What is the Best Mulch to Prevent Weeds?
Don't Work Yourself Up Over the Best Mulch for Vegetable Gardens. Lazy May be Best.
All Photos By Shelley Dedauw
The best mulch to prevent weeds depends on where the mulch lies, what else you need it to do…and, of course, cost.
What is the most important factor for a successful garden? I asked my friend, Kathy, a master gardener through the local university in Reno around the time I planned my first Nevada garden. I’d grown food under my mother’s tutelage before I turned 18, but this was the first time I depended on the soil to feed my own children.
Her answer was one simple, strong word: “Mulch.”
She didn’t tell me to wait until last frost or to avoid beefsteak tomatoes within our erratic growing seasons. Nor did she tell me to amend my soil yearly, adding copious amounts of organic material. These are also crucial factors. But her knowledge gleaned through the Cooperative Extension and her own experience, told me to cover my dirt.
Mulching is the simple act of covering soil with a protective layer. Materials can be organic or manmade, compostable or semi-permanent. Whether it’s applied to avoid drought, discourage weeds, or keep bulbs warm, the focus is on what lies beneath.
If you need more convincing, the U.S. Department of Agriculture’s Natural Resources Conservation Service says mulching is one of the easiest and most beneficial things you can do for your soil, and the Arbor Day Foundation says mulching is a newly planted tree’s “best friend.”
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Mulching Lessons Learned
Even after Kathy’s admonition, it didn’t immediately sink in. I never learned how to lay mulch in Mom’s garden. We pulled weeds both morning and early evening, and then rested when midday sweltered. Perhaps that was Mom’s way of keeping three teenagers busy during summer vacation. Mulching could have reduced that weeding tenfold. And mom didn’t worry about watering; we had a well, weren’t in a drought, and she had trained her kids how to efficiently move a sprinkler.
That year I grew jack-o-lantern pumpkins. Did I mention this was my first year cultivating in Nevada? Jack-o-lanterns are fun to grow, but they don’t have much culinary value. And I can purchase three jack-o-lanterns at the supermarket for how much I paid the water authority to grow a single plant.
Pumpkin leaves spread out full and green within June, fed by an intermittent sprinkler beneath the vines. But July was cruel. Plump and smooth in the morning, the leaves shriveled by midday.
I’m not proud of what I did. I watered more. That’s not the right answer when you garden in a desert. Sure, it plumps those leaves back up super-fast. But then you receive the water bill.
Kathy’s single word returned to me during the second week of wilting-and-watering. Dipping deep into the mower bin, I retrieved grass clippings and laid them on a tarp overnight. In the morning, I packed them tight around the stems. Leaves didn’t shrivel that afternoon and I didn’t turn the hose on until the next day. I could go two to three days between watering sessions instead of running out in panic to feed my failing pumpkins.
Why We Mulch the Way We Do
Moisture retention keeps plants alive, allows you to work elsewhere instead of answering to your garden’s every need, and promotes healthy fruit.
Did you know two factors for tasty heirloom tomatoes are variety and water control? The first is simple: some tomato varieties just taste better than others. But a second and newly discovered factor is how much water the plant receives when fruit forms. Well-irrigated tomato plants result in watery fruit. That’s why hydroponically grown produce is so tasteless. The secret is to give the tomato only the water it needs and not a drop more. But if you’re unsure of the amount, or have a busy lifestyle, “just enough” can easily become “holy cow, my plants are dying!” And compensating by overwatering after a dry stretch causes cracking.
“Just enough” water is made simpler by using drip lines and mulch. Run the drip line along the soil with emitters near each plant. Cover soil and hose with mulch. Then watch your plants for a few days to see how they fare. If they wilt in the heat, it’s more effective to add more mulch than increase water flow.
Summer heat upsets crops like carrots, which like warm tops and cool roots. Winter frost kills bulbs or pushes them out of the ground. A thick layer of organic material regulates soil temperature.
Weed suppression is a third reason to mulch, especially in gardens that get enough moisture. More water means more weeds. And the reason mulching suppresses them follows the basics of photosynthesis: plants need sunlight for growth. Vegetables above the mulch already stretch tall in the light but recently germinated seeds have to fight their way through. The best mulch to prevent weeds is whatever keeps back the light. If the layer is thick enough, weeds don’t stand a chance.
The Cheapest Methods
It’s not necessary to purchase expensive mulch unless you have aesthetic requirements. Homeowner associations may require you surround perennials with attractive bark or rocks. Vegetable gardening is different, especially if you’re growing food to save money.
The best mulch to prevent weeds is also the cheapest. Free materials that also benefit soil include compost, leaves, sawdust or wood chips, straw or grass clippings. Search online classifieds or get to know local farmers, offering to buy hay bales that have gotten wet. Collect leaves in the fall and store in plastic garbage bags to use in the next year’s garden. Contact tree care companies about receiving the chipped results of their labors.
Never use herbicide-treated grass clippings. A good friend accepted lawn trimmings from her church and used them as garden mulch. When her vegetables died, she realized the church had applied a weeding/feeding solution to the lawn but had failed to tell her. Though she disposed of the clippings, some remained in her soil. Those herbicides mean she can only plant bladed grasses, such as corn, within those spots for a couple of years.
If you’re using straw, look for bales that don’t have seed heads still attached…unless you want to grow wheat. I didn’t mind so much when grains sprouted beside my garlic. I let them ripen then pulled them for the chickens. But the next year’s bales had even more seeds and wheatgrass became the first crop of spring. Also, find organic bales if possible, because some wheat is sprayed with glyphosate herbicide right before harvest so spikelets mature at the same rate. Glyphosate will kill your broadleaf crops.
Those Manmade Mulches
Weed cloth, tomato plastic, and rubber mulch promise weed suppression or increased growth, but do they really work?
I’ve used weed cloth once and was not happy with the results. If I’d spread it beneath perennials, out of walkways, I’d have been happier. But the black fabric heated up my soil in the summer and tore beneath my gardening shoes. I only used it once. But a tear-resistant weed cloth can help northern gardens with shorter growing seasons.
The same with paper weed layers. Advertising claims were promising: it would warm the soil to increase growth and could be tilled in after harvest. But it crackled and tore. Soon the soil heated up too much. Tilling was more of a hassle than just ripping the paper up and throwing it away. I didn’t purchase it again.
Layers made from recycled tires or plastics must be removed at the end of the season, or they can pollute the ground. To some gardeners, this is worth the work. Others would rather be organic with material that can eventually become more soil.
The only plastic mulch I’ve ever used is that red tomato film, which promises increased yield because it reflects the right kind of light onto the plants. And though I’ve used it for five years, I can’t testify whether it truly increases yields. More important factors came into play each year such as soil amendment and blossom drop due to high temperatures. Whether or not it actually works, I do like it for two reasons: It’s easy to unfold, pin into place, and plant the seedlings into holes cut within the film. And it suppresses weeds everywhere except where light shines through the holes. If you do use plastic mulch, poke holes in it so water can pass through.
The Good, the Ugly and the Just Plain Bad
Every mulch material has its flaws. Straw can harbor insects that crawl into the little tubes. Grass clippings may mold and compact. Peat moss may be unsustainable and wood chips may turn sour or attract termites.
Some gardeners use old carpet, leaving it in the garden year after year instead of removing it as it drops fibers. Carpet can disintegrate with frequent watering. Recycled paper may be used as a weed barrier but it’s necessary to use newsprint with soy-based black ink. Decomposing paper can raise soil acidity as well.
A highly debated form of mulch is cocoa shells. This may be the best mulch to prevent weeds if you don’t have pets…but avoid it if you do. Cocoa shells retain a little theobromine, the ingredient in chocolate that’s toxic to pets. Some companies treat their cocoa shells, removing the fat that carries the theobromine, which also diminishes the sweet smell. If you use cocoa mulch, be sure it’s treated so it’s nontoxic.
And though some gardeners will tell you to never use hay because it contains weed seeds, others prefer it because it adds more nutrients to the soil when it decomposes.
In my experience, the best mulch to prevent weeds is whatever improves soil after harvest. This includes compost, straw, and leaves. The worst are those that must be removed because it can be difficult to get every piece. Removing mulch after harvest adds unnecessary labor if compostable material can be used instead.
What you use for mulch depends on where you’re using it, your budget, if you intend to remove it or till it in, and whether you want organic or manmade products. Research the pros and cons to each type before choosing the right one for your garden.
Lazy Desert Mulching
After reading article after article and trying product after product, I learned to keep it simple. I work hard on my garden to attain maximum yield, but I don’t have time to waste. I don’t need to create more work.
Seeds sown into bare ground grow a couple inches before they meet the mulch. Grass clippings land around tiny carrots while leaves pack against tall, slender onion greens. Transplants sink into the soil and, within minutes, straw packs against the stems. Potatoes grow six inches, are hilled up and grow again. When I cannot hill any more, I apply straw to reduce watering and allow even more growth. And deep mulch gardening doesn’t end there. When summer heat reaches triple digits, soaker hoses point downward and more straw lays atop to keep every precious drop where it belongs.
By harvest, I’m exhausted. I’ve spent hours each day cultivating, weeding, watering and preserving the vegetables. With sagging shoulders, I scan the tired and frost-damaged plot while chickens pace behind me, eager to reach fallen tomatoes. Autumn cleanup is simple: Remove the plants chickens can’t eat. And open the gate. Poultry claws dig deep into that organic layer, separating it so my hens can find pests hoping to overwinter.
Then cold weather hits. I’m not worried. I used to be embarrassed by my lazy cleanup techniques until I read an article about how keeping a cover is crucial for soil health. The entire land gets a rest.
And in the spring, the shovel digs deep, mixing chicken droppings with decomposed leaves, straw and grass. It all rests beneath the surface to feed beneficial microbes and create nitrogen for the next round of crops.
What have you found to be the best mulch to prevent weeds? Let us know in the comments section.
Originally published in Countryside July/August 2016 and regularly vetted for accuracy.