There is not a lot of good that can be said about thistles.  They are, for the most part, unsightly and many livestock species would rather gnaw on an oak tree than try to digest a mature thistle plant.  Unfortunately, thistles can quickly become a primary weed pest in pastures, along roadsides, and in CRP land.  To launch an effective thistle control program, you must first get to know your enemy because not all thistles are created equal and several species are present in Fond du Lac County.  The two most common are the bull thistle and the Canada thistle.  They also provide a good contrast in life cycle and control measures. 

Bull thistle

Identification:

        The bull thistle has deeply cut leaves with spines that run perpendicular to the leaf surface.     The flower is flask-shaped.  Each plant originates from a single seed and rarely do you see bull thistle growing in large, dense patches.

Life cycle:

        The bull thistle is a biennial.  It requires two growing seasons to complete its life cycle from seed to flower.  Seeds germinate during the spring and summer when soil moisture and temperatures are favorable.  After germination, they form a leafy rosette ranging from 4 to 18 inches in diameter before becoming dormant in the late fall of the first year.  Exposure to cold winter temperatures (vernalization) is necessary to trigger bull thistle to flower during the second year. 

        In the spring of the second year, bull thistles resume vegetative growth.  They normally bolt (send up that huge flower stalk) and flower in late-June or July. Each plant can send up several stalks and produce many flower heads, each with many viable seeds.  After flowering or with the first frost, biennial thistles die in the second year.  Biennial thistles reproduce only by seed.  Musk and plumeless thistles are also biennial species but are less common in east central Wisconsin. 

Control:

        The key to control is to stop seed production.  This can be done mechanically with a hoe or by mowing before seed heads develop.  Chemical control is also an option.  The best time to spray bull thistle is in the fall of the first growing season when plants are still in the rosette stage.  At this time herbicide is actively moved to the crown and roots.  It’s likely that you’ll have first year plants where you’ve seen or controlled second year plants.  Growth regulator herbicides such as 2,4-D or dicamba alone or mixed together are very effective in controlling biennial thistles.  Once thistles bolt in the second year, herbicide effectiveness is reduced.

Canada thistle

Identification: 

        Canada thistle has leaves with crinkled, spiny edges. The flowers are smaller than bull thistle and are pink to purple in color.  Canada thistle grows in patches that spread rapidly because of underground rhizomes (roots).  It is the rhizomes that can quickly convert Canada thistle from a small problem to a large problem. To measure how many roots Canada thistle can produce, researchers in three states planted either a single root segment 12 inches long or a 6-inch diameter plug of Canada thistle plants in 4x4x8-ft above-ground boxes filled with soil. No tillage or irrigation were done, nor were crops planted in the boxes.  Within 12 to 16 months, buds on these roots produced an average of 174 shoots and 930 feet of new roots, illustrating this weed's ability to spread unless adequately managed.  

Life cycle:

        Canada thistle is the definition of perennial, reproducing by both seed and rhizomes.  

Control:

        Eliminating Canada thistle in pastures and non-tilled areas is more difficult than bull thistle.  Mowing just seems to get them mad and makes them come back even stronger.  The good news is that chemical control options are improving. Be sure to read label directions for livestock withdrawal times and rotational restrictions.  Canada thistle herbicide applications are most effective during the bud to early flowering stage of the plant.  Here are some options: 

Clopyralid (Stinger) – very effective but also the most expensive.  A mixture of clopyralid and 2,4-D is also available and marketed as ‘Curtail’.  It’s a more economical alternative. 

Aminopyralid (Milestone) – a relatively new herbicide that is very effective in controlling Canada thistle.  It’s cheaper than clopyralid. It is also available in a premix with 2,4-D and marketed as ‘Forefront’. 

Growth regulators – dicamba and/or 2,4-D are economical but less effective and multiple applications over a period of years are needed. 

Glyphosate – offers good control of Canada thistle but is non-selective for other species. 

        It should be noted that a natural organism, Pseudomonas syringae, may infect Canada thistle in non-disturbed sites like pastures and roadsides.  Once infected, the organism produces a toxin that inhibits chloroplast formation and severely weakens plants. Infected Canada thistle populations often diminish and on occasion completely disappear over a period of years.