What will kill thistles but not grass?

Thistles are a common weed that can quickly take over lawns and gardens. They have deep taproots and sharp thorns that make them difficult to remove by hand. Many homeowners want to get rid of thistles in their yard but don’t want to harm their existing grass in the process. The good news is that there are several methods and products that can selectively kill thistles without damaging nearby grass when applied properly.

Why Get Rid of Thistles?

There are a few reasons why killing thistles without harming lawn grass is desirable:

• Thistles are aggressive weeds that can take over open areas.
• Their sharp thorns can be hazardous to people and pets.
• They looks unsightly and ruin the aesthetics of a nicely manicured lawn.
• They can reduce the health and density of desired lawn grasses.
• Some thistle species are noxious weeds regulated by law.

Allowing a thistle infestation to get out of control requires more intensive removal methods that may damage the lawn. That’s why it’s best to act early while the thistles are still small and manageable.

Why Not Harm the Lawn Grass?

There are several reasons to avoid harming existing lawn grasses when trying to remove thistles:

• You want to maintain a healthy, attractive lawn.
• It requires a lot of time and effort to reseed or replace dead grass areas.
• Some lawn grass varieties are expensive to purchase and re-establish.
• Killing the grass allows more weeds to invade the open areas.
• Many people want to avoid using broadcast weed killers for environmental and health reasons.

Selectively removing thistles by sparing nearby grass takes more precision but allows you to get rid of the weeds without destroying the lawn in the process.

Methods for Killing Thistles in Lawns

There are several effective options for killing thistles while leaving lawn grasses unharmed. Proper application technique and timing are key for these targeted control methods.

Spot Spraying Herbicides

Spot spraying weed killers directly onto thistles is one of the best ways to maximize control while minimizing off-target damage. The herbicide can be precisely applied to the weeds without contacting the surrounding grass. This requires an appropriate herbicide product paired with targeted application equipment.

Some key tips for effective spot spraying include:

• Use a selective broadleaf weed killer containing active ingredients like 2,4-D, MCPP, dicamba, or triclopyr.
• Spray when thistles are actively growing and small for best results.
• Completely cover the foliage and stems but avoid contact with desired grass blades.
• Use a pointed spray nozzle to produce a narrow, focused stream.
• Add a dye to the tank mix so you can see which plants have been treated.
• Repeat applications may be needed for full thistle control.

This precision weed control method requires more time and effort than broadcast spraying but allows desirable grass to remain unharmed.

Applying Natural Herbicides

Vinegar or clove oil based weed control products can provide effective thistle control when applied directly to the plants. These natural herbicides work by burning down foliage and stems on contact.

Tips for using natural herbicides on thistles include:

• Spray thistle plants on a dry, sunny day for best results.
• Thoroughly wet all foliage but avoid runoff onto grass.
• Use full strength vinegar for tougher weeds.
• Repeat applications will likely be needed as thistles regrow.
• Test in a small area first to check for lawn grass sensitivity.
• These work best on younger, smaller thistles that have less wax on their leaves.

Natural herbicides break down quickly without residual soil activity so they won’t prevent new thistles from germinating. But they provide a non-toxic control option when applied carefully.

Manual Removal

Digging or hand pulling thistles is effective on small infestations and scattered plants. Use a shovel, fork, or weed pulling tool to pry up the entire root system and remove the plant. This prevents regrowth from any remnants left behind.

Some tips for manual thistle removal include:

• Water area first to loosen soil and make digging easier.
• Use a shovel to cut roots 6+ inches down around plant.
• Leverage tool against base and slowly rock to loosen roots.
• Seal root pieces in plastic bags to prevent re-sprouting.
• Fill holes with new soil and reseed to prevent exposing bare areas.

Manual methods are labor intensive but provide precise, grass-safe thistle control. Repeat monitoring will be needed to watch for new seedlings.

Smothering with Mulch or Tarps

Covering thistles with a thick layer of mulch, cardboard, or a tarp can effectively kill them while leaving grass unharmed. This blocks sunlight and causes the smothered plants to die over a period of several weeks.

Application tips include:

• Water area first to promote seed germination.
• Cover at least 4-6 inches deep with mulch, overlapping sheets of cardboard, or an opaque tarp.
• Overlap seams and edges to prevent light from getting through.
• Leave barrier in place for at least 6-8 weeks.
• Check for and remove any surviving thistles.

This is a slower but low-effort method. It avoids herbicide use but leaves areas covered for an extended time.

Choosing the Best Products

Using the right herbicide, natural product, or manual control technique is key to effective, grass-safe thistle management. Consider the following factors when deciding on control methods:

Herbicide Product Selection

When choosing a traditional broadleaf weed killing herbicide, look for these traits:

• Selective formulas labeled for use on lawns.
• Contains 2,4-D, MCPP, dicamba, and/or triclopyr.
• Avoid products with high volatility.
• Low risk of grass injury when applied properly.
• Works on actively growing thistles.
• Rainfast within several hours after treatment.

Products like Trimec, Weed-B-Gon, and Ortho WeedClear are good selective herbicide options for spot spraying in lawns. Always read and follow label instructions carefully.

Vinegar and Clove Oil Cautions

When using natural herbicides:

• Choose horticultural vinegar with 10-20% acetic acid for effectiveness.
• Test clove oil products at weaker rates first and increase if needed.
• Never exceed label recommended concentrations.
• Avoid conditions that will allow drift like high winds or excessive sprayer pressure.

These natural herbicides can cause grass injury if misapplied so precision is important.

Manual Removal Considerations

For manual thistle removal:

• Use tools like shovels, spades, weed pullers, or stand-up weed poppers.
• Select tools with comfortable grip and leverage for prying roots.
• Consider soil conditions and depth of thistle roots.
• Wear gloves and protective clothing to avoid skin irritation.

Having the right manual removal tools makes the work easier and improves success.

Timing Treatments for Best Results

Proper timing of thistle control efforts maximizes effectiveness on the weeds while reducing lawn grass injury risks. Follow these guidelines:

Herbicide Application

• Treat when thistles are young and actively growing.
• Early spring or fall is ideal timing when grass is dormant.
• Spot treat in summer when thistles are visible but grass is semi-dormant.
• Avoid herbicide use if grass is heat or drought stressed.

Herbicides work best on smaller, actively growing thistles. Reduce grass injury risk by treating when it’s semi-dormant.

Vinegar or Clove Oil

• Treat on a sunny day to maximize leaf burn.
• Apply when thistles are at the bud to flower growth stage.
• Avoid treating during drought stress conditions.

Sunlight helps these natural herbicides be most effective. Hit thistles when growing strongly.

Manual Removal

• Dig when soil is damp but not soggy.
• Early spring is ideal before extensive root development.
• Pull young seedlings after rainfall has loosened soil.

Manual control is easiest when soil conditions are optimal for digging and leverage.

Smothering

• Apply mulch or tarp in early spring before thistles are too big.
• Water area first to encourage germination.
• Leave in place for 6-8 weeks minimum depending on material used.

Smothering is effective on young, emerging thistles before they become established.

Prevention of Thistle Regrowth

In addition to control methods, taking steps to prevent thistle regrowth will provide longer term, grass-safe management. Useful preventive measures include:

• Reseed treated areas to keep weeds out.
• Fertilize and aerate lawn grass for competitiveness.
• Maintain a taller mowing height to favor grass over thistles.
• Use corn gluten meal as an organic pre-emergent herbicide.
• Hand pull new thistle seedlings before they get established.
• Conduct frequent monitoring to find any new plants.

Preventive maintenance and prompt follow-up treatment of any regrowth will help keep thistles controlled without harming lawn grass.

Sample Treatment Program

Here is an example integrated thistle management plan that uses multiple selective control methods paired with prevention:

Spring

• Spot spray emerging thistles with selective herbicide.
• Hand pull any remaining plants not controlled by herbicide.
• Reseed treated areas to fill in bare spots.
• Apply corn gluten meal across lawn to prevent new seedlings.

Summer

• Spot treat random thistles with vinegar or clove oil.
• Stay alert and hand pull new plants immediately.
• Mow grass taller to shade out weed seedlings.

Fall

• Spot spray with selective herbicide after overseeding lawn.
• Cover small patches with mulch to smother new growth.
• Aerate and fertilize grass going into winter.

Next Spring

• Inspect closely and spot treat any remaining plants.
• Continue prevention regimen to protect lawn grass.

Integrating multiple control techniques with preventive management provides the best chance of selective, lasting thistle removal without lawn injury.

The Impact of Proper Thistle Control

Here’s a summary of what successful selective thistle management without grass damage can accomplish:

Benefits

• Removes painful thistle plants from lawns and play areas
• Restores a uniform, attractive lawn appearance
• Prevents thistle domination and displacement of desired grass
• Avoids need to re-establish large sections of lawn
• Saves money from reduced grass reseeding or sodding costs

Savings Over Time

Selective thistle control provides ongoing cost savings by preventing the need for extensive lawn rehabilitation and replacement. Here is an estimate of potential savings per 10,000 square feet of established lawn over a 5 year period:

Year	Cost without Proper Thistle Control	Cost with Selective Thistle Management
Year 1	$1,500 – Overseed damaged areas	$200 – Spot spray and pulling
Year 2	$2,500 – Resod large sections	$150 – Spot treat regrowth
Year 3	$5,000 – Full lawn renovation	$100 – Monitoring and prevention
Year 4	$3,000 – Overseed and resod	$100 – Monitoring and prevention
Year 5	$2,500 – Extensive overseeding	$100 – Monitoring and prevention
Total	$14,500	$650

As shown, preventing the need for major lawn rehabilitation by using selective thistle controls can potentially save thousands of dollars over time.

Conclusion

Controlling thistles in lawns while avoiding damage to desirable grasses is achievable with the right control methods used at optimal timing. Herbicides, natural products, and manual techniques can all provide selective thistle management when applied properly. Integrating them into a comprehensive prevention plan provides the best long-term control with minimized risks of grass injury. The small investment of time and money into selective thistle management practices can prevent the major expenses of lawn renovation and replacement in the future. Taking steps to control thistles now using grass-safe techniques will pay off over time with a healthy, uniform lawn free of painful weeds.