Verdure: Poa management, Nordic style

Looking to thwart Poa annua invasion on red fescue greens, researchers experimented with blending more competitive grasses into the seed mix.


Filed to: Verdure, Poa annua

Red fescue (Festuca rubra L.) is used on Nordic putting greens because it is environmentally friendly, with low nitrogen and water use. However, it is not especially competitive, and annual bluegrass (Poa annua L.) can invade the relatively thin red fescue.

In an effort to find more competitive options, Norwegian turfgrass researchers examined combinations of red fescue and colonial bentgrass (Agrostis capillaris L.) and red fescue and velvet bentgrass (Agrostis canina L.), comparing them with red fescue alone. These species and species mixes were compared across three nitrogen and phosphorus rates, two mowing heights, and the presence or absence of arbuscular mycorrhizal inoculation.

Conducted for two years in Landvik, Norway, treatments were three species mixes: (1) 99% red fescue + 1% annual bluegrass; (2) 90% red fescue + 9% colonial bentgrass + 1% annual bluegrass; and (3) 90% red fescue + 9% velvet bentgrass + 1% annual bluegrass. The grass mixes all received nitrogen treatments of 1, 2 or 3 pounds nitrogen/1,000 square feet/year (5, 10 or 15 grams/square meter) and phosphorus treatments of 0, 0 + mycorrhizae, or 0.4 pound phosphorus/1,000 square feet/year (1.8 grams/square meter).

These grass, nitrogen and phosphorus treatments were all mowed at heights of ~7/32 and 5/32 inch (5.5 or 4.0 millimeters). Fertilizers were applied as liquids, as ammonium nitrate and phosphoric acid. The mycorrhizae (a mix of Glomus species) were applied yearly as a granular product, brushed into hollow-tine aerification holes.

Collected data included turfgrass quality, tiller density, surface hardness and root dry weight. Annual bluegrass competitiveness was evaluated by transplanting plugs of annual bluegrass into each plot and then measuring the diameter of that plug after two months of growth.

In both years, the plots that contained only red fescue had the poorest visual quality at either mowing height. The addition of either colonial bentgrass or velvet bentgrass improved quality, and all quality (and tiller density) assessments were improved at the higher mowing height.

Adding more nitrogen also improved quality and increased tiller density of the bentgrasses, with such results most noticeable at 2 pounds nitrogen/1,000 square feet/year rather than at the highest nitrogen rate. The addition of phosphorus, with or without the mycorrhizae, never affected visual quality, annual bluegrass invasion, turfgrass color or tiller density. In the first year, surface hardness was unaffected by any treatment, but by the second year, increasing nitrogen and a higher mowing height softened the surface.

The best grasses for resisting the competition of annual bluegrass were, in increasing order: red fescue < red fescue + colonial bentgrass < red fescue + velvet bentgrass. Neither nitrogen rate nor mowing height significantly affected annual bluegrass invasiveness. The only exception to this was in the red fescue plots mowed at the lower height, where annual bluegrass invasion was increased.

In general, the researchers recommended that adding no more than 10% colonial bentgrass to a red fescue seed mix could aid in warding off annual bluegrass invasion. Although the addition of velvet bentgrass was the most successful treatment for resisting annual bluegrass in this two-year study, the researchers did not recommend that blend, because it created the softest surface and because long-term growth would lead to a less desired dominance of velvet bentgrass in the sward.

Source: Calvache, S., T. Espevig, T.E. Anderson, E.J. Joner, A. Kvalbein, T. Pettersen and T.S. Aamlid. 2016. Nitrogen, phosphorus, mowing height, and arbuscular mycorrhiza effects on red fescue and mixed fescue-bentgrass putting greens. Crop Science 57:537-549.

Editor’s note: Read all of Beth Guertal’s recent Verdure columns.

Beth Guertal, Ph.D., is a professor in the Department of Crop, Soil and Environmental Sciences at Auburn University in Auburn, Ala., and the 2019 president of the Crop Science Society of America. She is a 21-year member of GCSAA.