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Inorganic Mulch: What You Need To Know
Is inorganic mulch ever a better choice than organic mulch? It depends – maybe you need a mulch that doesn’t blow away in high winds, float away in heavy rains, or burn your house down.
Newsflash! All organic mulches are flammable.
But rock or stone won’t burn. So they’re the best type of mulch to use next to structures.
There are advantages to some types of inorganic mulch, y’ know, like that non-flammable thing.
But the cons of inorganic mulch generally outweigh the pros. So it pays to learn as much as you can before choosing an inorganic mulch.
If you’re wondering what inorganic mulches are, they’re either man-made or mined. Rubber mulch, plastic sheeting, landscape fabric, or rock mulch are types of inorganic mulch.
Not sure if an inorganic mulch is the best type of mulch for you? Learn how to choose the right type of mulch for your garden in three easy steps.
If you’re considering using inorganic mulch in your yard or garden, I’ve got you covered.
I’ve reviewed seven different kinds of inorganic mulches, and I tell you what I recommend and what I don’t.
Types of Inorganic Mulch That I Recommend
Gravel, Crushed Rock, Pea Gravel, River Rock, up to 1.5″ in diameter
Rock, gravel, and stone mulches work well in certain situations, but you need to understand the downsides to rock mulches before choosing one as your mulch. Typically, the disadvantages of rock mulch outweigh the advantages.
Considering using large river rocks or lava rock? See my reviews below.
Rock mulch is best for rock gardens, pathways, dry creek beds, beds next to buildings in fire-prone areas, landscaping around fire pits, and in trouble-spots where water collects during heavy rains and would wash other mulches away.
Depth: 2-4 inches, depending on the size of the rock. Apply bigger rocks more deeply.
- Many are attractive and rocks come in a variety of colors.
- Rock mulch lets water in and prevents the evaporation of water.
- It’s much less likely to float away than other mulches.
- It won’t blow away (unless it’s lava or pumice).
- It takes a lot of manual labor to install and remove rock mulch.
- It heats up plants and soil. Excessive use of rock mulch is contributing to the increase in nighttime temperatures in urban areas of the Southwestern U.S.
- It won’t add organic matter to your soil like organic mulches do.
- And it doesn’t suppress weeds.
Yes, people. You heard me! It. Does. Not. Suppress. Weeds.
Still want/need to use rock mulch? Then don’t install it over landscape fabric if you plan to grow any plants in the rock mulch. See my discussion of the evils of landscape fabric below.
That landscape fabric under your rock mulch is a false sense of security. Despite the claims of many manufacturers, landscape fabric isn’t a long-term solution for weed prevention, no matter what you top it off with.
Plastic mulch is best for heat-loving vegetables like tomatoes, eggplants, melons, and peppers, and warming your vegetable garden soil in the spring.
- It warms the soil for heat-loving crops.
- It suppresses weeds during the growing season.
- It reduces evaporation.
- You can only use plastic one time if you’re using them during the growing season. Re-using plastic mulch can increase problems with soil-borne diseases.
- Plastic mulch has a high environmental cost. It’s a one-use item that gets put into a landfill at the end of the season.
- When you use plastic mulches during the growing season, you must have drip irrigation installed.
Types of Plastic mulch:
- Clear plastic mulch is the best type of plastic mulch to use to warm up your soil in the spring. NOTE: It must be removed before planting. And, it can also be used to solarize your soil. Choose a UV-stabilized plastic so you can use it for several years.
- Black plastic mulch can be used to warm your soil when planting heat-loving crops. It also suppresses weeds.
- IRT plastic mulch heats the soil better than black plastic and also suppresses weeds as well as black plastic. This is my personal choice for heat-loving crops.
- Red plastic mulch isn’t a mulch I recommend. See my reasons below.
Large River Rocks (over 2″ diameter)
Why I don’t recommend large river rocks:
- The size of the rocks makes it difficult to weed. As I mentioned before no rock mulch suppresses weeds. And these large stones make weeding even more of a chore than it usually is.
- Like all rock mulch, it doesn’t add organic matter to your soil, it takes a lot of manual labor to install and remove, and it heats up areas around it.
- Large river rocks are almost always out of scale to any plants around them.
Why I don’t recommend using rubber mulch:
- The safety of rubber mulch is VERY questionable.
- It smells pretty bad (like ground up tires).
- Weed suppression isn’t as good as what you can get with organic mulches.
- It can attract cockroaches.
- And, just for good measure, it’s highly flammable.
Why I don’t recommend lava rock or pumice:
Lava rock shares the downsides of all rock mulch, (doesn’t add organic matter to your soil, is a lot of work to install and remove, and it heats up areas around it).
But lava rock has some downsides that are particular to it.
- Lava and pumice have small pores in them, so they absorb water, and don’t release it to your plants. So, you end up watering more than you would with regular rock mulch.
- It’s lightweight, so it blows around easily. Because of this, you should place it 1″ below grade to contain it.
- The sharpness of the rocks makes them unpleasant to work around.
Red plastic mulch was developed to increase yields for tomato farmers.
Why I don’t recommend using red plastic mulch for tomatoes in home gardens:
- Unfortunately, the research results for red plastic mulch are inconclusive.
- It doesn’t appear to have the benefits of increased yields that the initial research showed.
- IRT plastic mulch or black plastic mulch are better choices for cold-climate or short season gardeners who want to create the warm soil conditions that tomatoes prefer.
Why I don’t recommend using landscape fabric under any mulch:
- Contrary to manufacturers’ claims, the pores in it eventually clog, cutting off air and water to the soil below. This creates an unhealthy soil environment for your plants. Plant roots need air and water to survive.
- Weed suppression quickly declines! A year or two after you install it you’ll find yourself either weeding a lot, spraying weed-killer all over the place, or cursing the day you became the victim of a supervillain with a diabolical plan to cover your yard with weeds – especially if you put it under rock mulch.
- You can’t garden with it. You can’t move plants around, or easily divide perennials.
- It doesn’t add organic matter.
- If you change your mind later, it’s very difficult to remove.
- When the mulch on top blows or floats off, it’s butt-ugly!
So, don’t buy into the myth of landscape fabric.
Download Your Free Cheat Sheet and Choose the Best Garden Mulch
I cover both organic and inorganic mulch in the cheat sheet. And you’ll choose the perfect mulch for your garden in three easy steps:
- Focus on what plants you want to mulch.
- Choose the top three things you want this mulch to do for your garden.
- Once you have the answers to those questions, look through the list of mulches to find the perfect mulch for your needs.
The chart includes details about 26 different mulches. They’re grouped by mulches that I:
- recommend with reservations, and
- don’t recommend
Here’s a sneak peek of your printable cheat sheet:
Are there any types of inorganic mulch that I missed? If so, let me know in the comments and I’ll add them to the list.
Hi, I'm Cheryl. I'm a certified gardener, bird lover, and spreadsheet enthusiast. I believe that with a few smart strategies and a little know-how, you can grow your dream garden and still have time to enjoy it. I teach online gardening courses and write articles that help you save time and money in your garden.
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