You like Poa. You really like it.

Editor's note: The January "Verdure" column is Dr. Beth Guertal's response to comments she received from her November column about Poa annua. 

So, recently I wrote a "Verdure" column that focused on the ways in which we rid ourselves of Poa annua. The article was entirely, 100 percent centered on eliminating Poa annua and viewed Poa annua with the same derision we might heap on cockroaches or toe socks.

A superintendent wrote to let me know that I was coming from the wrong direction, that in many regions Poa annua is viewed as a desirable turfgrass and that the quality of a well-maintained Poa sward is equal to any bermudagrass or bentgrass green. Suitably chastened, I told him I’d find a great article that focused on turfgrass management strategies that keep Poa hanging around, happy and healthy.

That article is a 2007 piece that examined nitrogen rate and form on the quality of a creeping bentgrass (Penn A-4)/annual bluegrass putting green. Conducted by Dr. Maxim Schlossberg of Penn State University, this two-year field study examined annual nitrogen rates of 1.4-8.3 pounds/1,000 square feet (69-402 kilograms/hectare) comprising various ratios of nitrate-nitrogen (NO₃-N) to ammonium-nitrogen (NH₄-N). Each nitrogen rate was prepared using combinations of ammonium nitrate and calcium nitrate or ammonium sulfate to provide an array of ammonium to nitrate ratios. All nitrogen fertilizer treatments were applied semi-monthly as liquids, and irrigated following application. To ensure sufficiency of calcium and sulfur, gypsum (CaSO₄) was applied at 5.9 pounds/1,000 square feet (290 kilograms/hectare) in the fall before each study year.

Collected data included clipping yield/vigor, nutrient content in clippings, color and nitrogen uptake. The researchers evaluated the response of the combined bent/Poa mixture to nitrogen rate and form.

One objective of their study was to identify any significant treatment interaction(s). Interaction is when one treatment (such as nitrogen form) behaves differently at one level of another treatment (such as a low nitrogen rate) than it does at another level (such as the high nitrogen rate). That’s what happened here – there was interaction between the nitrogen rate and nitrogen form treatments.

When the nitrogen was applied at a low rate (<3 pounds/1,000 square feet [<147 kilograms/hectare] per year) the bent/Poa mix that was fertilized with more NH₄-N (>50%) had reduced clipping yields and poorer color. When moderate rates of nitrogen were applied (3-5 pounds/1,000 square feet [146-244 kilograms/hectare] per year), nitrogen form (the ratio of NO₃-N to NH₄-N) did not significantly influence turf color or clipping yield. However, at higher nitrogen rates (>5 pounds/1,000 square feet [>244 kilograms/hectare] per year) the bent/Poa plots fertilized with more NO₃-N showed reduced vigor and poorer color. At these higher nitrogen rates, color was maximized by treatments containing >50% NH₄-N. Unlike color and clipping yield, nitrogen content in the clippings was only affected by nitrogen rate (increased as nitrogen rate increased), and was not affected by the form of nitrogen. However, both magnesium and manganese tissue content were influenced by rate and form of nitrogen.

So, what was going on? Well, the authors posed a couple of theories. The first was that, despite having sufficient leaf magnesium and manganese across treatments, significant enhancement of vigor and color may be associated with the greater leaf magnesium and manganese observed in plots treated primarily with NH₄-N. The second theory, while hypothetical, was that assimilation of NH₄-N by a plant requires less energy than assimilation nitrate. Thus, perhaps the intensive nature of putting green management simply allowed the bent/Poa green to more efficiently use nitrogen fertilizer containing a greater percentage of NH₄-N.

The result? Frequent fertilizer at high rates of nitrogen (>5 pounds/1,000 square feet/year) containing >50% NH₄-N enhanced vigor and green color of a bentgrass/Poa putting green sward. However, when fertilizing at a more moderate yearly nitrogen rate (3-5 pounds/1,000 square feet), nitrogen form did not affect turf color, clipping yield or nitrogen uptake.

Source: Schlossberg, M.J., and J.P. Schmidt. 2007. Influence of nitrogen rate and form on quality of putting greens cohabited by creeping bentgrass and annual bluegrass. Agronomy Journal 99:99-106.

Beth Guertal, Ph.D., is a professor in the department of agronomy and soils at Auburn University in Auburn, Ala., and the editor-in-chief for the American Society of Agronomy. She is a 17-year member of GCSAA.