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The amount of useful light an area of vegetation receives over a day. It is measured by totalling the amount of photosynthetically active radiation (PAR), in the 400-700nm range, received in a square metre over a 24-hour period: the sum of the photosynthetic photon flux density (PPFD) in μmol m−2 s−1 over a full day and is typically expressed as a figure for mol m-2 d- (e.g. DLI of 20). The amount of sun hours, daylight, cloud cover, shade cast, latitude, light intensity and time of day can all affect the total figure. The term is often shortened to DLI.

Perennial ryegrass has an estimated DLI of 23 “to maintain optimum healthy turf quality” (in the USA), needing a minimum of 10 for survival and growth (in the UK).
(Kirby, G. ‘Spotlight on shade', The Groundsman, May 2019, p25)

Maintaining an acceptable quality of golf putting green surface for a creeping bentgrass sward has been estimated as having a DLI of about 30. (Russell, Travis R.; Karcher, Douglas E.; Richardson, Michael D. (2019). Daily Light Integral Requirement of a Creeping Bentgrass Putting Green as Affected by Shade, Trinexapac-ethyl, and a Plant Colorant. Crop Science, 59(4), 1768¯1778. doi:10.2135/cropsci2018.08.0501)

[See also ‘Russell, Travis, "Determining the Light Requirements of Various Turfgrass Systems and Investigating Techniques to Quantify Photosynthetically Active Radiation" (2018). Theses and Dissertations. 2778. http://scholarworks.uark.edu/etd/2778']