Features Agronomy
It may be expensive, but supina bluegrass can help maintain turf coverage in high traffic

It's a balancing act for sports turf managers to provide quality turf
density throughout the growing season without having to reduce a
field’s hours of play.

Excessive play often nullifies even the best attempts to restore a worn field into playable condition. It’s not just the presence of weeds which concerns athletes, but the number of bare spots on the field.

The University of Guelph’s Kathleen Dodson addressed the issue of maintaining turfgrass coverage under high traffic conditions in September at the Sports Turf Association’s annual field day in Vaughan, Ont.

With traditional pesticide products no longer at the disposal of sports turf managers in Ontario, combating weeds has become more of a challenge, and other strategies often must be adopted.

White clover can produce a slipping hazard, especially when wet, while plantain reduces a field’s stability and can lead to an increase in knee injuries. Prostrate knotweed makes for a clumpy field, contributing to tripping hazards. The knotweed grows in a horizontal, or prostrate, manner and can lead to potential sports injuries as the season reaches its later stages.

“In the spring it seems all right, but, as the season continues on, we start having these lumpy patches of the weed expanding, and then, as it dies off—it is an annual weed—we start having these patches of dying weeds that are very easily tripped upon by the players, more in the late summer and early fall.”

Dodson said overseeding is really the key toward rectifying weed and bare turf issues, now that more traditional herbicides can no longer be used. She said the idea is to introduce more desirable species into the turfgrass community to help fill in some of the bare ground and weak stands of turf. The introduction of such species will also help to outcompete weeds which are causing injuries to athletes.

“We want something that has a quick germination time, something that’s relatively inexpensive so that we can put lots of it down frequently, and we want it to have seedlings that can have a moderate wear tolerance so that they will thrive as the fields are still being played on.”

Annual ryegrass has been used as an overseeding option in the past, but many sports turf managers are turned off by its light green colour and its inability to persist within the field from one year to the next. It may aid in maintaining turfgrass during one playing season, but Dodson wondered if a municipality’s money wouldn’t be better spent by overseeding with a more perennial species “so that it has that staying power in the field.

“If I’m going to send out two employees to go overseed my field, I would like to know that those man hours will last and will be useful for a longer period of time when we consider our budgetary restraints.”

More and more sports turf managers are turning to perennial ryegrass for their overseeding purposes. Research has shown that perennial ryegrass does not affect the number of weeds, but the influence of the weed on the overall sward is reduced. By increasing the competition, the weeds aren’t growing as big and therefore aren’t contributing as much toward the potential for injury.

One of the drawbacks with perennial ryegrass, however, is that it is susceptible to climatic extremes, Dodson said. Elevated temperatures during the summer can stress out perennial ryegrass while cold winter temperatures can leave the turf sward open in the spring when weeds such as prostrate knotweed are germinating.

“What can we have that will compete in the spring, but may have some staying power throughout the season?”

Dodson’s research at the university has included studies of supina bluegrass which has been used in Germany on sports fields since the 1930s. Its attributes include a high traffic tolerance, a stoloniferous growth habit, and an early spring greenup.

“We can see it greening up in March or April, if it even goes dormant at all.”

It has a late dormancy of November or December which is ideal for fields in Ontario because people are anxious to get back onto the fields as soon as the snow melts, she said.

When initially establishing a field, mixing about 10 per cent supina bluegrass with Kentucky bluegrass will, over a few years, produce a surface in which the supina bluegrass is apt to take over the sward due in part to its stoloniferous growth habit as well as its ability to self-seed itself.

“Usually during the first two years after you’ve seeded with supina bluegrass, you’ll start to find seed heads that will last for about two weeks in mid-spring.”

One reason why supina bluegrass has not readily caught on with many sports field managers is the belief that its shallow rooting requires a lot of water, but Dodson said research shows its water needs are essentially the same as that of Kentucky bluegrass.

“So that worry is not such a big deal anymore.”

Supina bluegrass’ biggest drawback is its cost. Because it is limited for breeding, with only two cultivars available, it is comparatively expensive. Dodson said she paid $37 a kilogram for supina bluegrass seed last summer whereas perennial ryegrass seed was a mere 40 cents a kilogram.

Because of its expense, she studied it as a “companion” overseeding species, looking at ideal seed rates in natural turf fields.

One plot, with a seeding rate of zero, had no supina bluegrass in the mixture. Another plot, with a seeding rate of one-half, was given half a kilogram of supina bluegrass seed per 100 square metres. Other plots had seeding rates of one, two and four kilograms of supina bluegrass per 100 square metres. Each of the research plots were given six kilograms of perennial ryegrass seed throughout the overseeding regime.
Dodson’s research also looked at frequency of overseeding, taking into account budget restrictions and labour issues which might allow for only one overseeding event to occur in a single year.

“If I can only get out once, how is that going to sustain my fields throughout the season?”

Her research compared the once-a-year scenario to introducing one-third of the amount of seed every six weeks throughout the growing season. The intent was to determine if turf coverage could be adequately maintained by adding less seed yet introducing it more frequently so that it totals the same amount of seed by the end of the season as compared to a single overseeding.

Mowing heights of both 11/2 and three inches were factored into the study to see how they might relate to the various seeding rates, all of which were compared to no overseeding control “so that I could see what happens over time to my field if I’m not overseeding at all.”

Dodson’s research into supina bluegrass overseeding involved five seeding rates times two seeding frequencies plus two different mowing heights. The research was replicated four times and repeated a year later.

The research plots measured 90 centimetres by 90 centimetres with two-metre alleyways in between to prevent supina bluegrass in one plot to spread into another.

Although the research was done on a field not being used, she simulated wear on the plots that would be equivalent to six soccer games played in a week.

“Every month I look at the quality of the stand, colour and density of the stand, and I also do a monthly count of the species that are within each individual plot.”

Dodson began her first counts in June 2010 and continued through July 2012. At the outset of her research, she had a healthy stand of turf.

The one-time overseeding produced an increase in turfgrass population over time, but then it decreased during the playing season. Where she overseeded, there was about 90 per cent turfgrass coverage. Overseeding with higher amounts of supina bluegrass, she still maintained turfgrass up and above 95 per cent turfgrass coverage. Comparing that data with having overseeded with the same amount of seed throughout the year, yet divided into thirds, Dodson said she found she could maintain turfgrass coverage more uniformly throughout the playing season.

Each of the rates resulted in turfgrass coverage between 95 and 98 per cent. The three-times frequency continued to give high turfgrass coverage while wear began to show with the one-time frequency.

“Being able to get out there more frequently seems to be a key to maintaining turfgrass coverage.”

When mowing, it is important the cutting height doesn’t get so low that it produces a weak stand, Dodson said.

“If we’re cutting off the solar panels of our plant, it’s not going to be very efficient at collecting that energy and creating more carbohydrates.”

Carbohydrate creation is vital for enabling the plant to get through stressful periods.

Turf should be mowed frequently enough to ensure no more than the top third of the blade is removed at any one time. Mowing too infrequently leads to self-shading during which the plant tends to focus more on vertical shoot growth, reducing lateral stem growth and leading to reduced wear tolerance. Not mowing frequently enough leads to scalping. More than the desired top third of the plant is removed, leaving the canopy more open and prone to weed growth.

Mowing the research plots at a height of 11/2 inches every four or five days was compared was compared to mowing once a week at three inches. Mowing height had no effect on species composition, Dodson said, but turf quality was more apparent with the lower height of cut.

One often wonders what becomes of the seed when overseeding is done all at once. Does it all germinate and, if not, where does it all go? As seed is added to a system, some will be lost to predation, some will be lost to natural degradation, and some will germinate and become established.

Part of Dodson’s research was to find out if, when overseeding, the seed remained in the seed bank. Depending on which study is read, annual bluegrass seed can remain in the seed bank up to six or seven years. She wanted to see if other turf-type grasses had the ability to remain in the seed bank. Additionally, she tracked the amount of weed seed coming into the field so that she could evaluate the competitive ability of the turfgrass seed she added.

“Poa supina (supina bluegrass) has a great ability to just stay and lie and wait within a seed bank.”

Increased seeding frequency resulted in more turf seed in the seed bank. Mowing height had no significant effect on seed bank composition. Overseeding with supina bluegrass resulted in it becoming the dominant species in the seed bank. Seed bank populations were relatively uniform across all treatments with no statistical differences among them.

According to research, Kentucky bluegrass and perennial ryegrass don’t have the same ability to remain in the seed bank as does supina bluegrass or poa annua.

“You’re better off timing your overseeding events so that you’re maximizing the germination ability of your turfgrass seeds.”

Dodson said overseeding frequency appears to play a more important role than the total amount of seed being added throughout the year.

If perennial ryegrass is being used as an overseeding species on non-irrigated sports fields, it is important to effectively time overseeding events to ensure there is adequate precipitation because perennial ryegrass doesn’t really lie in wait in the seed bank as effectively as other turfgrasses.

“There may be a place for supina bluegrass in our overseeding program in spite of its expense. In a non-irrigated environment, it has allowed for a faster fill-in after long periods of drought.”

Dodson said her supina bluegrass plots have filled in faster than her perennial ryegrass plots.

“If you take care of your turf, the turf will take care of the weeds and the bare ground in your fields.” –