Knotweed is a noxious, invasive plant often found on the side of roadways, riverbanks, and sidewalks.
These weeds, often resembling bamboo, have extensive roots and tend to outcompete other plants.
This plant is all over the world, on every continent except Antarctica. It is very easily spread, making it nearly impossible to eliminate.
Keep reading to learn all about knotweed, including the different varieties, how to identify them, where they came from, and the best ways of getting rid of them.
Knotweed is native to Asian countries, but it was imported into North America in the 1800s as ornamental plants. It quickly became an invasive species, taking over gardens, sidewalks, the side of roadways, and riverbanks.
It was often used as an ornamental plant because of its lovely flowers and as livestock forage, but it quickly got out of hand because it didn’t have any natural predators to keep it at bay.
These plants are known to be so resilient that they even begin to grow in areas that have experienced lava flow from a volcano.
Knotweed is often confused with bamboo because there are many similarities, but it is actually from the buckwheat family.
Each type looks a little different. There are four main types of knotweed, with three being very hard to distinguish from one another and sometimes even from other plants.
Giant knotweed is the biggest of each species we will talk about, with the stems of this plant growing up to seventeen feet tall but generally around six to sixteen feet long.
The leaves have a heart-shaped base that tapers at the end, with fine little hairs on the underside.
The flowers are off-white and grow in clusters near the ends of the stem.
Since its introduction into North America, it has escaped attempts to eradicate it and is considered a seriously invasive species. It drains nutrients from every plant nearby, and it grows nearly everywhere in the world because it is such an impossible weed to eradicate completely.
It is often found in sunny areas near rivers and other bodies of water, pushing up through sidewalk cracks and along the sides of roadways. It has been planted for erosion control and as food for livestock for centuries.
This weed is rampant because it spreads very easily, usually from particles that have been blown from the plant and landed in waterways or other plant parts that have been chopped up in the soil and relocated by accident.
Allelopathic chemicals from the roots of giant knotweed prevent other vegetation from thriving because it drains the nutrients from the soil nearby.
This variety is a hybrid cross between giant knotweed and Japanese knotweed. You can find it growing along the side of riparian areas, roadways, and grasslands.
It is a shrubby plant that grows up to ten feet tall, with stout but hollow stems that are reddish brown in color. The leaves can be shaped as a spade or heart-shaped, which is a mixture of its parentage, and the underside of the leaves will have broad hairs all over.
In late summer, it produces clusters of white flowers.
This species originated in Japan and was introduced to North America as an ornamental plant. It has since become a nuisance that is very difficult to eradicate.
It can cause damage to home gardens and infrastructure by growing through small cracks in foundations or draining the nutrients from the soil in your garden.
These weeds grow incredibly fast, and even small pieces of the plant can regenerate. It can be spread easily by waterways or by being blown from a tree and landing at a new site.
Japanese knotweed is one of the more well-known varieties of weed. It can be found growing in riparian areas as well as roadways and other populated areas.
This weed proliferates in dense strands that reach up to eight feet tall in just one season. The stems are often compared to bamboo because they are hollow and segmented. It has thick leaves that are about six inches long, and they are shaped like a spade.
A big difference between this variety and other varieties is that the Japanese knotweed plants’ leaves have no hairs. Instead, these leaves have small bumps on the veins. The plant produces long-branched clusters of white flowers during the summer.
This species is native to Asian countries – primarily Japan, China, Korea, and Taiwan. It is very invasive and can now be found all around the world.
This pesky plant can burst through the cement on sidewalks, pavement, and even the foundation of buildings. It can also block out the vital sunlight other plants need, effectively and quickly wiping them out.
The weed grows so fast and large that it can drain the nutrients from the soil, starving other plants of necessary vitamins and minerals.
Himalayan knotweed is unique in its appearance and shares the least amount of similarities with other types of knotweed.
The reddish-brown stems are smooth with twigs that zigzag from node to node.
It can be recognized by its leaves, which are long and slender. The leaves on this species are also large and leathery, growing up to twelve inches in length.
It can also be identified by its pink flowers that grow in clusters at the tips of the branches.
These plants are extremely invasive because they grow into dense bushes that isolate vital nutrients from other vegetation and alter the natural ecosystem. They can grow in nearly any conditions.
Knotweed grows primarily in riparian areas, on the sides of roadways, and in grasslands. It is also found in gardens, pushing through the cement of sidewalks and infrastructure foundations.
It can survive extremely wet conditions very well and can adapt to survive drought temperatures easily, so it is found all over the world.
It is difficult to remove from landscapes because it is spread easily through small pieces of the plant or seeds that have blown from the weed.
These plants thrive in full sun but can adapt to shady areas. The plant won’t grow to its maximum height and width in shady areas but will still grow relentlessly.
How It Spreads
These weeds can be spread vegetatively from fragments of rhizomes (or roots) or by seeds. Each node of the weed can produce roots and a new plant. This is the most common dispersion method for knotweed and is responsible for its rampant invasion.
The seeds are easily blown from the flowers, dispersing the invasive species through the air. The roots branch out and grow deep into the soil, making it impossible to remove every piece so it continues to thrive.
Getting Rid of Knotweed
It is extremely difficult to completely eradicate the plant because of its ability to regenerate from fragments.
You can chop it down, but unless you remove all the roots and the surrounding soil to ensure that no pieces are left over, it will grow back.
Getting every piece out of the soil would be a challenge for even the most experienced gardeners. It is important to contact a professional for removal.
Wrapping Up the Relentless Knotweed
Knotweed is an invasive species that you definitely don’t want to find in your backyard garden! The plant’s flowers can be appealing, but they aren’t worth the trouble they cause to surrounding ecosystems.
If you would like to learn more about invasive weeds and how to get rid of them, check out the Common Weeds page on our website for more blog posts and guides.