Copper sulphate (copper sulfate, in US spelling) cut Pythium root rot in hydroponic cannabis at 2.5 ppm, in a study that Zamir K. Punja, Li Ni and Cameron Scott published on 23 September 2026. Heavier treatments also left copper in the flower.
Whether you can use that result comes down to three questions every cannabis regulator asks:
- Is the product allowed on cannabis?
- Is there a residue limit?
- Can your records show what went in the tank?
Canada answers all three in writing, so it is our worked example. The UK and Germany answer them through different rules, and we cover those too.
What the 2026 study found
The paper is open access in the Canadian Journal of Plant Pathology. Every dose in its abstract is copper sulphate pentahydrate, the common blue crystal.
| What they tested | Result |
|---|---|
| 50 ppm in broth | Growth stopped for Pythium myriotylum, Penicillium olsonii and Alternaria alternata |
| 50 ppm in broth | Botrytis cinerea down 80%, Fusarium oxysporum down 30% |
| 2.5 ppm in hydroponic nutrient solution | Significantly less Pythium root rot, fewer oospores in the roots |
| 2.5 ppm in hydroponic nutrient solution | Taller plants, longer roots, heavier roots and shoots (fresh weight) |
| Four weekly 100 ppm applications on powdery mildew | 70% less disease |
| Four weekly treatments of flowering plants | 31–45 µg/g copper in dried flower, against 4–7 µg/g untreated |
Source: Punja, Ni and Scott, Canadian Journal of Plant Pathology, 2026.
The authors say those flower levels "may be noncompliant with current regulations". They put the low root dose forward as an option for managing Pythium.
It isn't the first lab result. In 2023, a University of Lethbridge team slowed Penicillium olsonii with copper sulphate, on agar and on sprayed plants.
Don't dose from an abstract. It leaves three gaps:
- whether the 2.5 ppm root dose raised copper in the flower, since copper was measured after the heavier treatment
- how long the solution held 2.5 ppm, and how it was topped up
- the dose at which roots or leaves showed damage
The pentahydrate is about a quarter copper by weight. So the paper's root dose adds well under 1 ppm of copper to the tank. Compare it with the copper line in your own recipe (1 mg/L is 1 ppm in water).
| Copper sulphate pentahydrate | Copper it adds |
|---|---|
| 2.5 mg/L | 0.64 mg/L |
| 10 mg/L | 2.5 mg/L |
| 50 mg/L | 12.7 mg/L |
| 100 mg/L | 25.5 mg/L |
Our arithmetic, from standard molar masses.
Copper against water moulds isn't new either. Guidance from Oregon State and the University of Florida puts copper at 0.5 to 1 ppm for pathogens such as Pythium in irrigation water. It goes in by ionisation, or sometimes as copper sulphate. The paper's root dose sits inside that range.
So the news isn't the cure. It's cannabis data at that dose, and the copper the heavier treatments left in the flower.
Can you use copper sulphate on cannabis in Canada?
Only through a product registered for that use. Section 81 of the Cannabis Regulations sets the rule. Cannabis "must not be treated with a pest control product unless the product is registered for use on cannabis". Authorisation under the Pest Control Products Act is the only other route.
A registered product can only be used against the pests and in the treatment areas on its label, following the label directions. The label names the crop and the treatment location, such as indoor or outdoor (Health Canada).
Residues have their own rule. Subsection 93(1) says a cannabis product must contain nothing but cannabis. Subsection 93(2) allows pest control residues only within a maximum residue limit set for cannabis under the Act (Health Canada).
Copper sulphate is also a micronutrient salt, and that's where growers get tangled. Health Canada says the Pest Management Regulatory Agency (PMRA) regulates all products designed to control pests. If copper goes into the tank to stop Pythium, treat that as a pest control question. Take it to the Pest Management Information Service before the first dose.
The paper says copper works. The label says whether you can use it.
Health Canada points licence holders to the PMRA pesticide label search, which can list every product registered for use on cannabis ("marihuana" in PMRA's terms). We haven't run that search for this post. Search for copper products, check that cannabis, Pythium and irrigation use all appear on one label, and file the result with the date.
A clean pesticide panel won't settle it either. Health Canada can act over an unauthorised pest control product "whether or not that product is listed in the List and limits". That is the list labs test for. Its options run from seizure and recalls to licence suspension and penalties up to $1 million. Mandatory pesticide testing has been in force since 2 January 2019.
Can you use it in the UK?
Growing any cannabis plant in the UK needs a Home Office licence. Pest control has its own rule. In Great Britain, a plant protection product can't be used unless the relevant authority has authorised it.
The law defines those products by purpose: protecting plants against harmful organisms. Copper dosed to stop Pythium serves that purpose, even though copper is also a nutrient.
HSE, which authorises products in Great Britain, is blunt about it. Without a GB authorisation, "it is illegal to use that plant protection product here". Authorisations also run by crop and situation, and those uses can end before the product itself expires.
We haven't searched HSE's product database for a copper product authorised on cannabis. Run that search before the first dose, and file the result with the date.
Can you use it in Germany and the EU?
EU law starts from the same regulation. A product must be authorised in the member state concerned, and used properly, which includes the conditions on its label. Copper compounds were renewed in the EU in 2018 as candidates for substitution, substances to replace where safer options exist.
Germany's Plant Protection Act allows a product only within the uses set in its authorisation. Other uses need a case-by-case permit, meant for minor crops and regional pests, and every such permit is time-limited. We haven't searched the BVL's database for a copper product authorised on hemp or cannabis.
Cannabis clubs under the Consumer Cannabis Act (KCanG) have a rule of their own. They must follow good professional practice and keep health risks from plant protection products, fertilisers and other inputs low. If a club suspects its cannabis carries such a risk, it must tell the authority and recall it.
Copper residue is never zero
The study's untreated plants still carried 4–7 µg/g copper in dried flower. Copper is a micronutrient, so every flower has some. The question is how much, against which limit.
In Canada, that limit is a maximum residue limit set for cannabis under section 93(2). In Germany, the medical standard for pesticide residues is method 2.8.13 of the European Pharmacopoeia, written for herbal drugs. The British Pharmacopoeia carries the same method.
The method counts as a pesticide anything "intended for preventing, destroying or controlling any pest". For a pesticide its own table doesn't list, it points to EU food residue limits. Those limits measure copper as total copper, whatever form it came in.
German clubs are still waiting for their limits. The KCanG lets the federal ministry set maximum levels for pesticides, fertilisers and heavy metals, and club cannabis above them can't be passed on. In February 2026, a Würzburg court noted that regulation hadn't been issued. It struck down a Bavarian permit condition that made a club test to the medical standards, including Ph. Eur. 2.8.13. The court allowed an appeal, so it may not be the last word.
A lab report shows one copper number. It can't say how much came from the nutrient line and how much from a treatment. Your log can.
| Question | Canada | UK | Germany and EU |
|---|---|---|---|
| Is the product allowed on cannabis? | Only if registered for use on cannabis (section 81) | Only if authorised in Great Britain for that crop or situation | Only if authorised in that member state, within its authorised uses |
| Is there a residue limit? | Only within a limit set for cannabis (section 93(2)) | Not confirmed for cannabis. The British Pharmacopoeia carries method 2.8.13 | Medical flower: Ph. Eur. 2.8.13. Clubs: federal limits not yet set |
| Can your records show what went in? | Inspectors may review fertigation and spray records | Growers must comply with GACP | Clubs must report and recall cannabis they suspect carries an added risk |
Sources: Health Canada, HSE, MHRA, PflSchG, KCanG.
Your fertigation programme is in the batch record
In Canada, inspectors checking the testing rules may ask for batch records. Those cover spray logs, sanitation, the fertigation programme, rooting products and anything used on the plants from seed or cutting to point of sale. They may also ask for the IPM programme and a list of every pest control product used (Health Canada, section 6.0). Records tied to mandatory testing stay on site for two years (section 5.4).
The UK is less specific, but the duty is there. MHRA says a site that only grows and harvests must still comply with Good Agricultural and Collection Practice, GACP. In Germany, a club can only recall the right batches if it knows which batches got what.
If it went in the reservoir, it goes in the log.
Every additive should trace back to a recipe, a date, a dose and the batches that drank it. Keep that wherever your team keeps run history: a binder, a spreadsheet or facility software. An undocumented additive is a records problem before anyone tests for residue. Already logging feed changes against water use, as in our post on late-flower uptake? The same log can carry every addition to the tank.
A trial needs authorisation before a dose
Health Canada notes that R&D with a pesticide may require a research authorisation from the Pesticides Regulatory Directorate. Cannabis from R&D can't be sold, distributed or exported unless it meets the testing requirements.
In Great Britain and the EU, testing an unauthorised product or use needs a permit for trial purposes from the authority where the trial runs.
If you get that far, here is what we would want in the trial file. It's our own checklist, not legal advice:
- who authorised the trial, and under which authorisation
- a separate room or an isolated irrigation system
- a dose log with tank concentration, EC, pH and solution temperature
- an untreated control on the same recipe
- root and leaf disease ratings on a fixed schedule
- copper tests on harvested flower, against the untreated control
- trial batches held apart from saleable lots
Paperwork first, copper second.
Prevention starts with the water
Health Canada's IPM best practices start with prevention. The list includes sterilising water and irrigation systems, with ozone, ultraviolet and reverse osmosis as examples. It also covers sterilising growing media before planting and testing inputs such as water and growing media for contaminants.
The same list asks for pest-free starting material, the subject of our post on moving genetics into tissue culture. It also asks for regular monitoring of pests and of the conditions that favour outbreaks, with records. None of it is specific to Canada.
Ozone and UV were on that list before copper made the journals.
That leaves three routes:
- Tighten water hygiene and records now. Neither depends on the paper.
- Watch your country's register for a copper product that names cannabis and irrigation use: PMRA's label search, HSE's product database or the BVL's database.
- Run an authorised trial with the file above.
We would start with route 1 today. An authorised copper product that covers cannabis and irrigation would change that advice. Until then, route 3 is a research project with a regulator in the loop.
Next step: search your country's register and file the result. Then pull last run's fertigation log and check that every input has a name, a dose and a date.