Can I Put Weed Barrier Over Weeds? Will It Kill the Weeds?

If you’re looking into Weed Barriers as a solution, you may be wondering whether or not Weed Barriers can be used to kill existing weeds in the garden. This depends on the age and type of weeds that are present along with several other factors.

Weed barriers can be placed over weed seeds and young weeds in the garden to smother them, but may not be able to stop mature weeds from pushing through them. Weed barriers are most effective when mature weeds are pulled from the bed before the weed barrier is placed.

Weed barriers are a useful tool in the garden if you know how to install and maintain them, but they have to be placed properly to be effective. Keep reading to learn more about weed barrier options and which ones can potentially kill weeds that are already in the garden bed.

Weed Barriers Can Be Placed Over Weeds

It’s possible to place a weed barrier over existing weeds in the garden, though the weed barrier may not be as effective at preventing the weeds from penetrating it.

The effectiveness of a weed barrier against mature weeds depends on several factors:

Sun exposure: Weeds that are growing in an area with direct sun exposure may be more resilient beneath a weed barrier than weeds growing in semi-shade or shaded areas.

Type of weeds: Some types of weeds, such as those that spread through rhizomes beneath the soil or strong perennial weeds, may be more resilient to weed barriers if they aren’t uprooted before the barrier is placed.

Type of weed barrier: Organic weed barriers like cardboard and newspaper are more permeable than barriers like landscaping fabric, making it easier for strong weeds to push through them.

Even though weed barriers can be placed over weeds, this doesn’t mean that the barrier will necessarily prevent all weeds from making it through if mature weeds aren’t removed first.

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Do You Have to Remove All Weeds Before Putting Down a Weed Barrier?

While it’s a good idea to remove as many of the weeds as possible from a garden bed before placing a weed barrier, it isn’t strictly necessary. A fresh weed barrier will kill many younger or weaker weeds outright, allowing the dead weeds to decompose and be returned to the soil as vital nutrients for new plants.

However, if large matured weeds are not removed, they may have enough stored energy to break through weed barriers and make it to the surface of the soil.

Not removing mature weeds before placing a weed barrier increases the chances that you’ll still have to pull weeds. You may not have to pull as many, but the stronger ones will make it through the barrier.

Will Weeds Grow Through Weed Barrier Fabric?

A weed barrier fabric or landscaping fabric can prevent most weeds from growing through it by blocking access to sunlight. This means that dormant weed seeds in the soil and weeds beneath the barrier will die.

However, a landscaping fabric does not prevent weed seeds that are blown into mulch and compost on top of the landscaping fabric to germinate and send roots down through the fabric.

Since weed barrier fabrics can’t prevent airborne weed seeds from germinating over landscaping fabrics, they aren’t a practical choice in beds where compost and mulch are used.

How to Kill Weeds Before Laying Landscaping Fabric

The most effective method to use a weed barrier fabric is to kill the weeds in the bed where you plan to install the fabric before installation.

Here are some of the most popular ways of killing existing weeds before placing a weed barrier in the garden:

Pulling weeds: Pulling weeds might be the most labor-intensive way of removing weeds from the garden, but it helps prevent mature weeds from leaving their seeds in the soil.

White vinegar: White vinegar is an acidic solution that can help safely kill weeds in the bed before placing a weed barrier fabric. Be sure not to get vinegar on any plants or grass you don’t want to kill.

Herbicide or pre-emergent sprays: Herbicides and other chemical weed eliminators are a quick and dirty way to kill any weeds in your garden bed. Use organic herbicides in beds where edible plants are going to be installed for food safety.

Once most or all of the weeds are killed in a garden bed before the weed barrier is installed, this makes the barrier much more effective at preventing weeds from breaking through.

Any new germinated weeds will quickly die out before they can muster enough energy to break through to the surface and reach sunlight.

Tips for Removing Weeds Before Installing a Weed Barrier

Since removing weeds before installing a weed barrier can make your weed barrier more effective, here are some tips you should follow to make the process less painful:

Weed when the ground is wet: In moist soil, it’s much easier to get weeds up with the root intact rather than pulling them off at the ground line. However, try not to walk around in the beds too much while wet to avoid compacting the soil.

Use plenty of mulch: Placing mulch over your weed barriers can help prevent new weeds from germinating on top of them, especially if you use a mulch like gravel or rubber mulch.

Deadhead mature weeds: If you have a hard time uprooting sturdy, tough weeds that have already gone to seed, remove the seed heads from the garden. This can help prevent the weed from spreading in the garden bed. 

Taking the time to perform this step and remove existing weeds before placing a weed barrier can mean the difference between a low-maintenance garden and one that needs to be weeded constantly. It can also help keep your weed barrier more intact.

Weed Barriers Over Weeds Are Not as Effective

While it’s possible to place weed barriers over existing weeds, it means losing some effectiveness in your weed barrier. Strong weeds that have already stored energy from sunlight may bust through your weed barrier, allowing other weeds to receive sunlight.

The best option when putting down a weed barrier is to remove as many weeds as possible before the barrier is installed. This can help prevent wasted time, energy, and money.