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Society for Ecological Restoration Ecological Society of America Delmarva Ornithological Society

Knocking Out Knapweed

Another July has come and gone, but not all of the fireworks are over.  The striking pink display of spotted knapweed, Centaurea stoebe ssp. micranthos, is still going strong in rangelands and old fields and along highways and railroad grades across the country.  Native to Europe, spotted knapweed was introduced accidentally to North America in the late 19th century, and has since become a problematic invasive weed, especially in the west.

The pinkish violet blossom of spotted knapweed.

The pinkish-violet blossom of spotted knapweed.

I first noticed knapweed blooming on our wildlife sanctuary property in Pennsylvania about five years ago, at the same time that I started to seriously consider the possibilities for habitat management on the site.  Knapweed control was one of my first projects on the property.  Since I knew that mowing was counter-productive (causing the plants to flower at a lower height), I decided to pull the knapweed by hand.  I’ve repeated the process for four years, and have eliminated spread of the plant while steadily reducing the seed bank at the main introduction site.  Because spotted knapweed seedlings assume a low-growing rosette form during their first year and don’t bolt and bloom until their second year or later, I’ve been able to track my success by noting a significant reduction in rosettes (a few dozen this year compared to countless hundreds during the first year of two of treatment).

Unfortunately, Spotted Knapweed is a highly favored bee plant, used heavily by native bumblebee species.  It’s always emotionally taxing for me to pull, bag and dispose of the knapweed with bumblebees crawling all over the blossoms as I work.  This brings me to one of my take-home messages in invasive species control and restoration: in degraded habitats, native species often rely heavily on invasive plants because the diversity of native plants has been reduced.  It is important to know what native insect and wildlife species rely on the invasive plant you are removing.  Then, when planning a restoration, try to plant (or enhance habitat for) native plants that provide resources at the same time of year as the invasive.  In my case, I’m working on restoring some meadows that will hopefully make up for (from a bee perspective) the eventual disappearance of my knapweed stand.

Spotted Knapweed is highly favored by bumblebees

Spotted knapweed is an invasive plant, but is used extensively by native bumblebees

Since I’ve been working on this population for several years with a good deal of success, I thought I’d share a brief description of my spotted knapweed manual removal method.

Here are a few important points:

  1. Get a good digging tool – I have tried various things – heavy clawhammers to grab and yank roots, various hand tiller/weeder/fork type things, etc.  I’ve recently settled on a well-balanced, heavy-duty, and inexpensive digging tool  (see the photo below) that I picked up at the friendly neighborhood home improvement warehouse.
  2. Pull plants during their bloom period and before they’ve gone to seed.  Be sure to bag and dispose of the plants in a landfill, to avoid dispersing seed or root fragments.  A good rain just before (or even during) pulling makes the job much easier.
  3. Be patient when pulling and learn to stop and dig more when you feel that the root is about to snap.  Pieces of root left in the ground will sprout the following year.
  4. Don’t dig right at the base of the plant – you risk cutting through the root, which often twists to one side or another a few inches down.  Instead, loosen the soil about 5-6 inches from the base of the plant.

Here’s the process illustrated:

This is my tool of choice: a beveled, pointed blade on one end (used the most) and a straight blade on the other.  The pointed blade sinks well into the shale soils where my infestation occurs.

This is my tool of choice: a beveled, pointed blade on one end (used the most) and a straight blade on the other. The pointed blade sinks well into the shale soils where my infestation occurs.

Many stems usually radiate from a single root crown of older Spotted Knapweed plants.  It's important to gather all the stems up to get a good grip on the plant for pulling.

Many stems usually radiate from a single root crown of older Spotted Knapweed plants. It's important to gather all the stems up to get a good grip on the plant for pulling.

Strike the ground about 5-6" from the base of the plant and loosen the soil, prying up with the tool while pulling straight up on the plant.  Pulling at an angle makes it more likely that you'll break the root.

Strike the ground about 5-6" from the base of the plant and loosen the soil, prying up with the tool while pulling straight up on the plant. Pulling at an angle makes it more likely that you'll break the root.

The root system should pull up cleany if you loosen the soil enough and slowly pull upward on the plant.

The root system should pull up cleanly if you loosen the soil enough and slowly pull upward on the plant.

The root crown can be quite broad on robust plants, or much smaller with fewer fibrous roots on stressed plants.

The root crown can be quite broad on robust plants, or much smaller with fewer fibrous roots on stressed plants.

Please let me know if you have had success with other spotted knapweed control methods.  I have considered trying Milestone, a selective herbicide that is supposedly quite effective on both knapweed and thistles.  So far, I haven’t needed to resort to spraying though, since manual removal has been working well and at this point only takes me about a day each year to complete.  Now if only I could eliminate my crown vetch, smooth brome, and reed-canary grass this easily!

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4 comments to Knocking Out Knapweed

  • Matt,
    I am working on several knapweed related projects in the New England region.
    We just introduced 2 weevils as biocontrol agents (knapweed root weevil and knapweed flower head/seed weevil) on one site which has an infestation of 500-acres. Scary! We are also using Milestone herbicide (w/Garlon 3A & Escort) at this site and several others with good first year success. I’ll be happy to share monitoring results with you as I go.
    For really small patches we hand pull with various garden implements, but I will try your suggested tool. Thanks for that!

    Chris Polatin

    • Chris – thanks for the feedback. I am familiar with biocontrol weevils you refer to, because I used to work with them when I was at Cornell. I had an undergraduate position in the lab of Dr. Bernd Blossey, and fondly recall having to make a number of forays to local knapweed infestations to collect food for the root weevils we were raising in the lab. I’ll be anxious to hear about your longer-term results with Milestone. I am thinking of trying it for canada thistle, so may test it on the knapweed while I’m at it.

  • Matt,

    Just a follow-up on the knapweed thread from 2009. The straight Milestone at 7 ounces per 100 gallon rate boom sprayed within a 20-acre densely infested area had tremendous results— we have to look quite hard to find a knapweed plant last week. Grasses are intact. Based on this result and several others I’ve witnessed, I think Milestone has great potential.

    Chris

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