There is a lot of information about making compost “teas” and spraying this on your plants. There is also information about using composted manure as a homeopathic remedy on your soils. Weeds, unlike most of our garden plants, contain anywhere from 200% to 500% the mineral content than other plants. Weeds are dynamic accumulators, unlocking minerals for use by other plants.
“A plant consists not only of mineral elements, or inorganic matter — these elements make up only 2-5% (in a few wild plants and weeds up to 10%) of its substances — but also of organic matter such as protein, carbohydrates, cellulose, starch, all of which derive from air (carbon dioxide, nitrogen, oxygen) and make up the major part of the plant mass aside from water, namely 15-20%. The greater part of the plant mass, some 70% or more, consists of water.” — Ehrenfried Pfeiffer, Introduction to Biodynamics, pg. 53.
As Bill Mollison, one of the co-founders of permaculture, liked to say, “The solution is the problem.” Weeds are doing good work, if you let them. Your problem (weeds) can be used to correct your soils if you put your mind to it. Simply collect your weeds and allow them to ferment in a barrel until the leaves have at least partially dissolved. If you would like an aerobic fermentation, add a small aquarium pump to the barrel and you will populate the entire mixture with aerobic bacteria. Strain this mixture carefully to avoid spreading seeds.
One particularly aggressive weed I had was Conyza canadensis or “Horseweed.” This weed is adapted to herbicides like glyphosate and can ruin field crops. But this destructive capacity means that Horseweed is exceptionally skilled at absorbing nutrients first, and doing so quickly. This year I will be composting weeds and fermenting them in vats in order to redistribute their wealth evenly across the field.
Weeds are connoisseurs of soils: weeds grow where nothing else can yet. After a few years, weeds accumulate enough nutrients in the topsoil so that hungry plants like grasses can crowd them out. Weeds thrive in very particular situations, but once they have grown, died, and exhausted the soils (for their purposes), they die. The minerals accumulated are now locked in organic compounds digestible by plants that otherwise would have no access to these trace elements.
Rainwater is not just water. It contains dust, dirt, minerals, bacteria, carbonic acid, and more. From what I’ve read, plants — especially weeds — absorb these trace elements through their leaves. In both conventional and alternative agriculture, foliar fertilization (feeding by spraying on leaves) is employed. This already happens with rainfall. Weeds, since they accumulate metals and minerals more readily than other plants, are more likely to help capture more trae elements otherwise inaccessible to plants.
There is some (reasonable) doubt surrounding foliar sprays; it is hyped and its results are often exaggerated. But Ehrenfried Pfeiffer, a biodynamic authority, in Soil Fertility, Renewal and Preservation indicates Spanish Moss is not a parasite, but in fact gathers all its sustenance from the air and rain. Though Pfeiffer does not address it, William Albrecht falsely calls Spanish Moss a “parasite” in his Soil Fertility and Animal Health. Spanish Moss is an epicyte, not a parasite. This indicates how some plants not only absorb nutrients from the air and rain, but find their entire subsistence from these sources. More research needs to be done to determine how much plants can absorb from rain and foliar sprays.
Making a weed tea is a good way to exploit the accumulation of minerals by these plants and redistribute it across your property. It doesn’t matter if the weed grew in only one place — it is my thought that the elements available in these plants make an excellent supplement to your property. Why? Weeds are pioneer plants. Weeds digest what nothing else can, and when they die they “unlock” elements for subsequent life. Spraying the essence of these weeds across the property unlocks additional compounds and supplies your plants with trace minerals otherwise out of reach. Making a “weed tea” invigorates your property. By spraying this tea, you are giving all of your plants some of the benefits they would have received if they had recently followed a set of pioneer weeds: the enzymes necessary to digest the weed are, not surprisingly, in the weed itself.
Warning: be careful not to distribute weed seeds! Use a fine mesh or cheese cloth when straining the concoction.
On the topic of weeds and weed teas, Weeds and What They Tell Us by Ehrenfried Pfeiffer and Weeds, Guardians of the Soil by Joseph Cocannoeur (the latter available free online here) are both accessible and useful.
Don’t buy the logic of foliar feeding? You don’t have to think foliar feeding is legitimate for a compost tea or weed tea to be a good idea for your soil. Good luck!