Does Preen Work in Rock Gardens?

Whether you’re taking preventative measures to protect your rock garden from weeds or have noticed pesky plants popping up between the rocks, keeping weeds out of your garden is usually an uphill challenge, especially if you don’t know what works. However, you can take some foolproof measures to ensure that your rock garden stays beautiful and unencumbered by unwanted visitors. 

Preen works in rock gardens. Preen is one of the most effective preventative weed killers for any garden, including rock gardens. However, you’ll need to apply the Preen at the proper time of year to prevent weeds from taking over your garden. 

Let’s get into the details and discuss how you can keep your rock garden free from weeds. I’ll tell you about several methods that work for preventing weeds and tell you more about the benefits of using Preen to help. I’ll also discuss how and when to use Preen on your rock garden for the best results. 

How Does Preen Work? 

Preen works by stopping cellular mitosis, which halts all seed germination. It contains a chemical called trifluralin that activates in soil once you moisten it. This chemical will pull apart the cells in seeds, stopping all growth. 

Preen will work in a rock garden as a preventative herbicide. It cannot kill or disrupt fully germinated plants, making it an asset for a rock garden (or any planting area). You can confidently use it without worrying about killing your rock garden’s flowers, herbs, and other vegetation. 

However, that also means that Preen will not kill mature weeds – you’ll have to remove those by hand or use another weed killer to get rid of them. Still, since Preen can stop those adult plants from spreading, it will go a long way in keeping down the population of pesky plants. 

Other herbicides may harm your rock garden’s plants, so opting for a product like Preen is your best bet if you want to keep your existing plants safe and healthy. Likewise, Preen doesn’t have the same bad rap as harsher herbicides like Round-Up, a product whose company has been sued many times due to potential health hazards. 

It’s also worth mentioning that Preen doesn’t kill all weeds – it only targets seeds. So, it can’t control weeds that spread via roots, such as dandelions, mallow, and thistles. You’ll have to find other methods to manage these plants, so it’s essential to ensure that you take a comprehensive approach to your weed-maintenance program. 

Keep reading to learn how to get rid of all the weeds from your rock garden. 

How To Keep Weeds From Growing in Your Rock Garden

To keep weeds from growing in your rock garden: 

  • Use pre-emergent herbicides. 
  • Layer landscape fabric over exposed soil.
  • Add edging to the rock garden.
  • Hand weed the rock garden. 

So, let’s look at each of these steps and discuss how and when to use each one. 

Use Pre-Emergent Herbicides

The first step you want to take to prevent and get rid of weeds is to use an herbicide. 

Applying a preventative pre-emergent herbicide like Preen before you plant your garden or add anything else to ward off the weeds is a fantastic method for clearing your soil. These herbicides are far different from most others on the market because they will not affect the plants that are already growing. They only make seeds infertile, preventing sprouts from popping up. 

To genuinely eradicate weeds with Preen, you’ll need to till your soil and remove all the weed roots. Then, you can apply Preen to the garden and place your fully-rooted plants in the ground. 

Layer Landscape Fabric Over Exposed Soil

Landscape fabric is a fantastic addition to any garden. Not only does it smother out unwanted weeds, but it also fights erosion and retains soil moisture, ensuring that your plants’ soil is in tip-top shape regardless of environmental conditions. 

Layering landscape fabric over soils treated with pre-emergent herbicides like Preen offers additional benefits. Landscape fabric will prevent weeds from popping up out of the ground. Since their leaves will never see the light of day, the landscape fabric can eradicate weeds that have already germinated or rooted in your garden. So, the Preen and fabric work to eliminate all weeds from your garden, killing both seeds and leaves. 

To ensure that no weeds have a fighting chance of moving into your rock garden, overlap the material where there are gaps or where you need to add another strip of fabric. Then, cut holes in the fabric and plant your plants as usual. 

Add Edging to the Rock Garden’s Perimeter

Landscape edging forms a barrier to ensure that your garden weeds don’t crawl and root deep below your rock garden. 

Edging is an excellent choice for rock gardens. It has a neutral color and is low-profile so that it won’t take attention away from your garden’s signature rocks. I use this one in my garden because it is affordable, and the color makes it easy to cover up and hide with larger ornamental stones. 

You can also opt for fancier edgings like wood or aluminum, but any small barrier will do to keep creeping weeds from crawling into your rock garden. 

Hand Weed the Rock Garden

Last but not least, it’s critical to note that, no matter how many herbicides you use and how many features you add, there will still be some weeds. Following the above tips and treatments will go a long way in reducing weeds, but you’ll still have to pick some out, especially in early spring when the weeds are most prevalent. 

However, the more weeds you pick out, and the more often you apply your Preen and hand weed, the fewer weeds reappear every year. So, if you are diligent and keep pulling your weeds, you will eventually have a self-weeding rock garden. 

When Should You Apply Preen to a Rock Garden?

You should apply Preen to a rock garden during spring when the temperatures are consistently above 55 °F (13 °C). If you have any existing plants in the area, they should be fully mature or at least 2 inches (5.1 cm) tall before applying Preen. It would be best if you reapplied Preen every spring. 

Preen works best when applying it to your topsoil on a warm, dry day. Since it is a powder that slowly releases into the soil as you water your garden, dry conditions will give you a more even application and ensure that it doesn’t stick to one damp spot. There should also be little to no wind since the powder may blow away on a windy day. 

It’s also best to wear gloves and a mask when applying Preen. Preen can enter the lungs as you handle it, and inhaling the chemical can harm your respiratory system. So, always take care and be safe!

However, once the Preen settles onto your topsoil, you’re good to go. Give your plants a spray with a watering can or hose, and your rock garden will no longer be host to little weed sprouts for the following year. 

Pro Tip: Apply Preen underneath your rock garden’s landscape fabric the first year, then use it on top of your rock garden for the following years. The Preen underneath the fabric will work to eliminate seeds below the soil, while the chemical on top will stop new weeds from germinating. 

Where Should You Avoid Putting Preen in Your Rock Garden?

You should avoid putting Preen near drains, irrigation systems, or natural water bodies in your rock garden. If the Preen runs off into the water supply or a water body, it will disrupt the ecosystem and prevent small aquatic creatures from reproducing. 

So, it’s best to keep your Preen away from storm drains, irrigation systems, creeks, streams, lakes, and ponds. If your rock garden is beside any water sources, you may want to avoid using Preen. 

Likewise, it’s best to avoid applying Preen to a garden if you will be expecting rain within the next 24 hours. Rain can wash off your Preen, which is bad for the environment and will leave your garden vulnerable to weeds. 

Final Thoughts

Preen is an effective product for preventing weeds’ seed germination in rock gardens. However, you’ll have to combine Preen with several other weed prevention methods to eradicate unwanted plants from your plots. 

For the best results, use Preen with landscape fabric and edging. Then, keep hand-weeding and reapplying the Preen every year to slow the spread of weeds. Eventually, you will be able to eliminate all of the weeds in your rock garden within a couple of years. 

Alexander Picot

Alexander Picot is the principal creator of, a website dedicated to gardening tips. Inspired by his mother’s love of gardening, Alex has a passion for taking care of plants and turning backyards into feel-good places and loves to share his experience with the rest of the world.

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