Like any other living creature, weeds compete for resources to foster growth and reproduction.
Weeds grow fast by having competitive genetics that allows rapid growth and reproduction. They are superior at outgrowing surrounding plants by adapting to a range of environmental conditions. Weeds are also dominant in reproducing by having very efficient seed-dispersing mechanisms. For these reasons, weeds can be found virtually everywhere and can cause a detrimental impact on the environment, if left uncontrolled.
Before we look at how weeds grow so quickly, we must first discuss what a “weed” is. Once we know what a weed is, we can look at a few examples of weeds and the mechanisms they have in common to foster growth and expansion.
What is a “weed”?
A “weed” is not a scientific term to identify a specific plant or a family of plants.
We often hear “weed” being used to describe a plant that has grown in an undesirable location. We often associate weeds with plants sprouting in inconvenient places, and we usually pull these out using our hands. They’re an eyesore to our gardens, and we spend time removing them one by one.
An example of a garden weed is the common Dandelions (Leontodon taraxacum) that propagate as wildflowers. These plants sprout bright yellow flowers that poke up in unexpected places in our back and front yards, driveways, and sidewalks.
Other weeds are more invasive and disruptive to local agriculture, such as noxious weeds. The government identified these as detrimental to natural habitats and ecosystems if not managed correctly.
An example of this is the Milk Thistle. These highly competitive and persistent plants rapidly invade abandoned fields, roadsides, and disturbed sites. One of these plants can produce about 6,000 seeds that can remain viable for nine years. You can imagine the difficult task of trying to control this weed!
What is and isn’t a weed is a matter of opinion. However, there are shared characteristics of commonly agreed weeds, such as:
- Plants that grow in undesirable locations
- Plants that grow very quickly
- Plants that have extensive root systems
- Plants that produce lots of seeds
- Plants able to live in extreme and poor environmental conditions
How and Why Do Weeds Grow So Fast?
We’ve all had that experience where we’ve spent time removing weeds from the garden, only find them sprouting again a few days later. What are the traits of weeds that allow them to grow, germinate and flower so quickly?
Factors contributing to weed growth
Some weeds grow extremely quickly because they have a short period of growth between germination and flowering. This means that they can complete seed production quickly soon after flowering. For example, the Canadian Thistle matures seed within two weeks of flowering.
Weeds grow from seedlings to adult plants quickly because they have a high photosynthetic rate to produce new leaves. In addition, they possess high light saturation intensity and a lower carbon dioxide compensation point. In other words, they’re able to take advantage of prolonged and intense levels of sunlight to convert into plant energy even with the presence of low carbon dioxide levels.
Competitive weeds develop extensive root systems and are tolerant to a range of soil conditions. Some weeds like the dandelion have “tap roots” where the primary root is very thick and dives deep into the soil, whereas others like the Japanese knotweed have “deep roots” where they extend more than a meter down the soil. Once these root systems are established, they are tough to remove.
In addition, competitive weeds are adaptable to a wide range of climatic and soil conditions. Their general purpose genetic structure allows them to grow well in any environment. Under adverse conditions, they’ll still be able to survive even if soil conditions are poor, acidic, waterlogged, and heavy.
The aggressive growth habits of weeds allow them to compete effectively with surrounding plants.
Take, for example, the Poison Ivy. The poison Ivy is a noxious weed in Minnesota that spreads rapidly by underground stems (rhizomes) and seeds. It grows tall quickly by twining on larger plants and trees. This suffocates surrounding plants and blocks sunlight for shorter plants. As competing plants die, the poison ivy flourishes in its environment by monopolizing resources.
Weeds grow and spread extremely quickly as they have no special environmental requirements for germination from seeds. They will grow in well-fertilized fields, parklands, forests, abandoned gardens, and under irregular rainfall.
Contagious weeds that spread quickly often have a long and continuous seed production period. For example, the Redroot pigweed can produce seeds when only 1 to 8 inches tall and even after it flowers.
Human practices have contributed to the speed of weed growth. Any kind of soil disturbance, such as plowing, clearing, excavating, mowing, and burning, allows weeds to prosper through increased light levels. Most crop plants and turf do not survive these practices of soil disturbance, but many weeds can.
For example, the Nightshade weeds are dependent on tillage, such as digging, stirring, and overturning, for establishment. As such, weeds are a problem in agricultural fields that grow crops because they compete for plant nutrients and water.
In addition, many weeds are resistant or grown to develop the tolerance of many chemical control methods.
How Do Weeds Spread
Weeds can be found everywhere because they have mechanisms to take advantage of their environment to spread their seeds effectively. How weed seeds are dispersed, include:
- By Air,
- By Animals,
- By Water, and
- By Human activity
Weeds are clever seed spreaders due to their structures and way of scattering their seeds around. Some weeds contain long stems holding clusters of flowers that make it easy for seeds to attach to the shoes of passing walkers.
For example, the Curlycup Gumweed contains bright yellow flower heads covered with sticky resin. The sticky achenes facilitate seed transportation.
Another example is the Tumbleweed that starts off as a plant and eventually breaks off. It forms into a ball and tumbles by the force of the wind, spreading its seeds along its path.
The Redroot Pigweed produces tiny, lightweight seeds dispersed to new locations by floating on floodwaters, irrigation, and tillage tools. The pigweed seeds have multiple dormancy mechanisms, which allows the seeds produced to germinate at different times of the year.
The seeds produced by most weeds are small and lightweight.
For example, the common dandelion that produces puffballs contains densely packed seeds. You would have picked these up as a child, blew them, and watched them float into the air. This is why dandelions are challenging to manage because their seeds are so buoyant and impossible to find until they germinate and emerge from the soil.
If weeds have bigger seeds that are more prone to the forces of gravity, it would be easier to determine the weed-infected area and therefore manage.
Apart from weed seeds that can be dispersed through the attachment on animal fur, the seeds can spread when consumed and excreted.
In one experiment (Beach, 1909), a Jersey cow was fed six pounds of flax seeds. The cow consumed the seeds, and a small amount of them found in the manure were still viable for germination, even after passing the digestive tract. Several other studies confirmed the successful passing of seeds through cattle consumption.
Viable weed seeds can also be retained for 8 to 12 hours in the digestive tract of birds. Birds are agents for weed seed distribution. Seeds of many weed species can remain intact and viable in the intestinal tract of birds long enough to be transported several thousand miles.
Weed seeds take advantage of the flow of water movements in rivers and lakes to spread themselves.
The Purple Loosestrife weeds thrive in moist and marshy areas, thus affecting streams and wetlands nationwide. These weeds form dense clumps which fill in wetlands and slow-moving water bodies.
The Purple Loosestrife weeds produce over 100,000 seeds per plant and are transported by water, waterfowl, and hiking boots. The roots survive even in the winter flooding and re-sprout in late spring when the water levels drop. Seeds are tiny, which makes them highly buoyant and transportable through rivers.
By Human activity
Some weeds have traveled internationally due to human activity such as imports and exports.
The Parthenium ragweed was first described in 1810, and it became a problem in the United States in the early 1960s when it was found in shipments of contaminated cereal grains. Today, the distribution of the Parthenium ragweed is found in 23 US states, Puerto Rico, the Virgin Islands, the District of Columbia, and in 23 of 67 counties in Florida. Outside of the States, it’s found in Australia, Bangladesh, Ethiopia, Taiwan, and Vietnam, just to name a few.
The spread of the Parthenium ragweed is a worldwide concern because it poses a serious threat to native fauna and flora and severe biodiversity reduction. For human health, it causes asthma, eye irritation, and hayfever.
Q: Can weeds grow into trees?
Yes, weeds can grow into trees. Certain species of weeds can grow large enough to give the appearance of a tree by having elongated stems and woody trunks. These types of weeds are often called “weed trees.”
A “weed” is not a scientific term, and this word is often used to describe a plant that is has grown in an undesirable location or purpose.
In botany, a tree can be defined as a perennial plant that has a trunk with supporting branches and leaves. This definition can be extended to include plants above a specified height such as tall bamboos or plants containing useable lumber.
So whether weeds or any undesirable plant can grow into the size of a tree or have an appearance of a tree, this all depends on the plant you’re looking at. Everyone has their own opinion on what makes an undesirable tree.
An example of a weed that grows into a tree is the Tree of Heaven (Ailanthus altissima).
Initially introduced in the United States in the 1700s, this weed was appreciated as a fast-growing ornamental shade tree. It is highly adaptable, withstanding a range of weather and soil conditions. It has rapid growth and grows into a large tree of up to 80 feet tall.
However, by the early 1900s, the tree became less appealing due to its root sprouting habits and foul odor. These weeds can now be found in cities and towns growing out of place, often on sidewalks and out of stone walls.
The Tree of Heaven is difficult to eradicate because it is spread by cloning and seeds dispersed via wind. The female trees have the potential to produce more than 300,000 seeds annually.
Another weed that grows into a tree-like figure is the Box-elder Maple (Acer negundo).
The Box-elder Maple is considered an environmental weed in parts of Australia due to the damage it causes to waterways by trapping sediment, which causes erosion. It also invades coastal vegetation, healthy woodlands, and forests. It causes blockages in guttering, gardens, footpaths, and driveways in urban areas.
The Box-elder Maple can grow up to 50 feet high and is a deciduous tree. It loses all of its leaves during autumn and winter, creating a blanket mess on urban grounds.
Fundamentals of Weed Science by Robert L. Zimdahl
Environmental Weeds in the Southern Highlands
Tree of Heaven
Weeds of Australia
Taylor Total Weed Control