By Trygve S. Aamlid, Anne Borchert, Trond Pettersen, Paula Lawicka, Iris Eik, and Ove Hetland, NIBIO Turfgrass Research Group
Despite decades of systematic research to avoid winter damage, many golf course superintendents in northern areas sometimes find themselves in situations where they need to reestablish putting greens in early spring. With golfers waiting for an early start to the new season, delayed development of new turf cover can be a real headache for turfgrass managers.
After not being able to prove the occurrence of persistent growth inhibiting substances in sand-based putting greens killed by ice encasement, turfgrass researchers have usually explained the slow (re)establishment of winter-killed greens in early spring as due to low soil temperatures. For this reason, most Scandinavian superintendents cover reseeded greens until a couple of weeks after emergence with spring tarps that increase soil temperature. However, after several years of WinterTurf reestablishment trials, we now realize that the protection of the small seedings against strong light (photoinhibition) may be just as, if not more, important than soil temperature.
Field trial at NIBIO Landvik, Southeast Norway, 2025
Creeping bentgrass ‘Luminary’ was seeded at 9.76 g m-2 on a USGA-spec putting green on 3 April. Before seeding, the green had received a preplant fertilizer application of 45.5 g m-2 of Sierrqablen Plus Renovator (ICL, 5 g N, 2.2 g P and 1.9 g K m-2). After the first seedling emergence on 15 April, all plots were fertilized weekly with 1.5 g N m-2 alternating between Greenmaster Cold Start 11-5-5 and Greenmaster 12-0-12, yielding a total N-input for the 8 week grow-in period of 14 g N m-2 (including preplant application). On days without natural rainfall, the trial was irrigated three times a day for a total of 8-10 mm water. The green was mowed for the first time to 12 mm on 15 May with a walk behind mower.
The experiment had four blocks, eight treatments and included 5 permeable spring covers in addition to an uncovered control (See also Fig. 1a):
- White Agryl fibre tarp (Agrisoft, LOG, Norway)
- Evergreen original (Evergreen Turfcovers, Mississauga, Ontario, Canada)
- Evergreen Radiant (Evergreen Turfcovers, Mississauga, Ontario, Canada)
- Black PVC cover, 50% shade (SC-BL50B, Greenhousemegastore.com)
- Black PVC cover, 70% shade (SC-BL70B, Greenhousemegastore.com)
There were also two treatments in which various covers were combined sequentially, but we will not go into those treatments in this short update.
The covers were installed immediately after seeding on 3 April and stayed on until 29 April. Percent bentgrass cover was determined weekly using Turf Analyzer from images taken in a light box. We also logged soil temperature at 3 cm depth and measured the reduction in photosynthetic flux density (PPFD, 400-700 nm) under the covers at noon on 25 April, a sunny day without clouds.
Soil temperature and shade effects
April 2025 had record warmth with a mean monthly temperature of 9.0 °C, which is 2.6 °C higher than the 30 yr normal value for the experimental site (reference period 1991-2020). Frost was not recorded at 3 cm depth on any of the plots during the 6w k cover period. The increase in soil temperature due to covers was significant with an average of 10.5, 10.9, 10.8, 11.3 and 10.8 °C, with Agryl, Evergreen, Evergreen Radiant, 50% shade or 70% shade covers as compared with 9.9 °C in the uncovered control.
On the sunny day 25 April, the PPFD on uncovered plots was 1345 µmol s-1 m-2, which is most likely higher than light saturation for individual, newly emerged cool-season grass seedlings. We recorded a 16, 55, 37, 55 and 71% reduction in PPFD under Agryl, Ecergreen, Evergreen Radiant, 50% shade and 70% shade covers, respectively.
Detailed photos of the newly emerged seedlings (Fig. 2) at tarp removal on 29 April showed remarkable differences among treatments. Seedlings on uncovered control plots had developed a distinct purple color and were clearly stressed by the high light intensity. Seedlings on plots covered with Evergreen or Evergreen Radiant appeared most healthy and had, for the most part, developed more leaves than seedlings that had been under the 50 or 70% shade covers.
Turfgrass coverage
The development of creeping bentgrass coverage is in the various treatments is shown in Fig. 3 and further illustrated in Fig. 1. Plots covered with Evergreen Radiant and Evergreen developed full coverage significantly faster than uncovered control plots, while plots covered with Agryl and the 50 and 70% shade tarps were intermediate.
Conclusion
The take-home message from this experiment is that photoinhibition may play a significant role when reestablishing creeping bentgrass greens in early spring. Use of the right covers is likely to mitigate this negative effect and accelerate the recovery of winter damaged greens.