I read about weed tea many years ago and I made some. It stunk so bad that I never made it again.
With all the interest in natural gardening it is making a comeback and the internet is full of gardening advice that says fertilizer tea made from weeds is great for plants. Let’s have a look at the science behind this gardening practice. Is it worth making weed tea fertilizer? What kind of nutrients does it really provide? Are there better ways to use your weeds?
Just to be clear – I am talking about the weeds you pull from the garden – not the kind you smoke!
What is Weed Tea Fertilizer?
Weed tea fertilizer is made by adding weeds to water and letting them ferment/decompose for a while. The resulting juice is the weed tea.
Here are some simple instructions for making it.
- Collect weeds and chop them up – smaller pieces decompose faster. You can also use other plant parts like deadheaded flowers and even grass clippings.
- Place the material in a bucket and fill it mostly with water.
- Some people cover it to keep out flies, but I think added fly eggs and larvae can only improve the quality of the tea. I doubt mosquitoes will lay their eggs in this stuff?
- Some people keep the pail in the shade, others in sun. The warmth of the sun should speed up the process and make a better tea.
- Wait two weeks. Stir when the urge triggers you to do so – this is a precise process!
- Remove the liquid and use it as fertilizer.
- Don’t store too long, it might rot out the bottom of the pail.
After only two weeks, most of the weeds will still be there; what do you do with the left over weeds? You can add them to the garden or compost pile.
WARNING: This stuff stinks!
Weed Tea vs Compost Tea
Weed tea and compost tea are prepared in similar ways, but they start with different raw materials; one uses manure or compost, the other uncomposted plant material. Some compost tea is made aerobically, with the addition of air.
My previous reviews of compost tea have shown that, in normal garden environments, the tea has no additional value over and above the compost. There is almost no scientific testing of weed tea, but I am sure the same can be said about it.
Both provide two things; nutrients and microbes.
Brewing a tea will not increase the total amount of nutrients available. It might speed up decomposition and therefore cause nutrients to be released faster from the organic material, but the total amount does not increase.
There is very little support for the benefits of the microbes in such teas. The reason is that garden soil is already saturated with microbes. Adding more does not increase or change the existing populations. There are some specific cases for disease control, but these are best left to agriculture and not gardens.
Using Weed Tea Fertilizer
Here is one problem with this method. Gardeners don’t know the NPK of their finished product. So, they don’t know how much to give to plants. Using this stuff is all a guess, but that does not prevent lots of self-proclaimed experts from telling you how much to use. Here is what some say.
Use it straight. Dilute it 50% with water. Dilute it 1:10 with water. You see my point; nobody really knows. All these “so-called experts” are just guessing.
You can use it in potted plants, containers, raised beds, or even as a foliar spray. Lots of online advice suggest foliar spraying is best because “leaves absorb nutrients faster than roots” making it more efficient. That is not really true, except for some micronutrients, and unless you are trying to solve a specific nutrient deficiency, adding it to soil is better.
Which Weed is Best?
Some people claim one weed is better than another. Some use grass because it has high nitrogen. Others swear by comfrey because it is a “dynamic accumulator” – it’s not.
I am sure some plants are better than others, but I have found not scientific evidence that one makes a significantly better tea than another. Since most of the nutrients are still in the plant and not in the tea, at the end of the process, I doubt it matters too much. Cutting up the plant, or mashing it up, probably has a bigger affect on the nutrient content.
Soft tissue will decompose faster than hard tissue, so soft leaves are better than hard rubbery ones. Green, fresh stems are better than older hard stems.
Use what you have and the more you use, the better the weed tea.
NPK of Weed Tea Fertilizer
You can find hundreds of online sites that tell you how to make the tea. None of the ones I looked at, told you the NPK of the tea. The NPK is the important part. Would you buy fertilizer without the NPK value on the container? Why make tea without knowing the NPK?
I only found one scientific study that made weed tea and tested the NPK. They tested water hyacinth (Eichhornia Crassipes), Russian comfrey (Symphytum officinale) and pig weed (Amaranthus retroflexus) plants. Each one produced about the same NPK value with the average being, 3-7-4.
Note that they incubated the weeds for 30 days, not just 2 weeks. You can expect to have lower values after 2 weeks.
A good nitrogen fertilizer level for plants is 150 ppm which is 0.015%. A 1:10 dilution of the above weed tea would give you a value of 0.3%.
The weed tea fertilizer contains a good level of nitrogen and would be a good way to fertilize potted plants and containers. You would need a significant amount of liquid to do a garden.
A good general NPK value for plants is a ratio of 3-1-2, so the phosphate level of weed tea is on the high side.
Best Way to Use Weeds
What is the best way to use weeds?
I see many people saying that they throw them in the garbage. That is just a waste. Even weeds that spread easily from runners can be dried a few days in the sun and added as a mulch on gardens without any problem, especially if placed on top of wood chips. This is the method I use – the cut and drop method.
Almost all weeds can be composted.
Making tea does NOT increase the amount of nutrients over and above these other methods for using weeds.
Making weed tea seems like a complicated process that takes extra time and space. Even when you have the tea, you still have to go around the garden and dispense it to plants. It might be a convenient way to fertilize containers.
Warning, don’t make it near the neighbor’s fence – did I mention – it stinks.