By Jerri Cook – To mulch or not to mulch; that is the question. The answer isn’t so simple. There are many different types of mulch, all falling into two categories, organic and inorganic. All have advantages and disadvantages. It’s also important to learn how to lay mulch for your gardening style and region.
Organic mulches tend to improve the structure and nutrient content of the soil as they break down. Some organic mulch such as compost, straw, leaves and grass break down in one season. While they add to soil sustainability, they also need to be reapplied at least every season and often several times during the growing season. These sorts of mulches make sense if you have them in abundance on your property or nearby.
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Persistent organic mulches like wood chips and shredded bark may last many seasons. They are more attractive than mulches which break down quickly. They are also more expensive.
Inorganic mulches such as black plastic, red plastic, crushed rock, gravel, vermiculite and perlite also come with a hefty price tag.
Considerations for Gardeners Using Plastic Mulches:
The plastic types, red or black, tear over time. After a few seasons growers find themselves picking up shredded pieces of plastic all over their gardens.
The color in plastic mulches fades with each use, decreasing their effectiveness.
The plastic types also need to be well anchored so they don’t blow away in the wind.
While there is anecdotal evidence suggesting red mulch improves yields of tomatoes and cantaloupe by 20 percent or more, there are heavy concerns about plastic. Some studies suggest that red mulch releases arsenic into the soil as it breaks down. While this may not be a concern in smaller urban gardens, those with bigger plots may want to think twice about using large amounts of it.
Instead of using red plastic mulch to capture more of the sun’s red spectrum light to enhance yield, gardeners can hang red Christmas ornaments from their plants or place sticks that have been painted red in with the plants. Studies show this works as well as red mulch, without the concerns of chemical seepage.
How to Lay Mulch
The effectiveness of any mulch is directly tied to the density of application. Too much mulch can choke off air to the soil, promoting fungal and bacterial growth. This is a concern when using heavier persistent mulches like sawdust and sand. Apply too little and you will find yourself with weeds coming up through the mulch material. To avoid fungal and bacterial problems while maximizing weed suppression, apply mulches that settle, like straw and grass clippings, heavily and be more conservative with the denser mulches. This tutorial on deep mulch gardening is a good place to start.
Long-term garden plants such as tomatoes, peppers, vine crops and beans require thick layers of mulch, at least six inches of denser mulches and nothing less than a foot if using straw. This helps conserve ground moisture as well as prohibit weed growth.
I use straw and sawdust as mulches. The straw I put on at least 18-inches deep, while the sawdust is a little over six-inches deep. I also like sawdust best for my perennial crops, like the raspberries. It keeps the weeds down nicely and helps me to control unruly shoot growth.
Mulching perennials helps guard them against frost heave in spring and protects roots against dangerous winter temperatures. Rose growers routinely dig a trench about 18-24 inches deep next to the plant and gently tip the entire bush into the trench. The bush is then covered with layers of soil and mulch to protect against heaving and sub-zero temperatures.
Permanent flower beds can be lined with landscaping fabric then mulched with only a couple of inches of rock or wood chips. This method is not recommended for seasonal beds and vegetable gardens.
I don’t use red mulch or black plastic, Hoss (my large tiller) eats it. No matter how hard I try to keep him from it, he always finds it and chews it up. What he doesn’t devour, the tractor finishes off. More than once, I’ve been left with a pile of expensive shredded plastic.
Beyond choosing which mulch to use, gardeners have to decide if learning how to lay mulch and applying it to their gardens is a good idea at all. Contrary to popular belief, there are times when mulching is a bad idea. It’s true that mulching with organic matter improves the soil, slows erosion, slows weed growth and helps prevent the loss of ground moisture; even so, mulching isn’t always appropriate. I found this out the hard way.
After settling into our new farm in northern Wisconsin, I set out to start a garden. The previous owners weren’t big on gardening and had never planted one in the 12 years they owned the place, leaving me with little choice other than to direct my husband to plow up part of a hay field. I knew I would have to fight the pasture grasses all season long, so I had plenty of straw and sawdust on hand.
One day, as I was out weeding and mulching, a neighbor came by to admire the garden. We talked about growing conditions in the area, and that’s when she said mulching in the springtime here was a bad idea because of slugs. She went on and on about how thick they were and how hordes of them devoured everything in sight. She insisted the problem was made worse by mulching. I thanked her for her advice, but didn’t take it. Boy, was that a mistake.
A few weeks later, while standing on my steps with a steaming cup of tea watching the rays of daybreak lifting through the mists in the pastures, I noticed what appeared to be huge drops of dew on the spinach and lettuce. Since I had never seen dew drops so big, I decided to investigate. In my pajamas and slippers, I tip-toed through the wet grass, trying in vain to keep my slippers dry. I reached the edge of the garden, preparing to gaze in wonder at the huge drops of dew glistening like translucent jewels. Wonder turned to disgust when I realized that the morning sun wasn’t bouncing off large drops of dew; it was illuminating hundreds of slugs who had made their way to the summit to wait for some heat units. The mulch had provided them with a warm, moist environment perfect for breeding, and breed they did. I haven’t mulched my greens since.
That same year we had an invasion of Colorado potato bugs. Again, mulching contributed to the problem. One potato bug fact I learned from the experience is that the bugs over-winter in the first couple of inches of soil. Mulching moderates the temperature and ensures their survival through winter. I quit mulching my potatoes, and we eventually got rid of the bugs.
As I mentioned, I do use some mulch in my garden, but not as much as I did when I lived in a different growing zone with lighter soils. Instead of applying enormous amounts of mulch in the fall, my husband drills in rye or oats or some other cover crop for garden applications to prevent soil erosion and spring heaving.
In areas where the soil is sandy or winter precipitation is low, mulching with organic mulch might be necessary to prevent wind-borne erosion.
In the end, researching how to lay mulch and which material to use is completely dependent on your gardening style and region. Like an audience lost in the nuance of a classical play, longtime gardeners know that observation is key in determining what role mulch will play in the garden.
Originally published in Countryside September / October 2008 and regularly vetted for accuracy.