Turf & Rec

Features Agronomy
Non-chemical means to fight weeds on lawns

May 14, 2013  By  Mike Jiggens

With cosmetic pesticide bans in place in some Canadian provinces and municipalities, lawn care professionals have had to rely on other means to keep weeds at bay on their customers’ lawns.

The University of Guelph’s Katie Dodson spoke to an audience of lawn care professionals in March, at the fourth annual Nutrite professional seminar in Guelph, about non-chemical methods to help stave off weed encroachment.

Because turfgrass mixes resist weed encroachment better than monospecies stands, selecting the optimum species combination is one way to discourage weeds from entering the system.

“How can we as managers get our desirable plants into that community and try to discourage some of the undesirable plants we have?”


In southern Ontario, Kentucky bluegrass is regarded as the “king” of cool season grasses because it has rhizomes and has many ecotypes.

“That gives us a large selection of cultivars we can choose from for different environments,” Dodson said.
These can range from low input to high cultural intensities.


Kentucky bluegrasses perform best in sunny environments, and their primary response to drought stress is to go dormant. The advantage of going dormant quickly, she said, is that it’s maintaining itself during that period of time, and when the rains come back it can quickly recover. But while it’s dormant, it’s not able to compete with other species that might be trying to germinate during that period.

Fine fescues, on the other hand, have a low to moderate cultural intensity, are shade tolerant, are drought resistant, and can hold their colour and stay growing longer when drought is present. Studies show it also exudes a chemical compound into the soil that inhibits some weeds from germinating.

“If we can take advantage of that, that is something we should be looking for and putting it more into our mixes.”

One of the drawbacks of fine fescues, however, is that it’s not highly traffic tolerant. It’s OK if it’s part of a lawn mix for a front yard which will see little or no traffic, she said, but used in a back yard mix where children and pets are more active, fine fescue is probably not the best choice as the dominant species in a mix.

The greatest advantage to using perennial ryegrass in lawns is its ability to germinate quickly. One of its appeals is its moderate cultural intensity, meaning it doesn’t require a lot of input. Some older cultivars, however, are susceptible to extreme weather conditions. When temperatures get particularly high and it’s dry, there will be some dieback in the population and not much will survive freeze-thaw cycles during winter.
With perennial ryegrass-dominant stands, weeds can encroach quickly when there is drought.

In general, if high amounts of nitrogen are put into a turfgrass stand, the amount of weeds will be reduced.
“However, timing of your nitrogen applications is important.”

If nitrogen is supplied at a time when weeds are trying to germinate, one is helping the weeds get a step up on the plant.

Dodson added that when irrigating, wetting-drying cycles can promote some weed seed to germinate.
Allowing grass to go too long into drought will result in a loss of the turfgrass stand which provides more openings for weed seed to germinate.

Dodson shared the findings of an experiment she conducted to determine the optimum species mix for each different type of lawn care environment. She used four different seed mixes:

• a mix for a sunny environment (80 per cent Kentucky bluegrass and 20 per cent perennial ryegrass)

• a shade mix (80 per cent Kentucky bluegrass and 20 per cent fine fescue)

• a standard three-way lawn mix which can be consumer purchased (60 per cent Kentucky bluegrass, 20 per cent perennial ryegrass and 20 per cent fine fescue)

• a mix requiring little nitrogen, or a low input mix (50 per cent perennial ryegrass and 50 per cent fine fescue)

All four mixes were subjected to different irrigation regimes, including standard deep and infrequent irrigation (one inch of rain per week), frequent and light irrigation (one inch of rain for the week, but divided into thirds), and no supplemental irrigation at all (leaving only what occurred naturally).

Fertility also figured into the experiment. Some plots received fall and spring fertilization (two pounds of nitrogen per 1,000 square feet for the year) and some received no fertilizer at all.

The study lawn was scalped to rid it of all vegetation to prepare it for the different mixes to be added into the plots. The experiment was conducted twice—in May 2010 and May 2011—but results from the second year went askew due to that spring’s extraordinarily wet weather. At the time of seeding for each year’s experiment, a blanket fertilizer of 10-18-22 at a half-pound of nitrogen per 1,000 square feet was applied at the time of seeding to ensure all plots got the same start.

A mini-drought occurred in May of 2012 which led to the Kentucky bluegrass becoming dormant. It broke dormancy in June but shortly afterward entered into a longer nine-week summer drought which produced “some interesting results.”

The long drought had an effect on the overall population, and the deep irrigation resulted in having more open stands. Even though the irrigated plots received an inch of water each week during the course of the drought period, it set back the Kentucky bluegrass populations enough that they did not recover by the fall.
Overall, added fertilizer encouraged turf growth over weed growth, Dodson said. But, she added, weeds which encroached into the turf plots that were fertilized included dandelion and plantain while the unfertilized plots saw clover as a dominant species in the population. When no fertilizer was applied, more leguminous weeds were found in the population.

The Kentucky bluegrass/perennial ryegrass mix had the highest turfgrass cover throughout the experiment. Dodson said this didn’t come as a big surprise because the research was done at the Guelph Turfgrass Institute where there were no shade issues. Adding the small amount of fertilizer in the spring and fall generally improved the overall colour.

On plots which received fertilizer yet no irrigation, turf stayed green longer when drought hit than plots which received no fertilizer at all. In the spring, greenup was much quicker with plots on a single fertilizer.
The three-way mix was the optimum mix when irrigating regularly and adding fertilizer in the spring and fall. With no irrigation, Dodson said the best bet is to go with a fine fescue/perennial ryegrass mix if weed infestation is to be prevented.

“However, if you want to promote a fine fescue lawn, you have to remember it doesn’t tolerate traffic.”
None of the research plots was subjected to traffic.

When turfgrass cover is down to 40 or 50 per cent, a renovation project may be considered to restore a full cover. In general, renovation is any sort of cultural practice that will give turfgrass a competitive advantage, Dodson said.

Cultivation techniques include core aeration and verticutting (often referred to as dethatching or power raking). It involves mechanically killing off existing vegetative material, tilling it under and adding soil amendments, if necessary, before re-establishing with seed. Six to eight weeks later, a herbicide is applied to knock back any weeds which might be competing with turfgrass seedlings.

But, under a pesticide ban, traditional herbicides cannot be used. Common questions which might be asked include:

• How can I renovate without pesticides?

• Is it still worthwhile to rototill under the existing vegetation?

• Can a better quality lawn be achieved by doing this?

• If this is to be done, when should it be done?

Dodson presented details of a research project which looked into four different home lawn renovation strategies. Renovation methods included core cultivation, scalping, rototilling, doing nothing at all, and adding seed into the existing vegetation material.

For post-renovation, the fall was looked at for core aerating and the spring for dethatching. Doing both was also explored and then a comparison was made to a positive herbicide control and to a no-treatment control. Mowing and fertilizing were done afterwards.

The process was repeated, twice in the spring and twice in the fall, to see what happened. Some plots were either rototilled or scalped or core aerated.

All renovation treatments lowered weed populations by five to 15 per cent. Specifically, rototilling proved to be most effective toward decreasing weed populations in the spring, however, it also decreased turf populations.

Spring renovations encouraged annual weed growth while fall renovations resulted in more perennial turf type weed growth. Annual weeds promoted in the spring were effectively mowed out of the spring renovation plots by August.

Results of cultural practices varied from year to year. Spring power raking was particularly effective in May of 2011. Core cultivation took place in the fall of 2010 and then dethatching was done in May of 2011.

“What was really interesting was that all of the plots that received this dethatching had really low populations of weeds (about 10 per cent).”

When the spring is cool and wet and there is reliable moisture, turfgrass can better handle dethatching while weeds cannot, Dodson said. Turfgrass recovered quickly and weeds were popped out, giving similar results to that if a herbicide was used.

The mini-drought in May of 2012 led to dethatching the plots again at the end of the month. The drought had lasted about three weeks and was hot and dry enough to send the Kentucky bluegrass into dormancy. The turf in the study couldn’t recover from the dethatching.

“What I saw was an actual dieback of my turf, and I had more weeds in the plots than I actually dethatched or power raked.”

Dethatching right before a hot, dry spell is not a good idea, she said.

Spring power raking can be as effective as herbicide use for controlling weed populations as long as there is a cool season for the turfgrass to recover, she added.

A denser turf stand was realized when overseeding, Dodson said, but added, in hindsight, it would have been helpful to also look at the effects of slit seeding or dimple seeding.

When overseeding, it is best to add smaller amounts more frequently if it is an irrigated environment, Dodson said. If overseeding is to be done only once with a larger amount of seed, it is best to time it when the seed has the best chance at germination.

Turf type seed does not have the ability to remain in the soil seed bank, she said. What is put down in the spring will germinate during the growing season, but doesn’t survive from one year to the next.

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