There's so much more to a great yard than just mowing the lawn.

Preventing Weeds in Rock Mulch

Now is the time to take action to prevent weeds in the areas of your yard that are covered with rock mulch or decorative rock. By following these directions you can save hours of weeding in the summer.

Why use a weed preventer?

Let’s talk for just a minute about why you need to prevent weeds in rock mulch. Isn’t un-planted rock mulch the best option for a maintenance-free yard? Nope. Sorry folks. Rock mulch is often one of the worst places for developing weeds. Well, what if the rock mulch has weed barrier fabric underneath? For the first few years, the weed barrier will shade out weeds in the soil. But over time, dirt, dust and weed seeds collect on top of the fabric. Add moisture to that scenario and presto! You end up with a flush of weeds.

Miracle Grow Weed Preventer

Many manufacturers produce a weed preventer with the active ingredient “trifluralin.”

Weeding can be a good way to reduce stress and take out your aggression in a constructive way, but if you have too many weeds, weeding can create a maintenance nightmare that consumes all your free time. And believe me, it is not a pleasant task to hand weed rock mulch. If you don’t maintain it, rock mulch looks awful in a few years.

So what can you do to avoid hours and hours of weeding? One of the easiest ways to control weeds in rock mulch is to use a weed preventer, which is also known as “pre-emergent herbicide.” These products are usually come in granules that you sprinkle on an area and water into the soil. They create a chemical barrier in the soil that kills the little weeds as they begin to grow.

The name “pre-emergent” is derived from the fact that this type of weed killer kills the weeds before they “emerge” from the soil. The benefit is it can save you lots of time weeding in the summer by not letting the weeds grow in the first place.

Will it work on all weeds?

Keep in mind these products only work on weeds that are growing from seed. Some common weeds that grow from seed include crabgrass, foxtail, cheatgrass, purslane, and spotted spurge.

Some weeds you see in your rock mulch are probably not growing from seed. Instead, they die back to the ground in the fall and regrow each year from an underground root system that survives the winter. Weed preventer will not work on these weeds. Examples of weeds that pre-emergent will NOT work on are dandelions, common mallow, thistle, quackgrass, smooth brome grass, etc. If you have these weeds, check out the segment on “spot spraying weeds in rock mulch.”

Closeup weed preventer

Look at the active ingredient list rather than the trade name to determine what you’re buying.

So, what I’m saying is that pre-emergent herbicide isn’t going to get rid of all the weeds in your yard, but if you apply it correctly, it will reduce the amount of new weeds that you will get.

Which product should you choose?

The first step is to purchase a weed preventer. Which product should you get? Preen Garden Weed Preventer is the most common formulation. It contains the active ingredient trifluarlin. EIQ of trifluralin is 18.83, so it’s realtively safe for humans, pets and kids. In addition to being safe, trifluralin lasts for 8-12 weeks. That means you only have to apply it 1-2 times during the growing season. Here’s an Amazon link if you’d like to purchase a bottle.

You can use it in unplanted rock mulch as well as rock mulch around desirable plants, like if you have a garden bed or rock garden with rock mulch. It will not affect plants that are taller than 3 inches, but check here, for notes on how to use it. Note that this product is not meant to be applied to lawn areas.

What if you don’t want to use chemicals? 

If you’d like to use an organic product, you can use a product made from corn gluten meal. Common products with corn gluten meal include Preen Organic Vegetable Garden Weed Preventer and Garden Safe Garden Weed Preventer.
Corn Gluten Meal Weed Preventer
Corn gluten meal is effective and pretty safe. The downside is that it only lasts a few weeks, so you have to keep reapplying it, at least 3 times in the spring. That can get laborious and expensive, but may be a good option where safety is the biggest concern.

Here’s an Amazon link for the organic corn gluten meal version.

I’ve personally tried corn gluten meal in rock mulch. My experience is it prevented the first flush of weeds in spring but lost its effectiveness pretty soon after that. I ended up spot spraying more than when I’ve used the formulation with trifluralin.

Corn Gluten Meal Weed Preventer Closeup

Notice the active ingredient list names corn gluten meal.

I would not recommend using a corn gluten meal product that also contains fertilizer in it. The last thing you want to do is fertilize any weeds that escape the weed preventer!

Where should you apply weed preventer?

Common places would be areas with rock mulch (both unplanted rock mulch and garden beds with rock mulch) and cracks in the sidewalk, driveway, or patio. Try to remember where you had a lot of weeds last year, and single out those areas. You might be tempted to spread it throughout all of your rock mulch. If you’ve had a pretty bad weed problem that might be appropriate. However, I would try to target the areas of rock mulch where you’ve had the most weeds. That will save you money and reduce the amount of weed killer you’re putting in to the environment.

Be sure not to apply this product in an area where you want to grow plants from seed, like an area where you want to seed with grass seed, or your vegetable garden. Don’t apply these products to lawn areas. Choose a weed preventer designed for lawns instead.

When should you apply it?

The best time to apply weed preventer is in early March. If you have an areas that warms up early in spring, like a south facing slope, put down the weed preventer by a week or two earlier. If you don’t get to it until April 1 or later, there’s a good chance that the weeds have already started to grow and it’s too late. It’s a good idea to reapply weed preventer around May 15 to June 1 to catch the warm-season weeds that emerge in summer.

How should you apply it?

Be sure to read the label and follow the directions. You basically sprinkle it at the recommended in the area you want to prevent weeds rate and water it in. The reason you have to water it is to dissolve the product so it creates the barrier that kills the newly growing weeds. If you don’t water it in, it will sit on the surface and not have any effect on the weeds. Don’t hit the leaves of desirable plants, so hold the bottle close to the ground when applying it.

Products to avoid

Take note that there are some products out there that contain the active ingredient imazapyr. An example of a common product that contains imazapyr is Ortho GroundClear Vegetation Killer.

Ortho Ground ClearThese products are tempting to buy because they prevent weeds for up to one year, but keep in mind that in addition to preventing weeds, they also kill any existing plant that takes the product up by its roots. Even if your landscape plants are close by, they might be affected. This is because imazapyr can move through the soil and across the soil’s surface with moving water from rain, sprinklers, or other sources. So, if any desirable plants are exposed to it, imazapyr can potentially kill them.

Here are additional details: Imazapyr is persistent and mobile in the soil and can move around with moving water. This is true for both sprayable and granular formulations. So, if any desirable plants are exposed to it, imazapyr can potentially kill them. The EIQ of imazapyr is 22.3. The EPA has determined that is relatively safe to humans, although it is a potent eye irritant and can cause irreversible eye damage. The biggest problem is that it is kills a broad range of plants, pre and post-emergently. You have to be very sure that don’t want anything growing in the applied area, plus keep it away from the root systems of large trees.

So ONLY use these types of products if you are very clear about what you are doing. Since it can be devastating if you apply it incorrectly, I would recommend that you stay away from these products unless you are VERY sure they are the right product for the effect you want to achieve.


Use weed preventers in rock mulch to prevent new weeds from growing from seed. Apply it to areas where you’ve had a weed problem in the past or where you anticipate having a weed problem. Choose a product that contains either trifluralin or corn gluten meal. Sprinkle it over the surface at the recommended rate, with the bottle close to the ground. Water it in to activate. Reapply if needed at a later date. By investing some time upfront, you can free up your time when the weather is nice for summer fun.


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