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MMWD Blog: Think Blue Marin

Welcome to our blog! Written by staff at MMWD, “Think Blue Marin” explores all things water in south and central Marin—water supplies, conservation, new projects, watershed management, and more.

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Oct 02

Save the Cardboard

Posted on October 2, 2015 at 8:52 AM by Ann Vallee

by Charlene Burgi

"Save the cardboard!" is the mantra around our house every time a package arrives in a sturdy box. What becomes of all those boxes? They go into the garden as sheet mulch—a simple solution for suppressing weeds, building healthy soil, saving water, and even replacing unwanted lawns.

By saving cardboard, one becomes an aficionado of the paper product. For example, our golden retriever Sassy must eat a special dog food that is sent via USPS. The boxes containing her food are extremely heavy weight compared to the boxes that hold her sister Misty's canned dog food purchased at the grocery store. Both boxes work well for sheet mulching, but I save the heavy-duty boxes for areas that get more traffic or have a more prolific weed population. The thicker the material, the more time it will take to break down and the longer it will hold moisture.

Saving cardboard for sheet mulching is a frugal way to do the job. Another option is to purchase rolls of cardboard available at your local irrigation supply house and at some garden centers. This rolled product saves the time spent removing staples and tape from boxes, and it is much neater to install. On the other hand, I view the box method as a great way to repurpose refuse. Either method will add nutrients to the soil as the cardboard decomposes.

So why sheet mulch? And when is a good time to start? Sheet mulching is an easy way to keep weeds at bay for an extended period of time. Laying cardboard over a wetted area of weeds and covering with mulch will save you the dastardly job of pulling weeds. Even with a layer of mulch applied 3 inches thick, weeds can come up through the mulch. But by adding cardboard first you thwart light from gaining access to the soil, preventing weed seeds from germinating. The trick is to overlap the cardboard to prevent light from going into the gaps. Once the cardboard is down, top with mulch and your garden can be weed-free. Over several years, the cardboard will break down and need to be replaced about the time the mulch requires a fresh layer. The bonus is the new cardboard and mulch can go right over the top of the old.

sheet mulching
 Sheet mulching over an unwanted lawn
The best time to sheet mulch is right now. Fall is often a time when new landscaping is installed. If you are replacing a lawn, sheet mulch over the top of your lawn. Sheet mulch in a newly designed planter or area prior to installing plants. Sheet mulch before refreshing the mulch in planters. Sheet mulch natural pathways to abate weed growth. Winter is coming and weeds will start to grow—especially if we get the rains that are predicted with El Niño. Couple a few good rains and some balmy days we know to expect in February, and you have the perfect recipe for optimal weed germination and growth during a time we tend to avoid working in the garden. By sheet mulching now, you can avoid weeding then. Check out Marin Municipal Water District's updated sheet mulch page for step-by-step instructions if this concept is foreign to you.

Another reason to sheet mulch right now is MMWD and Marin Master Gardeners are hosting two free workshops on "Alternatives to Our Thirsty Lawns" October 17 in Mill Valley and October 22 in Ross. The workshops will provide valuable information on replacing lawns, modifying irrigation systems, and designing creative landscapes in lieu of lawn. Space is limited, so pre-register for the date of your choice today. Additionally, California is offering rebates up to $2 per square foot for replacing water-guzzling lawns with water-wise gardens. Add to that a rebate up to $50 from MMWD for organic mulch to help with your sheet mulch project. Fall is the perfect time for planting. Take advantage of these workshops and rebates!


Donald Gregory
October 2, 2015 at 4:04 PM
What is the difference between mulch and compost and how are they used in the backyard garden?
Charlene Burgi
October 5, 2015 at 8:28 AM
Hi Donald. Good question~Compost is organic material that decomposes into a rich organic material with the texture of fine loamy soil. Seeds can easily germinate in this medium. Mulch, on the other hand, is a layer of organic or inorganic material that is used to cover soil. Inorganic material could be rocks or rubber-like bark. Inorganic mulch does not add nutrients to the soil but will hold moisture in the ground. Organic mulch comes from living matter that has not broken down but able to continue to break down as it makes contact with the soil and elements. Just to name a few, paper, cardboard, cotton material, and bark all fall into this category. Typically, mulch is applied in a thick layer to prevent light penetrating to the soil and allowing seeds to germinate. Mulch keeps the roots cool in summer and warm in winter. Organic mulch will break down and feed the plants in that process. I hope that helps! Sincerely, Charlene

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