Can Hemp Suppress Weeds? What Science Says About Hemp Allelopathy?
Hemp once covered 392,000 hectares, with India Asia's top grower. A review shows how it holds back weeds naturally and what it means for organic farming.
Hemp has walked beside humans for a very long time. The paper describes it as one of the first plants ever domesticated. It says hemp originally comes from Central Asia, and from there it travelled to East and South Asia and to Western Europe.
Behind that old story is a quieter one, written in the soil. Researchers noticed that fewer weeds grew back where hemp had stood. This led them to study allelopathy, the way one plant shapes the life around it through chemistry. According to the paper, this influence can hold growth back or encourage it, depending on dose: stronger concentrations tend to inhibit, gentler ones can stimulate.
Hemp has plenty of chemistry to offer. The paper counts 483 compounds in the plant, including 60 cannabinoids and 140 terpenes. Its essential oils attract pollinating insects, discourage other seeds from germinating and protect the plant from pests.
The studies the paper reviews are encouraging. Hemp extracts affected common ragweed and fat hen (Chenopodium album), and caused oxidative stress in the leaves of Johnson grass. They held back the germination of lettuce, wheat, rye and rapeseed. Two details stand out:
- In lettuce, the shoot extract inhibited germination, but the root extract had no significant effect.
- In pot trials, hemp grown as a cover crop was among the strongest at inhibiting lettuce, maize and weeds.
India's place in hemp's story is an old one. The paper records that in the 1880s India was the largest hemp producer in Asia, which held 162,000 of the world's 392,000 hectares of hemp. It also cites a 1982 study on hemp's effect on maize, presented at an Indian Society of Weed Science conference in Bihar. For India's hemp industry, the paper's message is that hemp's worth may reach beyond fibre and seed into the field itself. It could serve as a weed-suppressing crop for organic farming and as a source of natural bioherbicides. The paper also notes that hemp can grow on soils contaminated with heavy metals and is irreplaceable in organic crop rotation.
Some of the most valuable science is quiet work: years spent watching seeds sprout, measuring roots and comparing plant extracts. The paper behind this post is exactly that kind of work. The authors point out that there are still too few herbicide preparations based on plant chemicals, so studying how plants influence each other is both important and necessary. We are grateful to Konstantinović, Koren, Kojić, Samardžić, Sikora and Popov for gathering this research into one review, and to every researcher whose experiments it draws on. Their patience gives us a clearer view of what hemp can do in the field.
The authors conclude that hemp's allelopathic effects are clearly confirmed. They say more research is needed to understand how it works, and suggest the combination of its compounds, rather than any single one, may be what acts on neighbouring plants.
Source
Konstantinović B., Koren A., Kojić M., Samardžić N., Sikora V., Popov M. (2021). Allelopathic Properties of Hemp. Contemporary Agriculture, 70(3-4): 101-107. DOI: 10.2478/contagri-2021-0015
Phenomenal Fact
Bhanga/Vijaya has been crowned as a sacred plant among other five, immortalized in Ātharvaṇa Veda, chapter 11, one of the oldest sciptures, where it is invoked to free us from aṃhas.
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