of 2.5 gallons per acre. Another favorite organic fertilizer is chicken manure, which he values for its slow release and “zero burn potential.” Applied when the grass is wet, the chicken manure helps improve the cation exchange capacity of the soil. Once a week, the course also applies liquefied sea kelp to the greens. “It has a lot of trace elements, minerals,” he says. “All of these things naturally promote microbe growth.”

Recently, the major challenge for superintendents in the South Florida area has been an ongoing drought. As of early April, the area had only received about 4.5 inches of rain in the previous six months, which poses a significant problem for a region with sandy soils that is accustomed to an average annual rainfall of 60 inches. “We’re trying to find the right balance of holding enough moisture in the ground and letting the excess moisture go through,” Dore-Smith says.

During the heat of summer, Copperleaf has also been applying a biosolid product made from waste collected by a local municipal wastewater plant. The product is “very cheap and readily available,” Dore-Smith says. “More superintendents are going to be looking into this type of fertilizer, with municipalities seeking ways to utilize their waste products more effectively.”

Chris Tritabaugh, superintendent at Hazeltine National Golf Course in Chaska, Minnesota, has been using seaweed extract and humic acid for the past decade, both at Hazeltine and at his previous course, Northland Country Club in Duluth, Minnesota. He has had good results in the form of improved root development and drought tolerance. “It’s helped us create a healthy soil system,” he says.

Finding the right balance

During regular spraying, Tritabaugh applies the seaweed at a rate of 1 to 2 ounces per thousand square feet, and the humic acid at 1 ounce per thousand square feet.

Tritabaugh says there’s a common misconception among superintendents that using biostimulants can enable courses to significantly change the way they manage their turf and give them the ability to eliminate a lot of other things they use, like traditional fertilizers. “I don’t think these products work that way,” he says. “They can help make your turf better, but they don’t solve any one particular problem all by themselves.” As a result, biostimulants may work best as an adjunct to traditional products, he says.

Brian Nettz, superintendent at Presidio Golf Course in San Francisco, applies compost tea weekly at a rate of 50 gallons per acre. He also applies seaweed extract weekly at a rate of 3 gallons per acre. “Seaweed is like V-8 juice for grass,” he says. To reduce stress on plants in the Bay Area’s cool, damp climate, he’s found enzymes to be useful when balancing plant nitrogen and energizing the root zone.

“We’ve reduced the amount we use because the turf is so healthy now,” says Nettz, who finds biostimulants to be at their most valuable during the winter, when they can keep turf healthy despite the relatively low soil temperatures.

The organic materials help reduce the need for chemical fertilizers, as well as mitigate the severity of plant diseases, Nettz says. “We’re never going to get completely away from using fungicides on turf; [organics] are just another tool in the toolbox,” he says.

The Presidio course was an early adopter; it has used compost tea for more than 20 years and seaweed extract for about a decade, according to Nettz. Back in the early 1990s, Presidio participated in a field trial to evaluate the effects of compost tea applications on greens.

Greens were sprayed weekly for 12 months at a rate of 1 gallon of compost tea per 1,000 square feet. It was applied during times of high disease pressure, and biweekly during times of moderate or low disease pressure.

Tests showed the turf treated with compost tea had longer root length and less Microdochium patch than untreated turf. Treated turf did not differ from untreated turf in color, density, soil carbon dioxide, soil oxygen, weed infestation, soil nutrient levels, soil bacterial biomass, soil fungal biomass or mycorrhizal root colonization. The researchers noted that “further trials are needed to evaluate various application rates and application methods to determine the full potential effects of compost tea on golf course turf and soils.”

While much more research is needed on the best ways to use biostimulants, it’s easier said than done. In recent years, the USGA has offered $10,000 grants to scientists who want to test biostimulant products, but few have taken them up on the offer, says Mike Kenna, director of green section research for the USGA.

“Scientists get a little frustrated” when trying to test products with a number of different ingredients, both organic and chemical, he says, because it’s difficult to tell which ingredients are responsible for results. “Biostimulants have so many ingredients, how do you compare products? It’s difficult,” he says.

When considering a biostimulant product for golf course use, Ohio State’s David Gardner recommends “trying to find solid data, and talking to someone who has used it.” While it won’t produce immediate answers, setting up a test plot is a good way to determine if a product will be beneficial to your turfgrass. He advises applying the product to a small test patch, with an adjoining, untreated patch for comparison purposes.

Gardner’s mantra is, “When in doubt, try it out – but make sure you have a test plot.”