RASEN · TURF · GAZON 4/2024 79 all. During the drought period and the following three week recovery period (25 June – 17 July) after resuming irrigation, digital images were taken 1 – 2 times per week from a fixed position in each plots. In order to avoid confounding effects of ambient weather conditions, the images were taken in a lightbox (Photo 1) and per cent coverage determined using the computer program ‘Turf analyzer’ (www. turfanalyzer.com). On the same days, visual turfgrass quality was assessed using a scale from 1 to 9 where 9 is the highest quality and 5 the lowest acceptable quality. The experiment was mowed at 20 mm height with a reel mower two times per week during the first four weeks of the drought period, but the mowing frequency was reduced to once per week when the drought symptoms started to appear five weeks into the drought period. Results All plots retained their initial coverage and turfgrass quality during the first 2 – 3 weeks of the drought period. Then the drought symptoms started to show up, first when analyzing the digital images and later from the visual assessments of turfgrass quality. This order highlights the benefit of using new techniques such as digital imaging for the early detection of drought stress. During the following weeks, coverage declined steadily to an average of 60 % (variation 50 – 75 %) after eight weeks. Concurrently, turfgrass quality declined to a minimum level below 5, i.e. lower than the lowest acceptable value. It is, however, noteworthy that this threshold was not reached until the very last day (25 June) of the eight-week drought period for the most drought tolerant species which were tall fescue and tetraploid perennial ryegrass. After termination of the drought period on 25 June, the experiment was irrigated, first to field capacity and then deficit irrigated two times per week corresponding to 80 % of the evapotranspiration (ET) value as calculated from the weather station that was placed under the rainout shelter. The response to irrigation was very fast, and within two weeks, most species had reestablished to almost the same coverage as before the drought period. The recovery of turfgrass quality was, however, slower because many plots were less uniform and with more dicot weeds than before the drought period. Table 2 shows a ranking of the eleven species for (1) ability to tolerate drought and (2) ability to recover upon natural rainfall or irrigation. For this ranking we have put equal weights on percent turfgrass coverage as determined by ‘Turf Analyzer’ and on the visual turfgrass quality. Because of the variation in coverage and turfgrass quality at the start of the drought period, the ratings are expressed relative to the initial values. Drought tolerance and ability to recover from drought of various turfgrass species Among the eleven species / subspecies in this experiment, tall fescue was the species least affected by drought. Based on available turfgrass literature, we believe this was due to tall fescue having a deeper and/or more extensive root system than the other species. However, despite selection for finer leaves, the plots seeded with tall fescue had a rather coarse leaf texture, which, together with supposedly limited tolerance to close mowing, slow establishTable 2: Eleven turfgrass specie ranked, first, for drought tolerance and second, for ability to recover from drought. Values for coverage, turf quality and their means have been expressed relative to their respective scores at the start of the experiment – Values used for ranking have been indicated in red. Photos 2a,b: Two out of four reps just after irrigation to field capacity at the start of the drought period on 30 April (top) and at the end of the drought period on 25 June (bottom) Green plots in the foreground of the bottom photo are tall fescue (one plot in column 2 and three plots in column 3 from left) and perennial ryegrass (two columns to the right). (Photos: T.S. Aamlid)
RkJQdWJsaXNoZXIy ODM4MTc=