Fermented nettle tea, cold extract & HLVd: how big is the risk really?

Nobody tests their nettles in a laboratory before setting up a batch of fermented nettle tea. What does that mean in practice for hop and cannabis growers – and for everyone else? An assessment based on the current state of research.

What is this about?

Hop latent viroid (HLVd) is regarded as the greatest biological threat to cannabis and hop cultivation worldwide. A survey by Dark Heart Nursery (2021) found that around 90 % of all cannabis facilities examined in California were infected; the estimated yield losses run into the billions. In Europe the situation in cannabis has so far been less well documented, but in hops HLVd has been detected since 1988 in the Hallertau.

Interesting – and relevant to every gardener – is an often overlooked detail: the stinging nettle (Urtica dioica) is one of the few natural hosts of the viroid. Alongside hops, Japanese hops and cannabis, the nettle is considered an established fourth host.

And this is exactly where the problem lies: fermented nettle tea and nettle cold extract are two of the most widely used home remedies in the organic garden. The plant is practically never tested before use.

In short: what is HLVd anyway?

HLVd is not a virus but a viroid – a naked ring of 256 nucleotides of RNA, without a protein coat. The Saxon agricultural service describes viroids as “infectious particles that consist of a nucleic acid” and that, unlike viruses, have no protein coat. Because they are so small and stable, viroids survive in the environment for a surprisingly long time – and they can hardly be eliminated with conventional disinfectants.

Symptoms in cannabis: stunted growth, small buds, up to 50 % fewer cannabinoids and terpenes. In hops often symptomless, but with clear losses in yield and quality. In the nettle: no visible symptoms – it is the classic silent reservoir.

The real problem: nobody tests their nettles

The official recommendation of all specialist sources comes down to RT-PCR tests or meristem cultures. Feasible for a professional operation. For the gardener who cuts nettles by the wayside and sets up a batch of fermented nettle tea with them: unrealistic.

The honest question is therefore not “Do I test my material?” but: “Under which conditions does the risk matter at all – and how dangerous is each preparation then?”

The three preparations compared by risk

Fermented nettle tea (2 weeks of fermentation)

The classic. Fresh nettles, water, a bucket, wait two weeks, let it ferment at 15–25 °C.

Risk: medium, rather low. Reasoning: there is at least one relevant study. Hagemann et al. (University of Hohenheim) tested HLVd-infected hop material in biogas fermentation and compost. The result:

“Infectivity experiments with inocula from HLVd-infected hop plant residues that had been composted, ensiled or treated in biogas digestate did not lead to transmission of HLVd to viroid-free plants.”

Caution: the study ran at 40–50 °C, a garden batch of fermented nettle tea at 18 °C. The conditions are not identical. But the direction is clear – fermentation, microbial activity and the drop in pH work together against the RNA.

Nettle cold extract (24 hours)

Chopped nettles, covered with cold water, left to steep for 24 hours, must not ferment, then sprayed undiluted onto the affected plants. The classic aphid spray.

Risk: theoretically much higher than fermented nettle tea. Reasoning: a study by Punja et al. (2025, Plants) measured HLVd stability directly:

“The viroid survived 7 days in crushed leaf extract (sap) and 4 weeks in dried leaves or roots at room temperature.”

24 hours lie comfortably within this 7-day window. And that is exactly the crux: functionally, the cold extract is fresh plant sap plus water – without heat, without microbial activity, without a shift in pH. None of the mechanisms that could break down viroid RNA in fermented nettle tea come into play here.

For comparison, what actually destroys HLVd (according to the same Punja study): 70–90 °C for 30 minutes, UV-C for 3–5 minutes, or 5–10 % sodium hypochlorite. All conditions that by definition do not occur in a cold extract.

Important limitation: so far there is no study that has explicitly tested nettle cold extract → susceptible plant. The risk assessment is a deduction from (a) “nettle is an HLVd host” and (b) “HLVd survives 7 days in plant sap” – not a direct experimental confirmation. But the mechanisms clearly speak against the cold extract.

Nettle brew / tea (hot water)

Pour boiling water over chopped nettles and leave to steep for 15 minutes up to overnight.

Risk: lowest. Boiling water does not reach the 70 °C over 30 minutes used in laboratory studies – but a 15-minute infusion with water that has been left to cool lies well above what fermented nettle tea or a cold extract achieve. For hobby users who want to play it safe, this is the cleanest option.

Who should worry at all?

A sober look is worthwhile here. The risk is not equally high for everyone:

High risk:

  • Hobby and commercial cannabis cultivation (legalisation is increasing the number of growers massively)
  • Hop growers, especially in the hop regions of Hallertau, Tettnang and Spalt – in the Hallertau HLVd has been detected since 1988
  • Gardeners with susceptible plants who collect nettles directly at a hop or cannabis field or in its surroundings

Practically no risk:

  • The classic vegetable gardener without hops or cannabis in the garden – HLVd has a very narrow natural host range. Tomatoes, cucumbers and the like can only be infected experimentally under laboratory conditions.

What to do without going to the lab?

Six practical rules of thumb that follow from the state of research:

  1. Pay attention to where the nettles come from. If hops or cannabis are grown nearby, collect nettles from another, distant area if possible (edge of a wood, your own garden without cannabis/hops).
  2. Avoid cold extract on sensitive crops. Anyone feeding or strengthening hops or cannabis should rather use fermented nettle tea or tea brewed with hot water. The cold extract is theoretically the riskiest preparation.
  3. Tool hygiene is a bigger risk than fermented nettle tea. The specialist literature almost unanimously names contaminated tools as the main route of transmission. A 10 % bleach solution reliably inactivates HLVd. Anyone who disinfects scissors between plants has done more for viroid management than any decision about fermented nettle tea.
  4. No shared irrigation or hydroponic water between crops. HLVd demonstrably travels through water systems, even without root contact between the plants. A cannabis plant watered with fermented nettle tea in a shared drip tray with other clones – that is the worst case.
  5. Establish your own nettle patch that you harvest from repeatedly. If this patch demonstrably stays healthy (no proximity to hops/cannabis), over the years it is a more controlled reservoir than wild-collected material of unknown origin.
  6. Do not panic. The only infectivity study available so far could not demonstrate any transmission to healthy plants from fermented hop material – that is, from a starting material with a far higher load than wild nettles. The residual risk remains, but it is not the end of the world.

Conclusion

If you have no hops and no cannabis in the garden: carry on as before. Fermented nettle tea is an established, largely harmless home remedy.

If you grow cannabis or hops: rethink nettle cold extracts critically, use tea brewed with hot water or fermented nettle tea that has fermented for longer instead, and above all take tool and water hygiene seriously. There, fermented nettle tea is not the biggest problem – the gardener with his scissors is.

And research is lacking: a study that specifically tests nettle cold extract for HLVd infectivity is still outstanding. As long as it is missing, every statement about the cold extract remains a reasoned risk estimate – not an established hazard assessment.

Sources