The common nettle, or stinging nettle is a herbaceous perennial plant that has been used for a variety of purposes throughout the millennia. It seems unbelievably useful at first glance. You can make a lot of from it – boil some tea with dried nettle, mix it into a pie with some cheese, mix it with chicken feed as a supplement. Or, you could use nettle water as a fertilizer or pesticide on your other crops.
Introduction / Background
But how effective is nettle for all of these purposes, really? And what are the downsides of leaving nettle to grow in your backyard?
Many farmers and gardeners have problems with stinging nettle since it’s very hardy as a weed. But it’s also relatively easy to clear out since the plant stems are not woody. Their growth rate is the real problem.
Nettle plants grow fast, and form clumps, taking nutrients from other plants. But what sets them apart from other weeds is their sting and the history of their usage. Even though it’s known for its sting, some variants don’t actually develop the stinging spikes, and the plant can be used for a wide variety of purposes, making it one of the rare “desirable weeds” among farmers who know how to benefit from it.
Stinging nettle has established an almost cult following among organic farmers. It can be eaten as well, or fed to chickens but what we are going to focus on in this article is usefulness to your other garden plants. We will be examining two instances where it can be used as a gardening resource:
- as a pesticide, and…
- as a fertilizer.
Use Cases For Nettle Water And Slurry – Sources and Studies
Searching for recipes, we found one video that covers all of the different recipes and use cases above, but of course we wouldn’t consider it on the same level as actual scientific studies. So take its claims at face value. The use cases are cited beneath each recipe in the section below.
Another exceptional resource is this video about fermented nettle, explaining why and how to produce an “FPJ” or “fermented plant juice” by adding sugar to nettle or any other plant mass. It also explains why nettle is the best for this – it simply contains every one of the most important micro and macro-nutrients in ample amounts.
Ideas & Recipes For Nettle Slurry
A word of warning: Don’t forget to use thick gloves when picking nettle! Pick only the parts of the plant that are above ground.
The main processing method for nettle is based on making a solution that you could then use on other plants (or soils). But something as simple as nettle can get very convoluted very fast, as you will see soon. There are several ways to make it. Here are just a few examples:
Recipe 1 – “The lowest of efforts”
- Take 1 kg of nettle plants (excluding roots) and chop it up or grind it
- Submerge in 10 liters of cold or room temperature water
- You may cover the bucket with a rag – do not seal it tight. It will start to stink after a while.
- 24 hours later use as an insecticide:
- Drain it out and do not dilute – just spray it on the plants as soon as it’s drained.
- The unused solution can be stored in a cold and dark place and applied as a fertilizer (at least once per week)
Recipe 2 – “A bit of witchcraft”
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- Take the same 1kg of nettle and boil it first for 20-30 minutes.
- Add as much water as needed to reach a total of 10 liters of water.
- Leave it to sit for 24 hours
- Drain the liquid into a separate container
- Add a bit of liquid soap to the solution (1:100 ratio maximum)[1]. The soap acts as a surfactant, improving the adherence and absorption of the solution by the plant leaves. Use an organic one or the one intended for babies.
- Mix it well.
- Before applying, dilute it further (1 part nettle-soap, 4 parts water)
- Prepared like this, it’s used primarily as an insecticide. It’s reportedly effective against various larvae, caterpillars and aphids.
Recipe 3 – fermentation slurry (fermented plant juice)
- Take 1kg of nettle and submerge it in 10L of water.
- Leave it to sit 10-15 days in a covered (but not sealed) container.
- Keep it in a dark place ideally at ~18C
- Drain the liquid part which should be olive to dark brown by now, and feel free to use the filtered nettle as compost.
- Fermented thick brown slurry very unpleasant smelling
- Repels aphids, mites, thrips, brown rot fungi.
- Warning: the longer it sits, the worse the smell is.
Effectiveness
There are a couple of separate assumptions we can make about nettle water/slurry, and then we will review some of the most prominent scientific papers to check which of those assumptions are true, and to what degree. The claims are:
- The solution possesses antimicrobial and insecticide compounds: They could be anti-bacterial, anti-viral, anti-fungal, insecticidal. This assumption is not far-fetched. Plants have been developing their defense mechanisms, fighting against microbes and insects for hundreds of millions of years.
- The solution has either organic or inorganic phyto-stimulants: these can come from the breakdown of the plant itself, like plant hormones (auxin, cytokinin, gibberellic acid, brassinosteroids, ethylene, abscisic acid, salicylic acid and jasmonic acid) or from microorganisms starting to form in the slurry.
- The solution provides absorbable minerals and nutrients: certainly true but to what degree of significance? And are they used by the plant or by the microbiome that indirectly feeds the plant?
After a bit of research, we’ve found some resources that address these points. So let’s start dissecting these bit by bit.
Antimicrobial & pesticide properties of nettle water
Many studies 1 2 3 show promising human-centered pharmacological potential of nettle extracts made using methanol, ethanol, dimethyl sulfoxide, acetone, ethyl acetate, hexane, and other organic solvents. They also show very high antioxidant activity. These extracts have been examined in crops as a fungicide. Here’s one study describing the methods and results.
But what about a plain old water extract?
This study found that water extracts do exhibit antimicrobial, antioxidant and even anti-ulcerous properties in rat trials.
As for plant applications, it turns out that there is a good reason to wait for days after submerging nettle and another good reason to add soap. Many of the antimicrobial compounds are only slightly soluble in water. Which is why you can speed up the process by boiling it first or reaching at least 80C before letting it sit for 24 hours.
This study found that an aqueous solution of nettle contains phenolic compounds in concentrations that have been proven to be antimicrobial and antifungal. In fact, they concluded that there is a significant difference between young spring nettle extract vs. autumn nettle. The concentration of these compounds was significantly lower in autumn nettle water. They did the extraction at 80 and 100 C, and for comparison, they used an ethanol extract diluted 20x to avoid the effects of ethanol itself on the bacteria.
Another angle to the same story would be this article from 1996 which used a cold water extract and found it to be not immediately efficient in the short term, and only selectively efficient in the long term against various species of aphids and pests. Aphids also migrate naturally, and reappear in other times whether they are treated or not. They are good at hiding and jumping to other plants so they need to be taken care of sooner rather than later.
So, for maximum antimicrobial and insecticidal effects in your garden:
- Boil the water for a short time and let it sit.
- Do it in the spring when the nettle is young. More phenolic compounds – better results!
For deeper research, this study has compiled a table (Table 1) of of 38 publications containing eligible data on the MIC of Urtica spp. extracts against bacteria or fungi.
Enjoy!
On to the next claim…
Organic or inorganic phyto-stimulants in nettle slurry
It’s interesting to note, as a general pro-tip: some plant hormones like Auxins are stable compounds and can even survive extreme heat treatment like autoclaving. Others, like Gibberellins suffer from heat breakdown and loss of activity. You can read discussions about thermal stability of specific compounds here and here.
Furthermore, here is a more complete list of plant growth regulators and their properties. Those that can only be filtered or are somewhat stable but not entirely are marked (F) and (CA/F) respectively. Those that can survive autoclaving as the sterilization method are marked with (CA)
Not only can some of these plant hormones survive boiling the nettle water, but they are directly enhancing the plants’ own heat resistance when applied. Vigor as a bonus effect to enhancing biomass and yield. That’s a win!
So, as you can see there are many plant hormones that can survive the nettle preparation processes involving heat. And for those that don’t survive the heat, nothing is stopping us from splitting the nettle batch into 2 parts, one cold-prepared and one heated – and mixing them later for a more complete solution.
Hormones are effective and can do their job in extremely small concentrations.
This is an open and shut case already, but after doing some due diligence, we’ve found a very limited pool of direct studies discussing this.
The composition of nettle water seems to be much more dependent of the time nettle was harvested than location – an old study has concluded that nettle water auxins might be impactful for crops. This was also mentioned in a later study.
In conclusion (surprisingly enough) there are very few studies pertaining to the exact measurement of plant hormone concentrations in nettle slurry.
This is a great research opportunity but also somewhat disappointing.
Mineral & nutrient content of nettle water
Compared Many of the studies mentioned elsewhere in this text also cover mineral content. Whether the mineral content is impactful and complete depends on many factors:
- The preparation process: Are you preparing nettle water, or regular slurry, or fermented plant juice (FPJ)?
- How long does it sit: the longer it sits, the more it stinks but also the compound breakdown leads to higher concentrations of bioavailable nutrients. Bioavailable being the key word.
- When did you pick the plants? Spring nettle and autumn nettle have very different mineral and hormone profiles.
Looking at just the recipes above, it shouldn’t be a surprise that results vary so much between studies and why people from the organic community might have very different results from beginners or some researchers. How detailed and thorough you are during application on the leaves themselves also has a huge impact.
Here are a couple of papers: [1], [2] covering the mineral content of different nettle water products. And just for good measure, the same video posted before, covering the preparation of a special kind of nettle slurry that doesn’t actually stink: the fermented plant juice (FPJ) which does require adding sugar, but the results speak for themselves.
Aside from the above, the odds that all of the researchers are going to be preparing their nettle in the same way and using it in the same way on the same plants are slim to none. And on that note it’s time to discuss…
Theories Vs Experiments
We can’t really expect to find a 1:1 comparison between studies anywhere, and there are probably very few (if any) replication studies. This article investigates the lack of replication in agricultural economics, but it’s safe to say the trend is not limited to just economics, and also much broader than just agri-sciences.
Which isn’t surprising since it’s kind of impossible to get them funded and set up, let alone published in journals with very limited space for new articles. And it’s not like there’s a nettle lobby funding all of this research, so there’s that…
It’s easy to theorize on what exactly is going on when nettle is picked, prepared, fermented, and applied, but the results from all scientific data have been mixed. For example, the nettle water recipes using liquid soap or detergents might work against aphids because of the soap, not because of nettle. Soaps and detergents are known to kill both insects and bacteria by disrupting cell membrane processes and the cell’s mineral/water regulation.
Does that mean we shouldn’t use nettle, just because it may not be the nettle compounds killing off the aphids? Absolutely not. In fact we’d be hard pressed to find a better candidate for the title of the most versatile (and legal) plant that grows literally everywhere.
In fact, being a weed actually does make it a double-edged sword. The gift of fire does come with a lot of responsibility, and this fire can burn (pun intended), so keep it away from both your kids, your pets, and your other plants.
Conclusion
Nettle water, nettle slurry and fermented plant juices (FPJ) are powerhouses of organic agriculture and deserve much more attention than they’re currently given.
Considering the fact that nettle grows everywhere, and that research has been ongoing since the early 80’s, it’s safe to say nettle is a good choice, as long as you can control it from spreading faster than you can manage it.
Don’t forget that insects are actually necessary for plants, some of them are beneficial, others are pathogenic. Never completely eliminate an entire class of living beings unless you are certain that you won’t introduce more imbalance into an already fragile ecosystem.
Footnotes
1. [soap]↩: before you start rolling your eyes at using soap in the cycle of food production, it’s worth noting that there are detergents and soaps specially made for agriculture. They’ve been used for decades in integrated pest management solutions with no ill effects on humans. since the compounds degrade after doing their job.
References & Resources:
- https://www.youtube.com/watch?v=ATfLeqnh9dE – the recipes video
- https://www.youtube.com/watch?v=DlLWanDZwuU – fermenting nettle – complete guide for FPJs.
- https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5944444/ – negative or insignificant results on plant yield
- https://www.mdpi.com/2071-1050/13/7/4042 – positive results
- https://onlinelibrary.wiley.com/doi/full/10.1002/aepp.13386 – the problem of replication in agricultural economics
- https://www.researchgate.net/post/Can_any_one_tell_me_which_are_the_heat_stable_plant_tissue_culture_hormones_and_which_are_all_the_heat_labile_ones – heat liable growth factors (plant hormones)
- https://www.intechopen.com/chapters/51590 – using detergents and soaps in agriculture
- https://link.springer.com/article/10.1007/BF01905863 – tried against aphids, mostly didn’t work
- https://www.tandfonline.com/doi/abs/10.1080/01448765.1985.9754444 – a composition examination of nettle watter, could be used as a feed for plants, good study
- https://www.tandfonline.com/doi/abs/10.1080/01448765.1986.9754482 – same author as above, another great study, although it’s old
- https://fiver.ifvcns.rs/handle/123456789/926
- https://scialert.net/fulltext/fulltextpdf.php?pdf=ansinet/pjbs/2009/58-63.pdf