CAN WE GO GREEN?

With increasing discussion on glyphosate and Roundup products for use in horticulture, Scott Drew for his final year thesis in horticulture at UCD looked to investigate the effectiveness of alternative herbicides currently available on the Irish market.

The European Food Safety Authority (EFSA) in their review said there was not enough evidence to link glyphosate to cancer

The debate on glyphosate licensing will be opening again soon. Glyphosate is the active ingredient in over 750 products, including Roundup.

Our Experiment

With increasing discussion on glyphosate and Roundup products for use in horticulture, Scott Drew for his final year thesis in horticulture at UCD looked to investigate the effectiveness of alternative herbicides currently available in Ireland.

Field experiments were carried out to evaluate the range of organic herbicides on the Irish market. (By ‘organic herbicides’, we mean herbicides developed from naturally occurring organic processes, or from plants and their oils. We were guided in our choice by the ‘List of active substances approved for use in EU organic agriculture under EU Regulation (EC) No 889/2008’.)

The products we chose were all purchased from off the shelf and are available from the major chain store, Woodies. The active ingredient of each organic herbicide is in italics below, with abbreviations in brackets: Hytrol: acetic acid (AA); No Glyphosate Roundup: pelargonic acid (PA); and Neudorff Weedfree Plus: pelargonic acid + maleic hydrazide, a synthetic, ie lab-generated, growth regulator (PAM).

These organic ingredients were compared against a control of glyphosate (G), from Fast Action Roundup. Three different plant species for the four different treatments were also selected to represent some of the most commonly-encountered weed species in gardens and landscapes.

Wild petty spurge (Euphorbia peplus) is a small annual plant, flowering from April to November. Each plant can produce over 260 seeds, allowing it to become very invasive in the correct conditions.

The second species selected was a perennial bulb, wild garlic (Allium ursinum), also known as wild ramson. The third species selected was ornamental bellis (Bellis perennis ‘Pomponette’), as a proxy species for lawn daisy. It is one of the most common broadleaf weeds or wildflowers found in Ireland, and is also in the same plant family as dandelions (Taraxacum officinale agg.), though without the deep taproot.

Each of the tests were replicated three times on each species. For the test, the organic herbicide products were added to standardized spray bottles with the same pressure. Three tests were done on the spray bottles to find the average release rate. This was 100ml/66 sprays (1.5ml/1 spray).

(See Table 1 below.)

Herbicides are the largest component of all the chemicals and pesticides used in gardens and landscaping.

Figures from Ross Cameron and James Hitchmough’s book, Environmental Horticulture: Science and Management of Green Landscapes, highlight that over 50 per cent of European and 70 per cent of American homeowners use herbicides for weed control.

In 2018, the license for one of the most widely-used herbicides, glyphosate, was renewed for five years by the European Commission. This was a very controversial decision, as the World Health Organisation had classified glyphosate a probable carcinogen in 2015, and cases have been successfully tried in the USA linking it to cancer.

In 2018 the European Chemical Agency (ECHA) and the

CHEMICAL COVERAGE

ACETIC ACID (AA) 100ml/m2

PELARGONIC ACID (PA) 525ml/11m2

PELARGONIC ACID + MALEIC HYDRAZIDE (PAM) 100ml/m2

GLYPHOSATE (G) 250ml/7.5m2

TABLE 2 HERBICIDE EFFECTIVENESS AFTER 14 DAYS

TREATMENTS PERCENTAGE

MORTALITY (%) AFTER 14 DAYS

WILD PETTY SPURGE (EUPHORBIA PEPLUS )

WILD GARLIC ( ALLIUM URSINUM )

POTTED DAISY (BELLIS PERENNIS ‘POMPONETTE’ )

ACETIC ACID (AA) 80% 45% 95% but regrowth after 14 days

PELARGONIC ACID (PA) 85% 50% 90% but regrowth after 14 days

PELARGONIC ACID + MALEIC HYDRAZIDE (PAM)

80% 55% 95% no observed regrowth during experiment time period

GLYPHOSATE (G)CONTROL 95% 85% 99% no observed regrowth during experiment time period

The results were recorded 14 days after the tests were carried out. (In many published papers on this subject, effectiveness was recorded after just two days, which does not give time for glyphosate to work, and is not an accurate reflection of efficacy.)

Before testing, pictures were taken and chlorophyll readings with a chlorophyll meter (spad 502dl) were carried out. After 14 days, the following results were recorded:

ACETIC

ACID (AA)

As highlighted in Table 2, AA controlled the spurge and daisy to great effect. Results could be clearly seen within three days after spraying, when nearly all foliage the AA had touched had died. The visual observation of mortality was clear when using AA. The results were similar to other research on AA at 80-100 per cent control.

Some regrowth was noticeable on the daisies at inspection on day 14.

Two of the three tests using AA on daisies showed small new green shoots emerging from the centre of what looked to be a dead plant. AA had the least effect on the wild garlic when compared to the other weed controls.

On average, less than 50 per cent of wild garlic showed any signs of mortality.

PELARGONIC ACID (PA)

Similar results were seen in PA to AA. These two completely organic herbicides struggled to get a high percentage in the visual observations of mortality when testing on wild garlic. This could have been expected from pelargonic acid, as labelled instructions state that it is “most effective on young weeds,’’ and the wild garlic was mature stand.

The PA showed visible effects on all the plants in less than a day, which seems to be common, as other researchers noticed results with this naturally occurring acid within hours after application. On inspection under the dead daisy foliage on day 14, some smaller leaves showed signs of regrowth, similar to AA.

PELARGONIC ACID + MALEIC HYDRAZIDE (PAM)

The results in the visual observation test provided in Table 2 show that PAM had similar effectiveness to AA and PA. The synthetic addition of maleic hydrazide (MH) stopped any signs of regrowth, which is a major factor in the effectiveness of this herbicide.

The chlorophyll reading of the PAM again showed similar data to the results of the visual test. Although MH is proven to have little effect on the most sensitive microorganisms below the ground, it is still not classed as an organic herbicide.

Analysis

The overall outcome of this short research project provided some insights into the effectiveness of naturalbased herbicides.

While the organic herbicides showed some promising results, the research highlights that many applications would be needed to achieve full mortality of the weeds. The addition of synthetic base chemicals to organic herbicides, as with PAM, can improve the effectiveness.

As expected, the most herbicide-resilient species was wild garlic. Other research has also shown that contact herbicides are not known to eradicate wild garlic, as the bulbs below ground remain unaffected.

Organic herbicides have poor effectiveness on perennial and bulbous weeds, as they only damage or ‘burn’ the green part above ground.

Glyphosate In Our Food And Our Bodies

Later this year, the EU will decide once again on the licensing for glyphosate.

Scientists in the University of Galway found low levels of glyphosate in the urine of one quarter of the people they tested, which is a concern. The people tested had low levels both of glyphosate and its main human metabolite, aminomethylphosphonic acid (AMPA).

Dr Alison Connolly, who conducted the research at Galway, said: “This study produced important results on human exposures to a chemical of public concern, and is particularly timely with the European Commission currently re-evaluating glyphosate.” (NUIG 2023 - https://www.universityofgalway. ie/about-us/news-and-events/news-archive/2023/january/ university-of-galway-investigates-glyphosate-exposureamong-families.html )

Galway researchers also highlighted that the higher detection frequency for AMPA in the population may be due to dietary exposure, ie, from residues in food and water. This is where the real issue lies for our usage of glyphosate going forward.

The most concerning and very often overlooked aspect to our usage of glyphosate, is that it is widely sprayed as a preharvest drying treatment on many cereal and food crops –such as oilseed rape, wheat, barley, soy and oats – especially in wet climates like Ireland and Scotland.

What Nature Tells Us

The Irish Wildlife Trust (IWT) do not support an outright blanket ban on glyphosate, though they advocate for tighter regulation on the use of herbicides and pesticides which would prohibit ‘over the counter’ sale.

IWT also highlights very widely that we still need glyphosate as the most effective herbicide in the fight against alien invasive plant species, such as Japanese knotweed and rhododendron.

The results of Scott's experiments with organic and synthetic alternatives show that while they are still a long way from achieving the same control effects as glyphosate, we really do not need to be using so much glyphosate in our home gardens and landscapes.

Recent studies from Trinity College Dublin, Dublin City University and University College Dublin also highlight how impacting glyphosate is to other animals. Bees are particularly impacted, as glyphosate affects their gut microbiome, the development of their eggs and larvae, and their ability to memorise.

Bees digest glyphosate the same way we do: as a contaminate of their food, the plant pollen of non-target species. If we look at glyphosate’s effects on bees and consider our own relationship to this herbicide, it becomes clear that we are all likely to be ingesting glyphosate on a daily basis.

Scott Drew’s experiment showed that glyphosate is still the best herbicide for dealing with tough herbaceous and bulbous weeds such as wild garlic. However, the organic alternatives are suitable for annual species. We still need glyphosate, but only as a ‘silver bullet’ for species that are difficult to control, such as invasive species.

The more worrying aspect of our use of glyphosate is the fact that we are consuming it in cereals and food products every day. ✽

SCOTT DREW, a UCD honours degree horticulture student, recently emigrated to the USA to work as a junior project manager in Cape Cod, Massachusetts, for Schumacher Landscapes. Through the pandemic years, Scott set up his own garden maintenance company and was awarded a number of annual contracts in the Wicklow and South Dublin area.

Noeleen Smyth

This article is based on Scott’s research, which was led and supervised by NOELEEN SMYTH, Assistant Professor in Environmental Horticulture at UCD.