by Linda Woodrow

How to harvest weeds for their best nutrients

Sometimes gardening seems to me like alchemy. Organic material that is of no value to us is converted into organic material of high value, and, like alchemy, the process seems almost magic.

Soil micro-organisms and plants do the converting, but they can’t do it without something to convert. The role of humans is to set up the system, supply the raw materials, and harvest the product.

The first law of gardening is the law of conservation of matter

There are very many sources of organic matter, but the kinds I look for are rich in a wide range of nutrient elements, concentrated, easily collected, and easily converted. One source that beautifully satisfies all these requirements is weeds.

Weeds are grossly under-appreciated lifeforms. They are, by definition, plants that are not valued and proliferate with no human work at all. Since they are not valued, they are free. Since they proliferate, they are very concentrated sources of bulk organic matter. Since they are plants, they contain all the major and minor plant nutrients, in a good ratio.

I harvest a big range of weeds, wherever I find easily collected sources, but my favourite weeds are azolla, cress, and stinging nettle.

All-time Favourite – Azolla

Azolla (Azolla pinnata or Azolla filiculoides) is my all-time favourite. It is a floating perennial waterweed, geographically very widespread, native to Australia and to a number of other countries. The small, fernlike leaves are about as big as a thumbnail and look like snowflakes. It likes still or slow-moving water like dams, weirs or ponds, and can cover them in dense mats. It often proliferates in the runoff from other people’s overuse of bought fertilisers. In spring it is green but as the weather heats up it turns a distinctive deep red colour.

Azolla has two features that make it my pick of the weeds. Firstly, it is symbiotic with a nitrogen-fixing alga called Anabaena. This alga, like the bacteria that cohabit with legumes, is capable of taking gaseous nitrogen out of the air and fixing it in the form of a solid. As azolla decomposes it releases copious amounts of nitrogen into the soil.

The beauty of azolla is that the nitrogen is not in soluble form, as it is in many high nitrogen fertilisers. Nitrogen is one of the major plant nutrients, but soluble nitrogen fertilisers are a bit of a worry. Plants cannot avoid taking in soluble nitrogen if it is dissolved in their drinking water, and they can easily overindulge. They will put on lots of lush new growth, but the new cells are thin-walled and prone to attack by fungal diseases and sap-sucking insects. However, served up as azolla, plenty of this major nutrient is available but is not being force-fed to your plants.

Earthworms and soil organisms, including thermophilic compost bacteria, relish this sort of high nitrogen food, since it is the basis of proteins. Azolla performs stunningly as a compost activator or as food for breeding worms. A couple of buckets full of azolla will replace animal manure in compost. I throw mine to the chooks, one bag of azolla and two bags of grass mulch. They scratch through it thoroughly searching for the little crustaceans trapped on the azolla. At the end of the day I have some very good eggs and a take-away compost pile.

The second feature of azolla is that it is so prolific and easily collected. In a good situation, it will double its leaf mass every week. Out of one small farm dam I have been collecting fifteen bags full, over half a tonne, per week for the last few months, and you can’t see where I’ve been.

I’m a bit of a wimp about cold weather. It takes a very inviting situation to get me in for a swim, but a dam full of azolla is very inviting. With someone on the other end of a six metre length of net, I can collect those fifteen bags in about half an hour. Even if I paid myself wages for collection time, it would be the cheapest fertiliser around. It is like picking up wet $20 notes!

Phosphorous Award Goes to …Watercress!

My second favourite weed is the mustard family (Brassica spp.)

Watercress is the weed-like, but mustard cress, shepherd’s purse and wild turnip are also prolific, easily collected, and more valuable than appreciated. Watercress is a good source of all the major plant nutrients and most of the minor ones, but its chief value is as a source of phosphorus. Of the ‘big three’ – the triumvirate of major plant nutrients, nitrogen, potassium and phosphorus – phosphorus is the rarest and hardest to get. One of the things phosphorus is responsible for is the formation of flowers, fruit and seeds. Since most of the product of gardens is flowers, fruits and seeds, this is pretty important! Legumes have a high appetite for phosphorus, and since growing legumes is one of the main ways of fixing nitrogen, lack of phosphorus can set up a vicious circle of nutrient deficiency.

Phosphates in rocks are not all that uncommon, but in this form they are only very slowly made available to plants, and I mean the kind of ‘slowly’ that geologists talk about. However, once they are taken up into a plant and converted into an organic form they are much more easily available, and can be recycled over and over.

In summer and autumn, watercress can become so rampant in shallow watercourses that it is a pest. I can collect it nearly as fast as azolla, and without going swimming. Like azolla, it is usually full of little crustaceans that the chooks love, but unlike azolla, they eat them cress and all. I get eggs with strong shells, since cress is rich in calcium. I get very orange yolks, since it is rich in vitamin A. And I get chook manure that is very rich in phosphates, from which I get tomatoes to fry with my egg.

Getting a Nutrient Sting

My third favourite weed is stinging nettle (Urtica dioica).

Stinging nettle concentrates a big range of nutrient elements, some of them quite rare, in easily collected and converted forms. It too is a rich source of nitrogen and of the other major plant nutrient, potassium. However, its biggest value is as a good source of some of the minor nutrients.

It is good that a dilute solution of fermented stinging nettle makes a foliar and soil fertiliser that is just about a cure-all for any nutrient deficiency. Instead of getting out those cumbersome nutrient deficiency keys and trying to work out whether the older or the younger leaves are suffering most, or whether the yellowing is spreading from the margins in or the veins out, I spend the time picking a bin full of stinging nettle. I cover it with water, let it ferment for a couple of days, dilute the resulting brew one to ten, and spray and water the patients with it. If this doesn’t fix them I give up on them!

To prevent deficiency diseases showing up in the first place, I also collect stinging nettle for compost piles and, after letting it sit for a few days to lose its sting, as mulch.

Stinging nettle has two other virtues as a weed. It seeds so prolifically that you can just about guarantee that where there is stinging nettle, there is a lot of it. And the sting effectively keeps many animals and other people away, so you don’t have to share it! With a mower and catcher, and long pants, boots and gaiters, or just with long sleeves and gloves, it is very easy to collect.

I put in azolla, cress and stinging nettle, and they get turned into tomatoes and lettuces and beans and eggs.

Seems like alchemy to me.