Creeping thistle (Circium arvense) is a particularly problematic weed on grassland as it can be difficult to eradicate once it is fully established. It can also affect other areas such as gardens and wasteland.
It is an aggressive weed that occurs on most soils but it grows more extensively on deep, well-aerated soils.
Creeping thistle is relatively indifferent to soil fertility but does grow better in richer soils. It can tolerate very low temperatures but is less successful in hot dry conditions.
This blog post will provide an in-depth guide on what Thistle are, how they grow, and how to eradicate them from your garden.
|Common names: creeping thistle, Canada thistle, Californian thistle|
Scientific name: Cirsium arvense
Flowering season: July to September
Habitat: grassland, uncultivated ground, trails, roads, vacant fields, farmland, wetlands, woodland, beds, borders, lawns, gravel paths or waste ground.
Creeping Thistle Identification
Creeping thistle can become a dangerous weed because it spreads by brittle lateral roots that quickly re-shoot if broken. You may want to restrict this plant in lawns and borders since these huge plants compete for light, water, and nourishment.
Thistles are easy to recognise by touch, however, they can be uncomfortable, so keep an eye out for the prickles. The spiny bracts that overlap right below their blooms are always present.
In comparison to their many relatives in the daisy family, this makes identification a straightforward task. The density of spine covering, as well as the size and shape of the leaves, varies with each thistle species. Soft prickles can even be seen on some.
The leaves of the creeping thistle have numerous spines, but the blooming stalks have very few spines or hairs. From July through September, its pinky-purple leaves bloom.
When the petals fade, fluffy hairs are left in their place, which is necessary for thistle seed dispersion, which is one of the main reasons creeping thistle spreads so swiftly.
What does Creeping Thistle look like?
Characteristics that make up the Thistle leaf, stem, root and flower are detailed below.
Creeping thistle is a perennial weed with soft green, heavily lobed, spear-like leaves that are covered with sharp barbs.
The flower is a purple pom-pom shape that will be produced in clusters at the top of the plant if allowed to flower. If the blossom is allowed to go to seed, it will turn white and fluffy, similar to the seed head of a dandelion.
Leaves: Leaves below the flower head are oblong to lanceolate with sharp spines along margins and on midrib veins on the underside of leaf surfaces. Leaves at the base of the plant are up to 6 inches wide. Leaves, stems and bracts at the flower head are covered with coarse white hairs giving them a greyish colour
Stem/Spikes: Flowering stems range from 30cm to 1m in height and are generally spineless and unwinged, although leaves are pinnately lobed and spiny. The perennial can reach up 5 feet tall.
Roots: Thistles produce horizontal white growing roots which are brittle but easily re-grow if they are snapped or broken, forming new shoots. As these roots begin to thicken root buds will be formed and grow into new shoots.
Unfortunately, cutting the grass, or aggravating them by digging the soil will only encourage new growth from these roots. Sometimes an individual plant will grow but they usually form a clump.
Flowers: Thistles can grow up to 1.5m tall and flower from July to September with dark small pinky-purple flowers.
Seeds: spread via fluffy wind-borne seeds and an extensive root system.
Thistles are pretty much self-fertile so will readily produce seeds and reproduce this way too. Seeds have rapid germination and higher temperatures during the summer months increase the germination rate. Thistles can take over a variety of areas including uncultivated soil and grassland including pastures.
The creeping thistle spreads quickly in lawns and borders, both through seed dispersal and via its lateral roots. It competes with other plants for water and nutrients.
On germination, it generates a tap root, which is followed by horizontally growing lateral roots. These fragile lateral roots create buds at regular intervals that mature into shoots. Spreading anywhere from 6 to 12m per year.
It regenerates quickly from broken pieces. Individual plants are dioecious (male or female) and nearly self-sterile, forming huge clumps. Male and female plants growing next to each other, on the other hand, will cross-pollinate and produce a seed crop.
Its seeds germinate quickly, especially during periods of variable temperatures or when daytime temperatures approach between 65 and 85 degrees F. (18-29 C.).
Creeping 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.
Creeping Thistle in Spring
In spring, creeping thistle plants emerge from an underground root system as a rosette of spiny basal leaves with purplish flower heads on long stalks (“stems”). In some cases, these flowering stems may be up to 12 inches tall where plants have not been mowed.
Steadily growing from new and existing stems, the flowers begin to form ahead of germination. Creeping thistles early growth is slow but once established, it will grow up to 6 inches per day during the growing season.
Creeping Thistle in Summer
As this season progresses, tiny yellow flowers will begin blooming from summer through to autumn. It grows in clumps roughly 30 cm high by 45 cm wide. In areas where there is no competition from taller plants, it can grow up to 60cm high.
After flowering, creeping thistle plants produce new basal rosettes of leaves that can reach 6 inches in diameter. These new leaves resemble the basal leaves except they lack spines on the mid-vein underside of leaf surfaces.
As these basal rosettes mature they develop sharp spines along margins and mid-vein underside of leaf surfaces.
During the summer is the best time to spray the weed. Do so in the evening to prevent the spray from evaporating and to give maximum time for the spray to be absorbed. In spring or if overnight dew is forecast, spray earlier in the day to allow the spray to dry before dew falls.
Creeping Thistle in Autumn
Creeping thistles are summer annuals that go dormant in autumn. However, when given ideal conditions this herbaceous perennial can live for up to 10 years and produce over 500,000 seeds per plant.
Coming towards the end of its flowering cycle, the seeds spread and germinate readily for the next season.
Creeping Thistle in Winter
In winter, the creeping thistle is a relatively short plant with dense white/grey hairs on the stems and leaves. The canes are green at this time of year as well. In cold winters, the leaves may die back to the ground only to reemerge in the spring.
The creeping thistle has a deep taproot which helps it survive periods of drought, heat and frost.
Flowers die away, but the foliage remains throughout the winter.
How to get rid of Creeping Thistle
The creeping thistle is difficult to remove by hand, as the lateral roots break easily and new plants will grow from any root segments left in the ground. Using weedkillers will give a quicker result, however, non-chemical control can work if done consistently over several seasons.
Method One – Herbicide Treatment
The best time to apply weedkillers to thistles is when they are growing vigorously, but have not yet flowered.
Apply herbicides to kill thistle, especially in spring and fall, before thistles can flower and seed. Use glyphosate for your garden, and use a broad-leaf herbicide containing 2,4-D or MCPP for your lawn.
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, Weedol Lawn 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
One application of weedkiller is unlikely to kill all the creeping thistles. You may need to spray once, allow the plants to die down, and then spray any regrowth again. Three or more applications a year, over a couple of years, maybe needed to completely kill it, depending on how extensive it is.
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.
Management of Creeping Thistle
To prevent thistles from growing in your garden in the first place, try the following:
- To prevent thistle seedlings from taking root, cover fresh beds with a thick layer of organic material (compost, for example). The loose nature of the mulch also makes it easy to pull up any small weed seeds.
- Regularly inspect the beds and remove any young thistle seedlings as soon as possible.
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.
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.