Bull thistle is an invasive weed that can take over your lawn, garden, or field. It’s important to identify the weed correctly before removing it so you don’t do more harm than good.

Bull thistle is an invasive weed that can take over your lawn, garden, or field. It’s important to identify the weed correctly before removing it so you don’t do more harm than good. 

This blog post will provide an in-depth guide on what Bull Thistle are, how they grow, and how to eradicate them from your garden.

Quick Facts

Common names: bull thistle, bank thistle, bird thistle, spear thistle, common thistle, Scottish thistle
Scientific name: Cirsium vulgare
Family: Asteraceae
Origin: native
Flowering season: June to September
Habitat: grassland, uncultivated ground, trails, roads, vacant fields
The bull thistle with its spiky structure

Bull Thistle Identification

Bull thistle is a weed that’s easy to spot with its thick, spiny stems and purple flowers. It can be found in many types of landscapes, from dry fields to wet marshes, but it prefers moist soil and will typically grow in clusters or dense patches. 

Bull thistles can invade gardens by sending roots down into the ground where they find water sources.

They also spread quickly because their seeds are carried through wind or animals’ fur coats. If you have bull thistle growing on your property, there are several ways you may want to remove it depending on how much time you have available for maintenance.

What does Bull Thistle look like?

Characteristics that make up the Thistle leaf, stem, root and flower are detailed below.

The bull thistle is a plant that can be found in fields, pastures, and along roadsides. It has spiny leaves and white flowers with purple centres. The plants grow to be around five feet tall and have large taproots which make them difficult to pull up from the ground.

Bull thistles are native species and should not be removed as they provide natural weed control for other plants like wheat or barley.

Bull thistles are often confused with lamb’s-quarters because of their similar appearance; however, bull thistles have taller flowering stalks than lamb’s quarters which makes it easier to tell them apart.

The Bull thistle leaves with their spikes
The Bull thistle leaves with their spikes

Leaves: Cirsium may be biennials or perennials with spiny leaves and typical thistle-type flower-heads. Leaves are deeply lobed and hairy – there are coarse hairs on leaf tops, making leaf feel rough to the touch, and woolly hairs on the underside.

Leaf bases extend down onto stems and form spiny wings along the stems. The hairy, prickly leaves overwinter to develop stems and branches of up to 2 feet (61 cm.) in spring

The spiky stem of the bull thistle
The spiky stem of the bull thistle

Stem/Spikes: branching and erect biennial weed that grows between 2 and 6 feet tall.

Long, sharp spines on the leaves at the midrib and the tips of the lobes. Bull thistle starts life as a spiny leaved rosette.

Roots: It has a deep taproot, which makes manual pulling a challenge as they can snap off and regrow.

Bull thistle flowering
Bull thistle flowering

Flowers: Rosettes form in the first year and bloom singly or in clusters from midsummer through to early autumn in the second year and each year thereafter.

Flower heads are pink-magenta and on each stem. Flower heads are “gumdrop” shaped and spines extend all around the base of the flower heads.

Bull thistle about to flower and seed
Bull thistle about to flower and seed

Seeds: The plant has the ability to produce around 5,000 seeds in a season.

The Problem

The bull thistle has a two-year life cycle, with the second year flowering and seeding. Seeds have a short life on the surface of the soil, but they can live for many years when buried, as in agricultural activities. Fall and spring are the most common times for seed germination.

Basal rosettes form in the spring and continue to grow into the winter, reaching a diameter of up to 3 feet. If the rosettes aren’t big enough by spring, they won’t bloom until the following year. Flowering normally begins in mid-June and lasts until early September.

Plants can be self-pollinated or pollinated by insects. The bull thistle does not have rhizomes and does not reproduce vegetatively.

Bull Thistle Seasonal Changes

Through the seasons, thistle weeds look drastically different depending on their growth stage, location and habitat conditions.

At ground level, most species of thistles have spiny leaves with prickly edges that help protect them from herbivores looking for a meal. Whilst retaining foliage all year round they dominant native plants and begin to invade an area extensively.

Bull Thistle in Spring

The thistle weed, also known as the common or Scottish thistle, is a flower that blooms during the summer and autumn seasons. It has a green stem with rigid spines and leaves  The flower itself can be blackberry pink in colouration.

Steadily growing from new and existing stems, the flowers begin to form ahead of germination.

Bull Thistle in Summer

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 and causes more problems as they spread easily.

Bull Thistle in Autumn

Coming towards the end of its flowering cycle, the seeds spread and germinate readily for the next season.

Bull Thistle in Winter

The thistle weed goes mostly unnoticed in the winter after the flowers and seed heads have died off. The first sign that you should be aware of is that it has moved its location.

Flowers die away, but the foliage remains throughout the winter.

Bull thistle taking over an area with its aggressive spiky structure
Bull thistle taking over an area with its aggressive spiky structure

How to get rid of Thistle

If you hand pull leaves behind any of the roots, the tenacious plant can rise from the ashes like Lazarus. Despite the foliar amputation, casual removal with this method is likely to leave behind the genesis of a plant.

Method One – Herbicide Treatment

The best time to apply weedkillers to thistles is when they are growing vigorously, but have not yet flowered.

In rough grass areas, apply Vitax SBK Brushwood Killer, a selective herbicide based on triclopyr formulation.

When treating borders and unplanted areas, use a systemic weedkiller containing glyphosate (e.g. Roundup Fast Action, SBM Job done General Purpose Weedkiller or Doff Advanced Weedkiller; or for spot treatment use Roundup Gel) just before the flower heads show colour.

  • Spray the foliage thoroughly, which should turn yellow after about a week
  • Watch for any signs of regrowth the following year. You may find that two or three treatments are needed
  • Glyphosate is not selective and any nearby plants or grass coming into contact with the spray may be killed
  • Ensure you follow the directions on the packaging of weedkillers

For grassy areas, it is best to use a selective broadleaf herbicide to keep the competitive grasses intact. After spraying, wait two weeks or more to give the herbicide time to work.

Method Two – Digging it out

Digging up creeping thistles is problematic because the roots will easily regenerate from broken pieces. Persistence will be needed to achieve permanent removal.

Plants will rapidly regrow from minor fragments that have been overlooked, so clearing by digging may take two or three seasons.

Weaken creeping thistle by chopping off the top growth over a long period of time. Because much of the weed’s food stores will have been used up in flower creation, it’s better to do this shortly before the flower heads turn colour.

A shovel can be used to dig up bull thistle. When the plant has bolted, removing the top couple of inches of the root is usually enough to kill it (produced stems).

Cut the leaves off one side of the plant using a shovel or other tool to provide easier access to the roots, which may then be dug up. To prevent viable seed formation, flowering stems should be removed and killed.

Leaves clump to form a difficult and painful way to manually remove
Leaves clump to form a difficult and painful way to manually remove

Management of Bull Thistle

We recommend treating Thistles when they are actively growing in early spring/summer before they start to flower. Depending on weather factors thistles normally flower between July and September.

As they can reshoot from broken roots it is important that the thistle is killed right down to the root leaving little chance of regrowth.

Therefore, it is often difficult to use non-chemical control methods such as digging the plant out as any damage caused to roots by a spade can promote regeneration.

Because bull thistle reproduces primarily by seed, avoiding planting and taking care not to disseminate seeds is essential for preventing new infestations. Because contaminated hay is a common way for this species to spread, be sure to buy weed-free hay or keep an eye out for new plants in the locations where hay is stored or distributed.

Cut stems of flowering bull thistle should not be left on the ground since they are likely to produce viable seeds after being cut.

In Conclusion

Thistle is a tough plant but with this guide outlining all areas for attack, we hope you’re ready to fight back against this enemy before they get too far into your garden again!

Thistle is a very adaptable weed that grows in our gardens today and is one of the most resistant. However, using one of the products mentioned below will eliminate it.

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