Gardening Topic for November 2011
Sheet Mulching to Create Raised Beds

Provided by the Western Massachusetts Master Gardener Association
www.wmassmastergardeners.org.


By Anna Perreault, Master Gardener

 

 

Whether you’re starting a new bed or converting your current beds to raised beds, sheet mulching is an excellent technique to use. Also known as lasagna gardening or composting in place, sheet mulching is a no-till method to create a nutrient rich area for planting.

At the most basic level, sheet mulching is a weed barrier covered by about a foot of organic material. Because you are piling up material, a raised bed is easily created. Raised beds have many benefits over traditional garden models. They can be customized to be accessible to gardeners of all abilities. Planting can begin earlier in the spring as the soil in raised beds warms up faster than the soil at ground level. Raised beds can easily be covered to offer protection against cold, wind and pests. They also lend themselves to being equipped with watering systems such as drip irrigation systems. As the mother of a young son, they have also served the purpose of defining areas off limits to walking and digging.

Sheet mulching requires that you gather many organic materials. For your weed barrier layer, all you need is newspaper or cardboard. The rest of your materials should be easily found and free. Autumn is a great time to sheet mulch as it also serves the purpose of finding a home for your fall clean-up yard waste. You will need to be sure to have both high-carbon (sometimes referred to as brown) materials and high-nitrogen (sometimes referred to as green) materials. Brown materials include straw, leaves, sawdust, and wood chips. Green materials include manure, grass clippings, garden trimmings, and food scraps. If you can’t find enough of these materials on your own, try asking around your neighborhood. Do you know someone with a rabbit or horse? If so, ask if you can take the manure off their hands. Offer to rake a neighbor’s yard, and you’ll have a friend for life. One note of caution: be aware of the source of your materials. If someone offers you their grass clippings, it is wise to ask if they have applied any chemical treatments, especially if you plan on growing food crops.

Before beginning any new garden, it is highly recommended that a soil test be performed. The Western Massachusetts Master Gardener Association offers pH tests at local gardening events and farmers’ markets seasonally. For a more in-depth test, the University of Massachusetts at Amherst can test for nutrient levels as well as heavy metals. The results of such tests will let you know if any specific soil amendments are needed. If amendments are required, put them down on your bed before you sheet mulch.

Now the real fun can begin. While starting a new bed often means a lot weeding, digging, or rototilling, with sheet mulching you begin by smothering any existing weeds or grass. You many slash down any tall or bushy vegetation, but leave it in place and pile the cardboard or newspaper over it. One sheet of cardboard will do. Be sure to remove any staples or tape. If you opt to use newspaper, use three to four layers and be sure there are no glossy sections that may leach toxins into your soil. Many people suggest wetting the newspaper down as you go so it won’t blow away.

Now you want to start your “lasagna” layers. Just like when you build a compost pile, the key is balance. The optimum carbon to nitrogen ratio (or brown to green ration) is 30:1. While all organic matter will have specific and varying C:N ratios, a basic rule of thumb is that you want to have roughly equal amounts of brown and green materials. There are some exceptions to this, such as sawdust, which is very high in carbon and will really slow down decomposition if you add more than a dusting. But don’t let the specifics throw you off; the beauty of this process is that you can do the best with what you have and still get great results.

As you lay down you materials, be sure to spray generously with water every couple of inches. Just like a compost pile, moisture is necessary. After 8 to12 inches of these layers, top it off with a couple of inches of either finished compost or soil (if you will be planting the beds the same season), or manure or other mostly green materials (if you plan to let the bed overwinter). Because the process is mimicking cold composting as opposed to hot composting, any weed seeds will not be destroyed. Therefore, a few more inches of seed-free mulch material added to the top will really help keep weeding to a minimum. You can use straw (be sure that it really is seed-free), bark mulch, or leaves.

Fall is the optimum time to create a new bed using sheet mulching because by the time you plant in the spring, the nutrients will be more readily available to your plants. If you plan on planting immediately, you may use the compost or soil layer as the seedbed. Either planting time, you ought to dig down below where any deep- rooted plants will be placed and slash though the cardboard or newspaper barrier.

Once you have your new bed, you can expect your plants to thrive in their nutrient dense surroundings. Your worms and other helpful soil organisms will also benefit because you will not be disturbing their underground networks as you do when you make compost in one place and then dig it up to cart it to your garden. Like any mulched bed, water retention will be much greater. Now all you will have to do is take up another hobby to fill all that time that used to be taken up with watering and weeding.

Sources used: Hemenway, Toby. Gaia’s Garden: A Guide to Home-Scale Permaculture. 2nd Ed. White River Junction: Chelsea Green, 2009.

 




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Provided by the Western Massachusetts Master Gardener Association
www.wmassmastergardeners.org