May 10, 2015

Spotted Knapweed

By Lawanda Jungwirth

If you’ve been reading “Plant Matters” for any length of time, you’ve probably realized that the majority of invasive plants are very attractive and that an inordinate number of them are purple.  The same is true of this issue’s featured invasive, spotted knapweed.

At first glance, knapweed looks remarkably like the bachelor’s button flowers you remember from your grandma’s garden.  It’s also similar in appearance to Canada thistle, another purple invader, but knapweed has no prickles.

What the heck does “knapweed” mean anyway?  Well, the weed part is obvious, but “knap” comes from the word “knop” which means a small knob or similar rounded protuberance.   Looking at the photos, you might wonder why it’s called “spotted” knapweed.  Well, at the base of each flower head are stiff leaves called bracts that have fine streaks tipped with dark fringes.  They kind of look like a spotted pineapple.  So the whole flower looks like a purple knob stuck on top of a spotted pineapple.  It’s more attractive than it sounds.

So what’s the problem with knapweed?  It overtakes open forests, oak and pine barrens, meadows, prairies, old fields, grasslands, gravel pits, agricultural field edges and disturbed areas.  Knapweed outcompetes native plants, decreases the water-holding capacity of soil resulting in increased erosion and water runoff, and reduces the quality of wildlife habitat.  Wildlife biologists at the Wisconsin DNR comment that when knapweed moves into an area, the nesting cover for grassland birds and other wildlife soon vanishes.

Knapweed has several methods of attack.  Each plant produces between 500 and 4,000 seeds that are viable for up to nine years.  Thousands of baby knapweeds are pretty hard to fight, since each one produces a taproot like a dandelion, but much longer and stronger.  Most of the seed falls near the parent plant, but is easily moved further away by people, animals, farm equipment and water.  Each tiny seed has a notch at its base and little hairs protruding from the top which makes it easy for them to hitch a ride on fur and clothing.

Besides producing abundant seed, knapweed has another, sneakier, way of taking over its environment.  Its roots emit a toxin that acts as an herbicide, thereby eliminating competition from other plants for water, nutrients and sunlight.  With a clear path, all those seeds can germinate their little hearts out!

Knapweed is a biennial plant, meaning it has a two-year life cycle.  The first year, it forms a rosette close to the ground, like a lawn thistle, but without the pickers.  The second year, multiple branched, wiry stems up to four feet tall appear, and each holds many flowers that bloom in July and August.  The flowers set seed in autumn and then, having done all it can to ensure survival of the species, the plant dies.

People always want to know how an invasive plant got here.  The short answer for knapweed is the same as for many of our worst invasive plants:  by accident.  It came to North America in the late 1800s in contaminated alfalfa and clover seed in soil that was used for ship ballast.  It took 100 years for knapweed to spread to just 26 counties in the Pacific Northwest, but in the following 20 years, by 2000, it had spread to 45 states.  Until the 1980s, spotted knapweed was confined to ditches and field edges in Wisconsin, but since then it has rapidly invaded many of the state’s prairies, barrens, pastures, hayfields and grasslands.

Spotted knapweed is listed as “Restricted” in Wisconsin DNR’s invasive species rule NR40.  This means that it cannot be transferred, transported or introduced.  You are not required to remove it from your property and if it is already pervasive you’ll have a hard time doing so.  The best we can do is to keep our eyes open and protect areas we hold dear by spotting and eradicating new invasions as soon as possible.  To prevent spreading of seeds, avoid travel through knapweed-infested areas if possible, clean your shoes, clothing and backpacks after hiking through infested areas, and use certified weed-free hay. 

Small knapweed populations can be pulled by hand or with a weed popper when the ground is damp.  Make sure to get the entire crown and root.  Flowering plants should be securely bagged and set out for trash collection.

There are other methods of control including burning, mowing, grazing, chemical application and biological controls.  Timing is everything with each of these controls and the methods are too detailed to fit in this space.  Take a look at the following link and click on the Control tab and then the Spotted Knapweed Factsheet page for detailed instructions on each of the methods:

After using any of the control methods described in the link, it is important to reseed the area with native plants, or the knapweed will move right back in.  Knapweed is a pioneer; a hardy species that is the first to colonize previously disrupted or damaged ecosystems.  And any place knapweed has been is definitely disrupted and damaged!

CAUTION:  Knapweed causes skin irritation to some people.  Wear long sleeves and gloves while working around spotted knapweed, and long pants and socks when hiking through it. 

TRUTH OR NOT?  There are anecdotal reports that people who have pulled spotted knapweed with their bare hands have developed cancerous tumors on their hands.  Some doctors have confirmed this to be the case; others have strongly disputed it.  What isn’t disputed is that knapweed contains repin, a neurotoxic compound that damages the brains of horses that graze it.  However, it is not toxic to cattle, sheep or goats.