Outforia Quicktake: Key Takeaways
- Weeds are plants that grow unintentionally and can cause nuisances or losses in revenue.
- Some plants considered weeds can be important medicinal or food plants elsewhere.
- Factors that make a plant a weed include vigorous growth, prolific seeding, dormancy, invasive spreading mechanisms, and being a pioneer species.
- Weeds can be controlled through biological, chemical, physical, mechanical methods or through permaculture.
- Chemical weed killers can have negative impacts on the environment and human health.
What some people consider a weed is often an edible or useful plant in another part of the world.
Plants known as “weeds” pop up everywhere, from allotments to verges and neatly mown lawns.
Some weeds, like Fat Hen (Chenopodium album) and Nettle, were used as food crops by Paleolithic peoples. Now, they are often cut down and sprayed with herbicides!
What Makes a Plant a Weed?
A plant is a weed if it is growing somewhere it hasn’t been intentionally planted. It is definitely a weed if it is causing a nuisance or loss of revenue in that place!
The same plant that is a weed in one place can be an important medicinal or food plant elsewhere! For instance, “Creeping Charlie” (Glechoma hedera) is an invasive weed that is edible. It was used in times past for clarifying ale!
Here’s some of the factors that make a plant a weed:
- Vigorous growth – plants that are more successful in a wet environment, for example, will overcrowd others that were intended to thrive.
- Prolific seeding – plants like Dock produce so much seed that they take over pieces of land.
- Dormancy – seeds can lie dormant for a long time in the soil. Once it is disturbed (i.e. by a gardener wanting to grow something else) they flourish.
- Invasive spreading mechanisms – reproducing efficiently and in many ways.
- Pioneer species – can grow in harsh and nutrient poor soils or cracks in tarmac!
World’s Most Interesting Weeds
You’ve got to hand it to these guys. They’re the gangsters and heavy muscle of the plant world. They’re strong, fast, and have enormous powers of endurance. They’re adaptable to change and reproduce like crazy!
Some have evolved amazing features to succeed where other plants fail. You’ve got to admire that, even as you rip them out of your window box or out of the foundation of your house!
Meet the world’s most invasive and successful weeds below!
1. Japanese Knotweed (Fallopia japonica) – Grows through concrete
Japanese Knotweed is the big daddy of the weed world! It spread across the world from Japan, Asia, and Korea when it was planted as an ornamental in European botanical gardens. It was introduced to the UK in 1825.
Japanese Knotweed is pretty. It looks like bamboo, with jointed stems, heart shaped leaves and strings of fuzzy white flowers. It starts coming up looking like red green bamboo shoots. You can find it in gardens, and on wasteland or embankments.
It spreads both vegetatively (by its extensive root system) and by seed. It likes to take over large swathes of wasteland and soil near waterways.
Japanese Knotweed has been known to grow through ⅔ in (5cm) of concrete or tarmac! You often need specialist help to remove it.
Check out this video of the damage caused by Japanese Knotweed.
2. Himalayan Balsam (Impatiens glandulifera) – Explosive seed pods!
Himalayan Balsam is another attractive import from the Himalayas.
It has blowsy pink flowers and a lovely scent. Bees love it! It’s actually a giant relative of “Bizzy Lizzies,” a popular bedding plant! It can be found in damp areas like riverbanks.
Himalayan Balsam spreads so successfully that it smothers native plants and wipes them out. It does this two ways.
It spreads rhizomes under the soil. Plus, its seed pods explode when ripe, flinging the seeds far and wide where they rapidly spread along river banks! Each plant can produce up to 800 seeds in its one year of life!
Check out the video below and see the seed explode in action.
3. Quack Grass (Elymus repens) – Chemical warfare
Quack grass is also known as “couch grass.” Quack grass has a folk reputation for being a medicine. The rhizomes have historically been taken as a tea for urinary tract infections. This grass is great for making children’s leaf whistles!
It’s an ashy blue-green color with large leaf blades. It has long, pale, straw-like rhizomes. You can find it in open grassy areas like lawns.
Quack grass is allelopathic. This means it releases chemicals that inhibit the growth of other plants near it! Now that’s playing dirty! It has no biological controls and can only be dealt with by digging out or using glyphosate.
4. Dandelion (Taraxacum officinale) – Paragliding seeds
Dandelion is one of the most iconic “weed” plants. However, It is well known as an edible and medicinal plant. It contains bitters, which help with digesting fats. It is also a diuretic, with the French nickname of “pissenlit.”
It’s a low growing plant that spreads long, toothed leaves from a central rosette. The flower is bright yellow with many petals. It grows in lawns and disturbed ground as well as in cracks in paving.
Its seeds spread from well known “fairy clocks.” These break up in the wind and are carried a long way by their silky parachutes. It also has a deep tap root that’s hard to remove.
5. Dock (Rumex sps) – Suspended animation
Dock is listed as one of the most “noxious weeds” by the United States Department of Agriculture (USDA). Dock root has traditionally been taken as a medicine for anemia. It is edible, but tastes disgustingly bitter!
Dock has a deep tap root which it uses to pull up subsurface nutrients such as minerals. It’s rich in iron and other minerals due to this!
Two common species are Broad-Leaved Dock (Rumex obtusifolius) and Curly-Leaved Dock (Rumex crispus). They have large cabbage-like leaves and deep red-brown seeds. You can find them in meadows and open grassland.
One dock plant can produce 60,000 seeds a year! They can lie dormant in meadows and fields until conditions are right to germinate. The seeds can still be viable 80 years later!
6. Ground Elder (Aegopodium podagraria) – Regenaration
Ground Elder is also known as “Gout Weed.” Gardeners hate this plant. This plant was introduced in many places by the Romans. They used it as a pot herb.
It is very nutritious and I find it quite tasty! It tastes like aniseed toothpaste. If you do try this, make sure no one has put weed killer on the ground elder.
It spreads vigorously from underground rhizomes and forms thick mats of bright green plants. Once it is mature it has white flowers that look like upside-down umbrellas.
Ground Elder’s superpower is that it can regenerate from just a tiny piece of rhizome left in the ground! Glyphosate herbicides are often used on it.
7. Burdock (Arcticum lappa/minor) – Velcro seeds
Burdock has a long history of medicinal and edible use. The root is edible and tastes like an aniseed parsnip. A blood-purifying tonic is made from the seeds by herbalists, and it is widely used in Chinese medicine.
The two species of burdock can be viewed the same way for most intents and purposes. Burdock is a large plant with huge wavy leaves that are silver underneath. It is biennial, which means it lives for two years.
Burdock seeds are one of the most irritating things known to mankind! They are covered with tiny hooks, like velcro.
They stick to the hair, fur, or clothing of anyone unlucky enough to brush against them. The plant does this so you will spread its seed. They are very itchy and hard to remove!
8. Cogon grass (Imperata cylindrical) – Stowaway weeds
Cogon grass is one of the most invasive weeds in the US. It was accidentally introduced to Louisiana in 1912. It comes from East Africa originally. It comes in packing containers after being used as packing for goods on ship voyages!
Cogon grass has long leaves and feathery white seedheads.
Cogon grass has spread across most of the Southeastern US. It is carried by tiny fragments of rhizome that stick to vehicles. The seeds are also spread by the wind.
It has been officially named one of the world’s 10 worst weeds! It’s causing major economic and ecological damage in US forests, cropland, rangeland, and natural ecosystems.
9. Palmer Amaranth (Amaranthus palmeri) – Invincible weeds
Palmer Amaranth has a history of being eaten by Native Americans. The leaves, seeds, and stem are said to be very nutritious. However, this plant is poisonous to livestock as it contains too many nitrites in the leaves.
Also known as “Pigweed,” this weed infests agricultural land. It can reach 6-8 ft (1.8-2.4 m). It has broad leaves.
Palmer Amaranth is the top problem weed in the US. This weed can quickly develop resistance to many of today’s herbicides! Many of the Palmer Amaranth plants are already resistant to glyphosate. Some are resistant to 2,4-D herbicide and dicamba at 18 times the normal dose!
Palmer Amaranth can cause crop yields to drop as much as 50% as it outcompetes crops for nutrients, water, and sun.
10. Stinging Nettles (Urtica dioica/Urtica urens) – Venomous needles
Nettles have a long history of use since prehistoric times. They are a traditional food of Romani gypsies. Accordingly, they are very nutritious and good to eat.
Once cooked, they lose their sting and become tender. They are very rich in iron, calcium, Vitamin C and many other minerals. The seeds are rich in fatty acids and protein.
Nettles grow in areas that are high in nitrogen. This means you will find them in farmland, allotments, and even ancient prehistoric burial sites!
One species of nettle is perennial and the other is annual. They both have stings full of formic acid! If you look under a microscope, you can see glassy needles that inject the acid when an animal or human brushes against it.
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What Can We Do to Control Weeds?
There’s a few different ways of controlling invasive weeds. There’s also advantages and disadvantages to each strategy! Sometimes, many types of control may need to be used with one plant to have any chance of success.
Biological Weed Control
This means finding a species that will eat weeds or otherwise hamper it and keep it under control. Sometimes the introduced control species will get out of control too, so this can be tricky to get right.
Biological control agents can be:
- Arthropods (insects and crustaceans)
- Bacteria and viruses
A great example of biological control is the introduced Cactoblastis cactorum moth. This moth helps control the population of Prickly Pear Cactus (Opuntia sps) in California. Another is the Common or Chinese Carp, which were introduced to eat water weeds that are clogging the waterways.
Chemical Weed Control
Weed killers fall in this category. Weed killers like glyphosate tend to work well for a while, but then many weeds develop resistance to the chemicals.
New chemicals need to be developed constantly to keep ahead of the weeds. This can be a risky option as chemicals can leach into the water table and cause pollution and illnesses.
Chemical weed killers have a few ways of dealing with weeds.
The ones below work by inhibiting amino acids in the body of the plant.
Then there are photosynthesis inhibitors – they stop weed plants from harvesting energy and making food.
Cell division inhibitors work by stopping root growth and cell division. They include
Last but not least, the synthetic auxin chemicals cause uncontrolled growth, which upsets the balance of the plant. You can see the effect of these in minutes after application!.
- 2,4-D Herbicide
Physical and Mechanical Weed Control
Manual weed control is often used in places like the rice paddies of SE Asia. These terraces are too difficult to reach for machinery and often too expensive. Check out how the rice paddies are farmed below!
Mechanical cultivation needs a lot of investment from the farmer and needs maintaining and updating. In today’s enormous American grain fields, enormous machines are the only viable option!
Check out the biggest farming machines in the world below!
This can include:
- Tine weeders
- Rotary hoes
- Flame weeders
- Weed zappers
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Problems with Chemical Weed Killers
There’s lots of problems with chemical weed killers.
- They can leach into the surrounding environment. They can pollute waterways and water tables and end up in drinking water.
- There is evidence that exposure to weed killers causes health problems, from dizziness to cardiac arrhythmia and coma! People can be affected by walking near chemical spraying, or by living near crops that are sprayed regularly.
- Weeds often become resistant to weed killers after a while. New chemicals then need to be found to replace them.
- Weed killers kill wanted plants too! They need to be carefully applied.
- Neonicotinoid weed killers kill bees,which are vital for pollination and plant health. These types of weed killer have been banned in many places.
Luckily, there are several pioneering new ways to control weeds, at least on a small to medium scale farm. Below is one of them.
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Masanobu Fukuoka and his “One Straw Revolution”
Japanese rice farmer Masanobu Fukuoka revolutionized farming and weed control when he experimented with permaculture in his fields in 1970.
People laughed at first, but they didn’t when they saw that his crop yield and health had improved and needed little maintenance!
Permaculture means permanent agriculture. It’s a way of farming where natural systems are mimicked.
One of the ideas is no dig farming. The soil is not dug up each time, which gives weeds a chance to grow. Instead, Masanobu allowed the dried stems of last year’s rice crop to remain as mulch and the new seeds were sown in the gaps. This protected the rice seeds and the soil.
He also let the irrigation channels become full of fish. He heaped the rich fish poop on his rice fields to fertilize them. His farm was a great success. Now permaculture is a global movement, especially in smaller farms and gardens.
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What’s the most troublesome weed in agriculture?
It depends on the country. In the US, the most troublesome weed is Palmer Amaranth in both corn and sorghum crops. Foxtail is the most prevalent weed overall. This is according to the 2021 WSSA Survey.
How much do weeds cost the global economy?
Weeds cost the economy roughly $40 billion in global losses. This includes reduced productivity and reduced access to resources, but also aesthetics and impact on human wellbeing!
Why do weeds become a problem?
Weeds reduce crop yield and can harm livestock if they are poisonous, like Ragwort. They outcompete crops for resources such as light, space, nutrients and water. Sme weeds are allelopathic, too. They produce chemicals which stop other plants growing!
How do weeds grow where plants are?
Weeds grow where there is available nutrients and bare soil. Bare soil is rare in a natural ecosystem. Most of the time the land is covered by some sort of plant already.
Only in modern agriculture is the topsoil continuously exposed to weed seeds on a large scale. Sometimes weeds will grow as they came in with the seed of the crop plant.
How are weeds classified?
Weeds can be classified by whether they are an annual (live or one year) or a perennial (live for a number of years). They can also be classified according to where they grow – wetlands, gardens, or dry lands.
They can also be classified by soil type, whether they are introduced or native, morphology, soil pH or cotyledon number!