Perennial ryegrass growing degree days (GDD)
Welcome to the first edition of the Northern Minnesota Turf Seed Growers Newsletter for 2026. The primary objective of this newsletter is to report on weather conditions, crop growth & development, pest management and chart the year-to-date perennial ryegrass growing degree days (GDD) compared to the previous eight years. This newsletter is scheduled for weekly distribution from the beginning of ryegrass green-up through swathing.
Suggestions on newsletter content should be directed to Dave Grafstrom
- Email: [email protected]
- Cell: 320-293-8722
Perennial ryegrass GDD’s will be tracked during the 2026 growing season with comparisons to the previous eight years. The accumulation of GDDs will begin after the snow has melted from the perennial ryegrass fields and continue through swathing. A base temperature of 32 °F will be used for perennial ryegrass (T - Base = 32 °F).
- Year to date GDD = 231 (Table 1)
- GDD last week (April 27 - May 3) = 60 or 8.6/day; long term average = 86 or 12.3/day
- GDD forecast for the next 10 days = 119 or 11.9/day
- Average GDD accumulation for first week of May = 104 or 14.9/day
- The 10-day forecast suggests cooler than average temps for the first week of May. Projected GDD is 11.9/day compared to the long-term average of 14.9/day
| Year | 2026 | 2025 | 2024 | 2023 | 2022 | 2021 | 2020 | 2019 | 2018 | 2026 vs. 2025 |
|---|---|---|---|---|---|---|---|---|---|---|
| March | 0 | 0 | 0 | 0 | 0 | 131 | 30 | 0 | 0 | 0 |
| April | 191 | 229 | 296 | 93 | 95 | 236 | 183 | 211 | 184 | -38 |
| May* | 40 | 812 | 653 | 959 | 649 | 640 | 600 | 548 | 815 | N/A |
| June | N/A | 932 | 859 | 1,064 | 959 | 1,007 | 995 | 919 | 1,007 | N/A |
| July | N/A | 1,040 | 1,119 | 985 | 1,104 | 1,174 | 1,179 | 1,067 | 1,100 | N/A |
| Total | N/A | 3,013 | 2,927 | 3,101 | 2,807 | 3,188 | 2,987 | 2,745 | 3,106 | N/A |
General crop condition
The 2026 season is one of those years with significant desiccation of the perennial ryegrass leaf tissue after snow melt. In many fields brown leaf tissue extends into the crown region. However, when the dead ryegrass leaf tissue is removed from the crown area green tissue is observed. Further, white roots are observed when plants are dug out of the ground which is a good sign. A better assessment of ryegrass winter survival can be made after the soil temperatures in bare ground and sod conditions move into the 50’s. On Thursday, April 30, the frost was out of the ground in a bulk seeding of perennial ryegrass and averaged 22 inches in sod conditions at the U of MN Magnusson Farm.
Biological activity in the soil begins to increase as the soil temperatures move into the 40 degree range. Soil temperatures at the U of MN Magnusson Research reached 40 °F on April 21 in both bare soil and sod conditions (Table 2). In a twelve-year period from 2015 to 2026, the range between bare soil and turf conditions was 0 days in 2026 and 23 days in 2021. In 2026, soil temperatures reached 49 °F in bare and 48 °F in turf conditions on April 23. However, because of the cool conditions of the last week of April the soil temperatures have returned to the high 30s at the end of April. The warmer temperatures in the first three days of May resulted in average soil temperatures back in the mid-40s.
| Year | 2026 | 2025 | 2024 | 2023 | 2022 | 2021 | 2020 | 2019 | 2018 | 2017 | 2016 | 2015 |
|---|---|---|---|---|---|---|---|---|---|---|---|---|
| Bare | 4-21 | 4-11 | 4-8 | 4-28 | 5-3 | 4-3 | 4-22 | 4-16 | 4-20 | 3-30 | 4-14 | 3-31 |
| Turf | 4-21 | 4-21 | 4-22 | 5-1 | 5-6 | 4-26 | 4-27 | 4-29 | 4-29 | 4-13 | 4-15 | 4-15 |
| Range | 0 | 10 | 14 | 3 | 3 | 23 | 5 | 8 | 9 | 15 | 1 | 16 |
Crop management
The following is a review of the critical soil temperatures for perennial ryegrass growth and development. This information is based on soil temperatures at a 4-inch depth that had air in the soil pore spaces (not waterlogged).
- 90 °F: Shoot growth ceases
- 77 °F: Root growth ceases
- 70 °F: Maximum temperature for expansion of root growth
- 60-75 °F: Optimum temperatures for shoot growth
- 50-65 °F: Optimum temperatures for root growth
- 40 °F: Shoot growth ceases
- 33 °F: Root growth ceases
- 20 °F: Low temperature that will kill plants if temperatures drop rapidly below 20 °F
Pest management
With the accumulation of heat and improved soil temperatures the growth rate of weeds and perennial ryegrass growth will accelerate. More on perennial and annual weed control in next week’s newsletter.
Next week’s newsletter will be released on May 12.