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A beneficial weed is any of various plants not generally considered domesticated, but which nonetheless has some companion plant effect, or else is edible or somehow beneficial. Beneficial weeds include a great many wildflowers, as well as other weeds that are commonly removed or poisoned.
Although erroneously assumed to compete with neighboring plants for food and moisture, some "weeds" provide the soil with nutrients, either directly or indirectly.
- For example, legumes, such as white clover, if they are colonized by the right bacteria (Rhizobium most often) add nitrogen to the soil through the process of nitrogen fixation, where said-bacteria has a symbiotic relationship with its hosts roots, "fixing" atmospheric nitrogen (combining it with oxygen or hydrogen) making the nitrogen plant-available (NH4 or NO3).
- Others use deep tap roots to bring up nutrients and moisture from beyond the range of normal plants so that the soil improves in quality over generations of that plant's presence.
- Weeds with strong, widespread roots also introduce organic matter to the earth in the form of those roots, turning hard, dense clay dirt into richer, more fertile soil.
- In fact, some common plants like tomatoes and corn will "piggyback" on nearby weeds, allowing their relatively weak root systems to go deeper than they could have alone.
Many weeds protect nearby plants from insect pests.
One way they can do this is to repel insects and other pests through their smell, as do alliums and wormwood. Another is to entirely mask a companion's scent, or the pheromones of pest insects, as with ground ivy and wild oregano.
Some also are unpleasant to small animals, because of their spines or other features, keeping them away from an area to be protected.
Some weeds act as trap crops, distracting pests away from valued plants. Insects seeking a food plant search by smell, and then land at random on anything green in the area of the scent. If they land on an edible "weed", they will stay there instead of going on to the intended victim. Sometimes, they actively prefer the trap crop.
Recent studies on host-plant finding have shown that flying pests are far less successful if their host-plants are surrounded by any other plant or even "decoy-plants" made of green plastic, cardboard, or any other green material.
- First, they seek plants by scent. Any “weed” that has a scent reduces the odds of them finding crop plants. Obvious examples are Crow Garlic and Ground Ivy, the former being “wild chives” and the latter a form of wild mint, both dramatically masking both plant scent and insect pheromones. They cut down Japanese beetle infestation, and caterpillar infestation, for example cabbage worm, tomato hornworm, and even squash bugs.
- Second, once an insect is near its target, it avoids landing on dirt, but lands on the nearest green thing. Bare earth gardening helps them home in perfectly on the victim crop. But if one is using “green mulch”, even grass or clover, the odds are that they will make what's called an “inappropriate landing” on some green thing they don’t want. They will then fly a short distance at random, and land on any other green thing. If they fail to accidentally hit the right kind of plant after several tries, they give up.
- If they are there to lay eggs on the crop, weeds provide one more line of defense: Even if they find the right plant, in order to ensure that they didn’t hit on a dying plant or falling leaf, they then make short leaf-to-leaf flights before laying eggs. They must land on the “right kind of leaf” enough times in sequence, before they will risk laying their eggs. The more other greenery is nearby, the harder it is for them to remain on target and get enough reinforcement. Enough “inappropriate landings”, and they give up, heading elsewhere.
One scientific study said that simply having clover growing nearby cut the odds of cabbage root flies hitting the right plant from 36% to 7%. Statistically, this is enough to dramatically change the productivity of one's crop, all alone.
Many plants can grow intercropped in the same space, because they exist on different levels in the same area, providing ground cover or working as a trellis for each other. This healthier style of horticulture is called forest gardening.
Among the obvious benefits are providing a wind break or shelter from noonday sun for more delicate plants.
Conversely, some intercropped provide a living mulch effect, used by inhibiting the growth of any weeds that are actually harmful, and creating a humid, cooler microclimate around nearby plants, stabilizing soil moisture more than they consume it for themselves.
Repel plants or fungi, through a chemical means known as allelopathy. Specific other plants can be bothered by a chemical emission through their roots or air, slowing their growth, preventing seed germination, or even killing them.
A common companion plant benefit from many weeds is to attract, or be inhabited by, beneficial insects or other organisms which benefit plants.
- Some beneficial weeds, such as lamb's quarters and purslane, are edible. This list of edible flowers includes many wildflowers that are considered weeds when not planted intentionally. Dandelion is an excellent example of an edible weed (see dandelion wine, dandelion coffee).
- A number of weeds have been proposed as natural alternate sources for latex (rubber), including goldenrod, from which the tires were made on the car famously given by Henry Ford, to Thomas Edison.
- Cocklebur and stinging nettle have been used for natural dyes.
- Studies have shown that milkweed is actually a more effective insulator than goose down.
Some plants seem to subtly change the flavor of other plants around them, in a way humans find desirable, for example stinging nettle, besides being edible if properly cooked, seems to increase essential oil production in nearby herbs.
- Clover is a legume. Like other beans, it hosts bacteria that fix nitrogen in the soil. Its vining nature covers the ground, sheltering more moisture than it consumes, providing a humid, cooler microclimate for surrounding plants as a "green mulch". It also is preferred by rodents over many garden crops, cutting down on the amount of desired vegetables that get eaten.
- Dandelions possess a deep, strong tap root that breaks up hard soil, benefiting weaker-rooted plants nearby, and draw up nutrients from deeper than shallower-rooted nearby plants can access. They will also excrete minerals and nitrogen through their roots.
- Crow garlic, the wild chives found in sunny parts of a North American yard, has all of the companion plant benefits of other alliums, including repelling japanese beetles, aphids, and rodents, and being believed to benefit the flavor of solanums like tomatoes and peppers. It can be used as a substitute for some alliums in cooking, if its slightly more bitter flavor is masked by sweetness or lipids.
- Bishop's lace (Queen Anne's Lace) works as a nurse plant for nearby crops like lettuce, shading them from overly intense sunlight and keeping more humidity in the air. It attracts predatory wasps and flies that eat vegetable pests. It has a scientifically tested beneficial effect on nearby tomato plants, and when it is young it has an edible root, both of these are true because the domesticated carrot is simply a cultivar of this "weed".
-  Horticulture Research International, Wellesbourne : “Insects can see clearly now the weeds have gone.” Finch, S. & Collier, R. H. (2003). Biologist, 50 (3), 132-135.
- Ussery, Harvey. "Plant an Edible Forest Garden". Motherearthnews.com. Retrieved 2012-06-13.
- "Journal of Chemical Ecology , Volume 9, Number 8". SpringerLink. Retrieved 2012-06-13.
- Burnett, Bruce. "Stinging Nettle: Companion Plant and Medicinal Herb". BCliving. Retrieved July 21, 2013.
- Anon. "Companion Planting for Vegetables & Plants". Country living and farm lifestyles. countryfarm-lifestyles.com. Retrieved 2011-03-07.
- "Scribd". Scribd. Retrieved 2012-06-13.
- Schoonhoven, L.M., J. J. A van Loon, and Marcel Dicke. 2005. Insect-plant biology. Oxford University Press, London.
- Cover Crops - Living Mulches