Features Agronomy
Sugar maple leaf mulch shows promise as organic herbicide

MAPLE leaf mulch as an organic herbicide? Research studies suggest it’s
worth considering, especially if conventional herbicides have been
outlawed in your area.

Alexander Kowalewski, assistant professor of environmental horticulture at Abraham Baldwin Agricultural College in Tifton, Ga., spoke in March at the Canadian International Turfgrass Conference and Trade Show in Vancouver about the progress made over the past 20 years to learn why maple leaf mulch—especially that of the sugar maple—has proven effective in weed control.

A research project under the direction of Dr. Thomas Nikolai began in 1991 at the University of Michigan in response to a local ordinance which banned fallen leaves from going to landfills. Leaves were mulched into the ground in the fall as an alternative disposal method. No detrimental effects to the turfgrass were observed.

After about 10 years into his research, Nikolai observed that areas of the campus mulched with maple leaves produced greener grass with fewer dandelions, but whether or not the leaves provided control against the weed was still in question.

The campus managed its greenspaces with fertility and herbicide use, “so the question was, is there herbicide residue on these leaves that could be inhibiting the dandelions?” Kowalewski said.

In Nikolai’s research, leaves were separated by species and not genus.

“We wanted to see if there was a particular genus of maple leaves that could provide better control of dandelions from others,” Kowalewski said.

The objective of Kowalewski’s work was to quantify the effectiveness of maple leaf mulch as an organic broadleaf weed control method and identify the maple leaf species which provided the most effective weed control.

“Is there a species that provides better control than others?”

The research was conducted on well-established Kentucky bluegrass on native Michigan soil at the MSU campus. Site preparation began in October 2003 with a broadleaf weed application put down to kill all weeds in the turf plots. Dandelion seed was then taken to seed the area, creating a seed bank for them to grow. No further herbicide applications were made afterwards.

Data was collected over the next two years, during which no further herbicides were used. Leaves used in the study were collected from pesticide-free tree plantations in a nearby forested area. The leaves were taken to MSU and applied in November, incorporated into the turfgrass canopy by use of a rotary mulching mower.

Leaves under study included red maple, red oak, silver maple and sugar maple as well as leaves from sugar maple trees with a high sugar content (those designated for maple syrup production).

Two particle sizes of mulched leaves were studied: 2.5 and 6.5 centimetres. The smaller particle size was achieved by grinding the leaves up. The difference in sizes were selected to determine if the chemical within the leaves might be released quicker in ground up leaves, thereby providing better control of the weeds.

Two application rates were used: half a kilogram per square metre, which is similar to the rate of leaves naturally falling onto a field, and 1.5 kilograms per square metre, which equates to a depth of about five centimetres of leaf cover. Additionally, a third plot was studied with no leaves at all.

The study plots received no irrigation, low fertility and were cut at a low mowing height to stress the turf and promote as much weed establishment as possible to determine if the mulched leaves worked. A worst-case scenario had been created, increasing the chances for weeds to infect the turfgrass.
Assessed were both spring greenup colour on Kentucky bluegrass on a one-to-nine scale, with six deemed acceptable, and the number of dandelions present in a three-square-metre plot.

Whether the particles were fine or coarse, there was no discernable difference to the turf or its effect on dandelion seeds.

Significant results were realized, however, based on application rates and species. The plot with no applied leaves at all showed the worst spring greenup.

“As rates increased, spring greenup got better and better,” Kowalewski said.

Fewer dandelions were also realized as a result of leaf mulch applications. Plots mulched with both sugar-content levels of sugar maple leaves provided the fewest dandelion counts, and as rates increased there were fewer and fewer of the weeds.

After two years of no herbicides used, results were similar in 2005 to the previous year.

“The leaves are not only making the grass greener and more vigorous, it’s also reducing the amount of dandelions in the turfgrass,” Kowalewski said.

Of significant note, he added, was the dandelions that were present, “for whatever reason,” were not producing seeds, resulting in fewer dandelions and weeds which weren’t blooming.

Dandelion count data was similar in 2005 to 2004, but counts exploded by August, whether or not the plots were covered in mulch.

“But you have to consider the management practices here,” Kowalewski said. “There’s no irrigation, low fertilization, and we’re scalping the grass. So we’re making great conditions for dandelions, and we eventually lost the control over two years.”

Results of the study over the two-year period showed that, in low-maintenance situations, increased leaf applications on Kentucky bluegrass produced better spring greenup. Increased leaf rates also showed better dandelion control. At its highest rate, about 80 per cent control was realized as an average across the leaf species. The sugar maple, and especially the higher sugar content sugar maple provided the best dandelion control. Control was eventually lost in the second year because of the stressful conditions.

Kowalewski said the question became, “How are these leaves inhibiting the dandelions?”

Answers were sought to the questions:

• Is it (mulched sugar maple leaves) an organic fertilizer because the leaves being put down at a rate of 1.5 kilograms per square metre is a significant amount of nitrogen?

• Are the leaves stimulating microbial activity in the soil, with the microbes eating the leaves and eating the weed seeds in the soil as well?

• Are the leaves just blocking out light, shading the weeds and preventing them from germinating, or are they smothering the weeds, making it difficult for them to respire, germinate and grow?

• It is allelopathy? Are the leaves making chemicals that are inhibiting the dandelions, preventing them from germinating?

A follow-up study was conducted to determine if the weed control was a fertilizer effect. The study was done using best management practices with maple leaf mulch as the organic weed control method. Whereas the original study was conducted by scalping the turf, different mowing heights were used in the newer study, along with different fertilizer rates, to provide optimum or better conditions. By allowing the turfgrass to thrive, it was hoped a reduction in dandelion populations would be even more evident than in the initial study.

The study, to see how leaves controlled dandelions at high maintenance levels, was conducted at Michigan State University from 2004 to 2006 on a mix of Kentucky bluegrass, perennial ryegrass and fine fescue grown on native soil without irrigation.

Leaves collected in October were applied in November. Under study were sugar maple, red maple (red maple provided the worst dandelion control among the various species in the original study) and a plot with no leaf mulch applied. Leaves were applied at the same high rate as the first study, amounting to about 15 grams of nitrogen per square metre.

Some areas received no fertilizer while others got the 15 grams of nitrogen per square metre.

“If it was organic fertilizer coming from the leaves, this 15 grams of nitrogen applied using urea should negate it. We’re getting the same amount of fertilizer as nitrogen applied with the leaves there. We should see the same effects if it’s an organic fertilizer rather than allelopathic chemical.”

Results showed that turfgrass quality increased as fertilizer rates increased, and there were fewer dandelions as mulched leaves were put down, and less yet when sugar maple leaves were used as compared to red maple.

“We also saw that increased mowing height actually increased the amount of dandelions we had per plot,” Kowalewski said, adding the fertilizer didn’t have an effect on dandelion populations, even though it increased turf quality.

“The leaf mulch is controlling the dandelions, but the fertilizer doesn’t have an effect, so it would suggest that there are chemicals within the leaves that are actually inhibiting the dandelions, or the leaves are covering and smothering the dandelions, stopping them from germinating.”

Kowalewski said fewer weeds are normally found as mowing heights increase.

“Why are there less dandelions in the area that has the lower mowing height? The answer is, we take a look at the crabgrass cover results.”

Intense crabgrass competition was evident in the low mowing height areas, he said, noting the crabgrass had actually choked out the dandelions.

Concluding that fertilization increased turfgrass quality, reduced mowing heights increased crabgrass populations, and the most leaves reduced dandelion populations, “This would suggest that the leaves are not providing fertilizer, and that’s the way the dandelion populations are being reduced, by increased turfgrass competition. It would suggest there is something else in the mulched leaves that are inhibiting the dandelions or there’s a different method that the mulched leaves are inhibiting the dandelions.”

One factor which needed to be considered in order to get to the bottom of the mystery was the fact no irrigation was provided at any of the plots. Kowalewski said it could be deduced that it wasn’t through organic fertilizer that the leaves were inhibiting the dandelions.

A turf physiologist at MSU continued with the research, using growth chamber studies with the use of maple leaf extracts. Maple leaves were crushed into tiny pieces, were mixed with water, were left to sit awhile, and then the crushed leaves were filtered from the water. This allowed irrigation to be done using water with chemicals from the leaves. Growth chamber studies done without soil prevented soil microbial interaction from taking place.

It was further deduced that weed control wasn’t the result of blocked light or smothering because leaves weren’t being put down on the dandelion seeds. Instead, a solution with maple leaf extract was being used.

The irrigation slurry made from maple leaf extract provided a much higher rate of dandelion germination control than that of just regular water, making a statement that allelopathy is the likely reason for control. Likely, a chemical exists in the leaves which inhibits the growth of dandelions, Kowalewski said.

He concluded that, for home lawns or municipal greenspaces, mulching sugar maple leaves will provide increased spring greenup and reduced dandelion populations.

The chemical has yet to be synthetically reproduced, he added, “but there’s a strong case for it.”