Thistle Control

Ag Biz News Column
Chad Gulley
County Extension Agent—Ag/NR
Smith County

 Thistles and Fall Control

             Texas is home to more than ten species of thistle.  Thistles are common in dry or moist soils throughout Texas.  They thrive in disturbed or overgrazed areas, in abandoned fields and along roadsides.  So what should I do to control thistles?

Bull thistle, Canadian thistle, plumeless thistle, and musk thistles are some of the common thistles found in East Texas.  Most thistles are considered a biennial plant.  This means the plant germinates from seed, sprouts up and produces a rosette and tap root the first year.  The second year the plant stem elongates produces flowers, produces seeds, and then this plant dies.  Control of thistles is best done in the rosette stage before stem elongation occurs.  The rosette stage is when the thistle is close to the ground and usually takes place in winter.  Stem elongation of the thistle usually occurs in spring.

Bull thistle, when mature, can reach 2 to 5 feet tall in height with many spreading branches with spiny wings from the leaves.  The flowers can be dark purple and somewhat clustered.  Bull thistle may be gray-green colored in the leaves and covered in spines.

Canadian thistle, when mature, can grow up to 4 feet tall.  The leaves vary from light to dark green and are oblong or lance-shaped, deeply cut.  At the leaf margins, these thistles are spiny-toothed with some hairs on the leaf as well.  The flowers are small bristly clusters and light lavender to deep rose purple in color.  Of the species mentioned, Canada thistle is considered a perennial plant.

Plumeless thistle is a branching plant with spiny wings that extend up to flowers.  The stems grow from 1 to 4 feet tall.   The flowers are purple to pink in color and the flowers are 1 to 2 inches in diameter.  The bracts are sparsely to densely hairy.

Musk thistles are freely branching plants that can grow to 7 feet tall.  The leaves are dark green with long, sharp spines.  Flowers are deep rose to purple in color and the flower can be three inches in diameter.

So how do we control thistles?  One option is to mechanically control these biennial plants.  In light infestations, a garden hoe or shovel can easily remove these from pastures or home lawns.  Mowing thistles is another option but timing is important to insure a good control.  Herbicides are another option.  While in the rosette stage, hand spraying or broadcast spraying can offer adequate control.  Left untreated, thistles can produce numerous seeds that can be blown in the wind making the population for next year even bigger.  In isolated areas, a hand-held sprayer, back-pack sprayer or one on the back of an all-terrain vehicle can offer adequate control while using low volume products.

Now is the time to scout pastures and begin looking for thistles growing in the low, wetter areas.  The number of thistles present will help determine which control option offers the most effective means for the economic return.   Ideally we recommend broadcast applications for plant densities of 125 plants per acre.  For plant densities less than 125 plants per acre, individual plant treatments may be a more economical option.  This will vary and determined by each manager as they decide control options for their operation.

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