Hemp Research

Can Waterhemp Competition Affect Hemp Growth?

Much is said about hemp being able to grow without herbicides or pesticides but is it really true?

Hemp is still a relatively new crop around the world. We need more research on the influence of weed competition on crop yield and quality.

Although hemp can be grown for seeds and fiber, most of it is aimed at producing flowers to then extract phytocannabinoids (mainly tetrahydrocannabinol (THC) and cannabidiol (CBD)) and terpenes to be used in therapeutic applications.

It’s very important to understand how external factors, like competing weeds, affect the chemical makeup of hemp because the 2018 Farm Bill established a maximum threshold of 0.3% of THC.

All hemp surpassing this amount of THC needs to be destroyed, which represents economical loss and potential legal repercussions.

We know that exploitative competition for resources can decrease the availability of water and nutrients, changing the growing conditions. We also know that some plants can sense their neighbors and adjust their metabolism and growth accordingly.

Another known phenomenon is that moderate drought and decreased nutrient availability can boost the THC content in hemp.

So, could competing weeds reduce water and nutrient availability and cause that same increase in THC? Some hemp producers believe it does but very little is known for sure.

Hemp grown outdoors.
Photo by Crispin Jones on Unsplash

This research study set out to evaluate:

  • The effects of waterhemp competition on the total hemp and flower biomass yields in plasticulture.
  • If waterhemp competition can influence the THC and CBD contents and the terpene profile of hemp.

This is particularly important since no herbicides have been labeled for use in hemp crops in the US.

The study was conducted at the Southern Illinois University Horticulture Research Center.

Seven treatment combinations were tested in a randomized complete block design with eight replications for each treatment. The experiment was carried out twice — crops were grown and analyzed in 2019 and 2020.

“We tested the effects of varying waterhemp and hemp planting densities on hemp yield, height, chemical composition, and aboveground waterhemp biomass in a plasticulture system.”

Key Takeaways

In both years, 2019 and 2020, there weren’t significant changes to the THC, CBD, or terpene contents due to waterhemp competition.

The terpene composition, total hemp biomass, stripped floral biomass, hemp height over time, and final hemp height weren’t affected by waterhemp competition either.

Interestingly, the opposite of what was expected happened. In both years, the total waterhemp biomass was consistently affected by the presence of hemp.

“We did not find evidence that competition from waterhemp is responsible for elevated levels of THC.”

Previous studies had already highlighted that hemp is a highly competitive crop. It can suppress the growth of aggressive competing weeds — even those for which management options are scarce.

“Waterhemp growth was strongly suppressed by the presence of hemp, but no significant yield loss of hemp was detected.”

In fact, the opposite happened. The waterhemp biomass was reduced. And since their mass is directly related to seed production, it was hypothesized that hemp could be used to reduce the number of troublesome weeds in other crops via crop rotation.

Thank you for reading :) Let me know in the comments if you have any questions. Sign up for my newsletter to stay up to date on the latest cannabis research.

About the Author

Inês Tavares is a graduate of Minho University, where she received her MSc in Textile Engineering. Plagued by the idea that a 9 to 5 would be her reality until retirement, she took the first COVID-19 lockdown as an opportunity to start her freelance writing business.

She has since been helping cannabis & psychedelics businesses increase traffic and sales by providing them with content strategy, SEO blog posts, email marketing strategy, lead magnets, and marketing emails.

She created MariGrama to fight misinformation by sharing reliable, trustworthy content on cannabis — no twisting facts for marketing, no dodgy sources, only science-backed information.

You can connect with her on LinkedIn or take a look at her portfolio.




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Ines Tavares

Ines Tavares

I provide blog management services to cannabis businesses looking for a boost in organic traffic and sales. Connect with me on LinkedIn: tinyurl.com/3r5k8vuc

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