Bull Thistle Control: Managing Bull Thistle Plants In Gardens

Spear Thistle
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By Bonnie L. Grant

Bull thistle (Cirsium vulgare) is a plant that is related to the sunflower family but has none of the charm and beauty of those sunny-nodding flower heads. It is a prickly biennial that grows freely in disturbed soils, pastures, ditches, roadsides and unmanaged spaces. The plant has colonized much of North America and is a pest plant in the garden and in agriculture. Bull thistle control can be manual or chemical with an emphasis on seed control. Learn how to get rid of bull thistle and prevent this prolific weed from taking over your garden.

What is Bull Thistle?

Bull thistle plants are native to Western Asia, North America and parts of Europe. What is bull thistle? It is a free-seeding weed with a prickly demeanor and rapid spread. The plant has the ability to produce around 5,000 seeds in a season. These bur-like seeds cling to animals, pant legs, machinery, etc. and get spread around with abandon. For this reason, bull thistle removal is a priority among farmers and meticulous gardeners.

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Bull thistle starts life as a spiny leaved rosette. The hairy, prickly leaves overwinter to develop stems and branches of up to 2 feet in spring. It has a deep taproot, which makes manual pulling a challenge.

In summer the plant grows a scented flower that resembles a spiny globe topped with fringed pink petals. The flowers are produced at the ends of the tangled stem growth and last for several weeks before producing tiny striped seeds capped with white downy hairs. These attach themselves to any object that brushes against them.

How to Get Rid of Bull Thistle Manually

The stubborn plant can arise like Lazarus from the ashes if hand pulling leaves behind any of the root. Casual removal with this method is likely to leave behind the genesis of a plant in spite of the foliar amputation.

Digging the plant out with a spade or hori hori is the best approach to mechanical bull thistle control. Take care to remove the entire fleshy taproot for best results. In order to reduce the seed population, cut off the seed head and tuck it into a sack to keep the fluffy seeds from dispersing.

Other Types of Bull Thistle Removal

In agricultural situations, the introduction of a bull thistle seed head gall fly has been proposed as a biological agent. However, it has been shown to have limited effectiveness. There is also a weevil that is an effective control agent, but it can also affect desired thistle species.

Chemical treatment is most effective on the first year rosettes of bull thistle plants. The types of sprays used in agricultural scenarios are dicamba, glyphosate or 2,4D. Glyphosate is the easiest preparation for the home gardener and found in such products as Roundup.

For widespread control, mowing twice per year has been effective in reducing the population by preventing seed heads. Of course, your battle with the plant will only be as effective as your neighbors because of the travel ability of the downy seeds.

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