Biochar from Japanese Knotweed

New here, and looking forward to learning a lot more about biochar from all the folks here. Figured I'd share a little project I did this past weekend.

I've been interested (some would say obsessed) with the idea of the use of invasive species as animal feed and/or biochar production. To that end, I wanted to do some experiments with making biochar with Japanese Knotweed.

After doing some small scale biochar production in my fire pit with metal trash cans and a small hotel pan, I wanted to give something else a try. I took a 30 gallon trash can with a tight fitting lid and drilled a few strategically placed holes in the side.

My son and I stopped at a roadside patch of Japanese Knotweed, and carefully cut and stuffed as much as we could into the trashcan. Again, knotweed can root from a very small fragment, so being super careful was key. Once the knotweed was in the can, I kept the cover on tight except to take a quick photo.



That's the can of knotweed, with the fire ring and the metal paint cans I use for smaller batches of biochar in the background.

Close-up of of the knotweed...we used the entire plant, from woody stems to smaller tops and leaves.



The knotweed was of course dormant, it being December in the northeast US. I did a small batch of "green" knotweed earlier this summer, which I'll share photos of as well. With this dormant knotweed, I expect it'll be dryer than the fresh plant.

I got a fire going. One of the reasons I'm interested in using invasive species for biochar is that good hardwood is going to create the most weight/volume of biochar, using good wood for biochar production feels not quite right. So, other than a few spruce logs from the bottom of my fire pit wood stash, all the wood we're burning are branches that fell in the yard and would need to be picked up anyway. The ashes will be spread in the woods out back to help feed the soil back there.



I built the fire up quite a bit bigger than that to make sure I had enough heat. My son loves a good campfire, so he was happy to hang in the backyard feeding the fire and grinding some of the biochar we made yesterday.

The holes in the trash can were in the side, facing the heart of the fire. That way, any gases produced in the char process would burn off. In this way, the biochar process actually helps itself along by literally "adding fuel to the fire".

After a few hours of feeding the fire while mowing up lots and lots of leaves for the chicken run, I pulled the can out with my welding gloves and opened it up.

The charring process was complete, which was nice to see. The volume had been greatly reduced, as expected.



30 seconds of crushing with the fire poker stick reduced the particle size and volume significantly.



Obviously a woody plant is not going to produce the highest yield of biochar, but something about creating something good out of a nasty invasive. I find that the biochar made from the knotweed crushes more easily than biochar made from harder materials.

The next step is to do a few more batches and try to get an idea for how long the process takes per "batch". Then I can get a few more barrels and set up a batch system, so we can do several cans on a single fire session.

I'll add the biochar into our compost to charge, and then use it around the property. Will making a few cans of biochar out of knotweed make a big difference? Maybe not, but if I can slow the spread or weaken a stand or two while enriching my soil and having a fun afternoon in the backyard, why not!?

A photo of the small-scale batch I made with fresh, green knotweed this summer, in a hotel pan:



And the results (again, very low volume):

Very cool, its especially neat to see it work on the green material.
Packing the feedstock tighter could help up the yield volume.
The dried  knotweed looks like it would easily crush up with a hatchet or hammer.

Neat! I've thought about this too, since there are several big patches of knotweed near our place and it grows like mad. My thinking has been more in the direction of a pit/trench burn, since that's the method I have the most experience with, and that's yielded the best results. However, since Japanese knotweed stalks are quite thin and hollow, I suppose you'd have to keep adding fuel all the time to keep it from burning out to ash if doing a trench burn. The volumes of fuel this would consume would make it impractical to transport the knotweed stalks any distance, so you'd basically have to make the burn next to the knotweed patch. Since we (fortunately!) have none on our land, we'll probably not do this anytime soon, but who knows? Maybe the people who have knotweed on their land will turn permie and want help with this...

Thanks folks for the replies!

I agree that the material could probably be packed a bit more tightly into the cans. I could see either stomping the material in to make more fit, or perhaps working with two barrels...one for woody stems and one for leafy tops.

And yes, due to the nature of the knotweed material, and it's propensity to spread easily from just a small fragment, I think in-vessel burns make the most sense to me. Thankfully, there's no shortage of knotweed around my area.

I live a few miles from an 8,300+ acre management area, and I have seen lots of knotweed along the roadside around the exterior of the preserve. The nice thing is that there are a bunch of car pull-offs in the area used by hikers and cyclists.

If we harvest a patch 2-3 times a season, can we slow it's growth? If nothing else, we're using a fast-growing material to create biochar without removing any native materials from the area.

One more thought, then one more set of  pictures of making biochar from a waste product. The idea of making biochar from invasives is very attractive for all the obvious reasons. I'm sure there are many more woody invasives that would make better feedstock, it just happens that knotweed is the one I see around here.

I did another test making biochar from another hard-to-compost material I had in quantity - avocado pits. For a few years, I was bringing food waste into my large chicken compost system at the rate of hundreds of 5-gallon buckets a year. There were a number of avocados in the mix, quite often. Everything would compost but the pits, so I started gathering them up and tossing them into a metal paint can. When the bucket got full-ish, I sealed it and threw it in the fire pit.

Before:



After:

Did you all know that Japanese knotweed is a very medicinal plant? In fact, it is the #1 source for the highly prized longevity supplement resveratrol.  I know, most people love resveratrol as the excuse to keep drinking too much red wine. I make medicine out of Japanese knotweed and I consume it every year.  I have seen several large patches of it in my area. Most are in someone's yard.  I am a little nervous about what sorts of pesticides they've thrown on it, and even asking for permimssion.  Here in the coastal PNW, we have bone dry weather in the summer, so that's the time to harvest it.  I killed a patch once by making the medicine.  I'm sure if you asked someone and let them know how invasive it is, they'd be glad to have you haul it off.

John S
PDX OR

Yeah, I'd heard it's high in resveratrol. How do you make your medicine, John? It must be a method that concentrates the resveratrol a lot, if you killed a patch with it? I can't imagine eating enough unconcentrated knotweed to kill the plant, although the young shoots do make a quite okay rhubarb substitute in pie or crumble...

Interesting, I had heard knotweed had medicinal qualities, but wasn't familiar with the specifics.

One of the reasons I like harvesting from public land is that I figure it's less likely the knotweed has been dosed with chemicals. The state isn't going to pay for expensive treatments on small patches of knotweed on the fringes, I imagine.

I'll probably grab a couple more trash cans next time I hit Walmart so that I can start making batches to increase volume of char created and volume of knotweed processed. Actually, there's a small patch of knotweed I'm aware of between here and Walmart, now that I think of it...

The resveratrol is concentrated in the root.  It's orange. You can eat the shoots, but you have to time it just right.  Since it is the main commercial source of resveratrol, I imagine that the companies get out there with serious equipment and have workers sift through the plants afterwards to get to the root.  I use a shovel.  I just chop up the orange root and pack it in a jar to extract the resveratrol in a glycerite (using glycerine). More people will use alcohol to extract it.  It extracts and preserves the medicine better than glycerine, but I have read that then the alcohol interferes with the healing you are trying to do in your cells. When you get to age 60, longevity becomes much more interesting.

John S
PDX OR