How much perennial ryegrass/Kentucky bluegrass in a mix?

With the renovation season here, questions abound about how much perennial ryegrass to use in a P. rye/K. bluegrass mix. Perennial ryegrass germinates quickly and provides quick cover and soil stabilization when used in a blend. However, too much perennial ryegrass leads to long-term problems from dollar spot, brown patch, red thread, and other diseases. Kentucky bluegrass has better disease, drought, and wear tolerance the perennial ryegrass, but is very slow to germinate. Hence bluegrass and perennial ryegrass are often used in the same mix to get the best of both worlds. Bluegrass and perennial ryegrass are blended by weight in mixes and we recommend including less than 10% perennial ryegrass by weight in a mix. The ratio by seed number is even smaller since Kentucky bluegrass contains about 1.4 million seeds per pound whereas perennial ryegrass contains about 240,000 seeds per pound. The most recent research on mixing Kentucky bluegrass and perennial ryegrass was done at the University of Wisconsin under athletic field traffic conditions. They found that even though the aggressiveness of the specific cultivar of Kentucky bluegrass made a difference in the final composition of the stand, the initial composition of the seed mix does not reflect the final population in the stand.

Table 1.  The effect of the Kentucky bluegrass:perennial ryegrass ratio in the seed mix on the composition of a turf stand two years after seeding. Means are averaged over 4 different types of cultivars (aggressive, compact, BVMG, and common) under sports turf conditions (University of Wisconsin). 
                                                                                                                                
% K. blue:p. rye                  % Kentucky bluegrass
by weight in                          in stand 2 years
original seed mix                         after seeding
                                                                                                                                
95:5                                           73
85:15                                          59
75:25                                          50
65:35                                          44
50:50                                          35
25:75                                          21
                                                                                                                               

Their research validated our recommendation to limit the perennial ryegrass in a seed mix to no more than 5% or maybe 10% to maximize the amount of Kentucky bluegrass in the final stand. Table 1 summarizes their results but the full report can be found at http://www.hort.wisc.edu/uwturf/pdf/athfdmix.pdf. However, their study was not done in the much warmer and humid Indiana under lawn conditions,  so we started our own study in 2007.

Our Study
Our study was seeded in mid-August 2007 (ideal for Indiana) with a perennial ryegrass blend (33% each of SR4600, Penguin, and Fiesta 3) or Kentucky bluegrass (20% each of Odyssey, Arcadia, Mercury, SR2100, and America. Seed ratios by weight are given in the following table, but target seed number/area was 12 seeds/sq in. The area was covered with a germination blanket to prevent washing and irrigated as needed. Starter fertilizer (6-24-24) was applied at 1.2 lbs P2O5/1000 sq ft at seeding and again in mid September, followed by urea at 1.0 lb/M in November. Annual maintenance includes mowing at 2.5” twice per week, irrigation to prevent stress, 3.5 lbs N/1000/yr, preemergence crabgrass herbicide in the spring, and broadleaf herbicide in fall. Corsair herbicide will be applied each June on individual plots to remove perennial ryegrass and allow easier and more accurate counting of Kentucky bluegrass.

Observations so far

• Fall 2007’s extended hot weather allowed for tremendous crabgrass germination into the plots. As the crabgrass died, annual bluegrass filled in the voids which would not be expected on most turf sites.

• As expected, mixes containing higher percentages of perennial ryegrass germinated quickly and provided more turf cover and less weed cover by October than did those treatments with little or no perennial ryegrass in the mix.

• By the following Aug, the 100% KBG treatments was comprised of 80% KBG and the rest of the plot was annual bluegrass.

• Similar trends occurred in our study as the Wisconsin study where mixes as low as 80:20  were comprised of only 51% KBG by the following Aug (64% of the 100% KBG treatment)

• This study was repeated in fall of 2008 which had much better conditions for KBG establishment.

• Each study will continue for five years to track the species composition changes from the seed mixes.

Table 2.  Effect of the Kentucky bluegrass:perennial ryegrass ratio in the seed mix on the composition of a turf stand 2 months and one year after seeding. Means are averaged over 4 replications (Purdue University).