How to Kill Thistles

Thistles are one of the UK’s most persistent common garden weeds. Not because they’re particularly tough above ground – a mower handles the tops easily enough – but because of what’s happening beneath the surface.

Creeping thistle roots can extend 5 metres horizontally and penetrate 2 metres deep. Colonies expand by 6 to 12 metres every single year. And here’s the problem: those roots are brittle. They snap easily. Every fragment longer than 5cm that you leave behind will regenerate into a new plant.

This is why digging up thistles often makes the problem worse. You’re not removing the infestation – you’re spreading it.

Why Thistles Are So Hard to Kill

Understanding how thistles spread explains why most control attempts fail.

Creeping thistle (Cirsium arvense) – the most common species in UK gardens – doesn’t rely heavily on seed to spread. It spreads through its root system. Underground stems called rhizomes push outward in all directions, sending up new shoots at intervals. What looks like a dozen separate plants is often a single organism connected beneath the soil.

These roots store enormous energy reserves. Cut the plant down and it draws on those reserves to regrow. Dig it up and leave fragments behind, and each piece becomes a new plant. The roots can even lie dormant in undisturbed soil for years, waiting for the right moment to emerge.

Seeds are a secondary concern, but still significant. A single thistle plant can produce over 5,000 seeds, and those seeds remain viable in soil for 10 to 21 years. This is why preventing flowering matters almost as much as killing existing plants.

Identifying Your Thistles

The UK has several thistle species, but two dominate gardens:

Creeping thistle has wavy, spiny leaves with a hairless upper surface. The stems are smooth and spineless. Flowers are lilac-pink, appearing in clusters from July to September. This is the problematic one – the spreader with the aggressive root system.

Spear thistle has rigid bristles on the upper leaf surface and spiny wings running down the stem. Flowers are larger and usually solitary. It spreads only by seed and has a deep taproot rather than spreading roots. Still a nuisance, but more manageable.

Both are classified as “injurious weeds” under the Weeds Act 1959, alongside ragwort. You can be legally required to control them if they’re spreading onto neighbouring land.

The Wrong Approach

Many gardeners reach for a spade when they see thistles. This feels productive but usually backfires.

Those brittle roots break with every dig. You might remove the visible plant and even pull up what seems like a satisfying amount of root. But somewhere in the soil, fragments remain. Within weeks, new shoots appear – often more than you started with.

Mowing helps prevent seed spread but does nothing to the roots. The plant simply regrows from below. Regular mowing might weaken an infestation over several years, but it won’t eliminate it.

Vinegar, salt, and boiling water burn the foliage but don’t reach the roots. The thistle looks dead for a week or two, then returns. These methods work on seedlings but are useless against established plants with deep root reserves.

The Right Approach

Killing thistles permanently requires getting herbicide into the root system. This means using a systemic weed killer that the plant absorbs through its leaves and transports downward.

Glyphosate-based products are most effective. The plant takes up the chemical through its foliage, carries it through the stem, and distributes it throughout the root network. Everything connected dies – including those underground runners you’d never find with a spade.

For thistles in rough ground or pasture, triclopyr-based weedkillers offer an alternative that’s particularly effective on woody and deep-rooted weeds.

Timing Matters

The best window is early spring when thistles reach about 15cm tall but before they flower. Plants are actively growing and moving nutrients upward, carrying the herbicide with them.

The second-best window is late summer into autumn. After flowering, thistles pull energy downward into the roots to prepare for winter. Herbicide applied now rides that same current deep into the root system.

Avoid spraying during drought or when plants are stressed – uptake is reduced and results suffer.

Application

Spray thoroughly, covering all leaf surfaces until wet but not dripping. Don’t cut or mow before spraying – you want maximum leaf area for absorption.

Leave the plants undisturbed for at least two weeks after treatment. Visible dieback takes time, and the chemical needs that time to work through the entire root network.

Large infestations may need a second treatment 6-8 weeks later to catch any regrowth from surviving root fragments.

When Manual Methods Work

Hand removal can work – but only on seedlings. Thistle seeds germinate readily, and young plants in their first few weeks haven’t yet developed the spreading root system that makes them so difficult.

If you spot a tiny thistle rosette, dig it out immediately. Go 10-15cm deep and remove the entire taproot. At this stage, before lateral roots develop, complete removal is possible.

Once a thistle has been growing for a season, manual removal becomes counterproductive. If you want to avoid chemicals entirely, repeated cutting over several years can eventually exhaust the roots – but this requires cutting every few weeks throughout the growing season, year after year.

Preventing Return

After clearing thistles, prevention matters. Those seeds in the soil bank will germinate for decades.

Maintain thick, healthy grass. Thistles struggle to establish in dense turf. Fill bare patches promptly and keep lawns well-fed.

Mulch garden beds thickly. A 10cm layer of organic mulch blocks light and makes it easy to spot and remove any seedlings that emerge.

Monitor regularly. Catch seedlings early when removal is still effective. Once you see those first spiny leaves, act immediately – before roots start spreading.

Common Questions About Thistles

How deep do thistle roots go?

Creeping thistle roots can penetrate up to 2 metres deep, though most of the regenerative root mass sits within the top 30cm. The horizontal spread is more problematic – roots can extend 5 metres or more in a single season, with colonies expanding 6-12 metres per year.

Will pulling thistles out get rid of them?

Rarely. Thistle roots are brittle and break easily during removal. Any fragment longer than 5cm left in the soil will regenerate into a new plant. Digging actually makes infestations worse by scattering root pieces throughout the soil.

When is the best time to kill thistles?

Early spring when plants reach about 15cm tall, before they flower. The second-best window is late summer into autumn when nutrients flow downward into the roots – any herbicide applied then travels with them.

Are thistles illegal in the UK?

Creeping thistle is one of five “injurious weeds” listed under the Weeds Act 1959. While you won’t be prosecuted for having them, you can be served with an enforcement order requiring you to control them if they’re spreading onto neighbouring land.

How long do thistle seeds survive in soil?

Thistle seeds can remain viable in soil for 10-21 years. A single plant can produce over 5,000 seeds, which is why preventing flowering is almost as important as killing existing plants.

Thistles share their persistence with several other deep-rooted weeds. If you’re dealing with multiple problem plants, you might also find our guides to killing docks, removing nettles, and controlling bracken helpful – all are persistent weeds that respond to similar treatment approaches.

For thistles that have truly taken hold, a systemic weed killer applied at the right time remains the most reliable solution. Get the timing right, spray thoroughly, and those underground colonies finally stop spreading.