Sheet mulching is a fantastic - and quick - way to start a new garden packed with native plants! Sheet mulching can be used to convert a small (or large) turf-lawn areas into a tidy, blank canvas for creating a new flower bed or food garden.
Sheet mulching can also be successfully used to build the soil profile of a site that is difficult to work with, has been degraded or that is very weedy. The incredible thing is that it is super easy to do (no digging!), uses up recyclable materials and makes excellent use of natural materials that you likely already have on hand!
The method of sheet mulching is not new. It has been around for as long as people have been growing plants. Essentially, it is a method of smothering unwanted plants using layers of highly compostable, biodegradable materials. As the layered-up materials decompose, they leave behind an enriched soil that will provide fertile growing conditions for new crops. This also helps to kill off grasses and annual weeds (like Garlic Mustard, Alliaria petiolata) quite easily. In Southern Ontario, you may still need to keep a close lookout for Creeping Charlie, Glechoma hederacea, and Field Bindweed, Convolvulus arvensis. These are two difficult weeds that will likely find their way up through the sheet mulch but can be weakened by continual hand-pulling and the smothering of the sheet mulch layers (scroll down to find out more about invasive species and sheet mulching)!
It can take around one full year, sometimes longer, for a sheet mulched area to decompose and be ready for planting food crops. However, when planting pollinator gardens using perennial wildflowers - like milkweed - trees and shrubs, you can plant into the sheet mulched area immediately if you want to! All you have to do is move the mulch aside, cut a hole into the cardboard, plant, then replace the sheet mulch layers and mulch. You can keep an eye around the base of the new plants to ensure that weeds do not find their way up through the opening in the cardboard before they are smothered.
This is the simplest version of sheet mulching, and it works very well for Ontario native plants! Most perennial wildflower and grass species do not require extremely nutrient-rich soils in which to grow. They will however, appreciate some organic matter and for the most part, having good water-retention is a trait most plants like in their soils.
Basically, aim to use up as much of the biomass (organic material like grass clippings, green plants, wood chips, chopped stems and twigs, used potting soil, weeds with seed heads removed etc.) that you have available on the property where you are installing the new garden.
Cardboard, not newspaper or yard waste bags, is essential for this method. Paper will decompose too quickly to be an effective weed barrier!
This method is a must if you are planting a vegetable garden or are hoping to include native plants that grow in a forest-setting (woodland gardens) where the soils are richer and more nutrient-dense. If you are working with a seriously degraded landscape site, this method will also be beneficial for really helping to improve the quality of the soil.
With this sheet mulching method, you can make every layer different, or keep repeating the same few "ingredients" over and over! It depends on what you have available to use!
The drawing below is just one example of many combinations that could build your garden lasagna. Just try to keep in mind the carbon-rich, nitrogen-rich layering system which will help with evening out the rate of decomposition. See the lists in the drawing above for some ideas of carbon-rich versus nitrogen-rich materials.
The "Minimalist" Method is an illustration of Sheet Mulching, Method 1, described above.
This is an illustated example of Sheet Mulching, Method 2, described in more detail above!
The Fungal Network
Perhaps one of the single most convincing arguments for sheet mulching, or "no-till" gardening, is in defense of the precious network at work beneath our feet. An entire universe of microorganisms are at work within the soil. We are only beginning to scratch the surface of understanding the intricacies and interdependent systems and cycles that make soil a living, breathing, moving entity. Our soils are often misunderstood, often overlooked and quite profoundly underappreciated.
When you sheet mulch, you do not have to dig up and disturb the soil microorganisms that already exist in your gardens. Instead, you add so much fuel for them to continue to feast upon, increasing their reach and powers: powers which we are only beginning to understand. If you have ever pulled up a clump of soil and seen a network of ‘veins’ resembling a central nervous system of interconnected webs, you have seen the fungal network. Helping trees and plants to communicate with one another and possibly delegating, exchanging and managing carbon and other nutrients, the fungal network - or “the wood wide web” - is the life network of our soils.
Texture and Tilth
An easy and highly effective way to improve soil quality in your gardens is to use leaf mould. In the autumn, leaves can be piled up into leaf compost heaps that will decompose into rich hummus which can be added back onto gardens. Or, you can even leave the leaves where they fall and where they blow, if your property is amenable to this. If you have lots of high-wind, you may need to contain the leaves if you want to try to compost them. This is similar to the nutrient-rich, robust, micro-organism rich soil that you would find on the forest floor.
As leaves (and other organic matter) decompose, they produce organic acids, like humic acid, which improves a soil’s ability to aggregate or ‘stick together’. With a high carbon content, humic acid feeds soil microbes and makes it easier for a soil to hold onto nutrients that are needed for plant growth, instead of seeing them leach away. This is called the “cation exchange capacity” or CEC of a soil. For more information about the CEC, you can watch “Cation Exchange” (minus the ending spiel about chemical fertilizers that are unnecessary in a sustainable ecologically-planned and supported garden!).
The Soil Microbiome
Gardeners should always compost the biomass from their own property - tree leaves, vegetable garden remains, grass clippings, plant cuttings - instead of letting it be hauled away in leaf and yard waste bags. Some may argue that this spreads plant pathogens from year to year. However, when you build a strong, biodiverse garden with lots of plants and insect life, these types of pest problems tend to find ways to balance themselves out naturally.
Pesticides and herbicides become unnecessary in a garden where predatory insects make their home. For example, there are so many different insects that will prey on aphids (i.e. predatory wasps, lacewing and hoverfly larvae, lady beetles to name a few!). Planting native plants, like milkweed species, is a great way to attract these species! If you remove the biomass of the plants (fallen leaves and spent perennial stems and leaves), you remove habitat of overwintering species and microbial life. You end up robbing the system of a lot of its opportunity to take care of itself by cycling important nutrients and microorganisms back into the site.
Leaf mould is also excellent for improving soils’ tilth which refers to its ability to both percolate and hold water, to hold its shape without compacting and to be friendly for root growth (with good aeration) and nutrient absorption. Additionally, leaf mould is rich in microorganisms! We are just starting to understand the complex interrelationships between soil life and plants, but we do know that soils that harbour lots of micro-lifeforms are healthy soils!
Soil microorganisms work to decompose organic matter into nutrients that plants can use, as well as helping plants to communicate with one another. Leaf mould is one type of compost that is used to inoculate soil (much like how humans can take a probiotic supplement to inoculate the digestive system with beneficial microorganisms). “How your fallen leaves can be used to fight climate change” is an article from CBC News, written in November 2021, which discusses the immense fungal network that is present in leaf mould.
Leaf mould is highly effective for:
Plus, the application rate is only 2 kg per hectare as opposed to regular compost which is applied at a rate of tonnes per acre!!! (1 hectare = 2.47 acres)
For further reading, check out:
Suzanne Simard, “How Trees Talk to Each Other” https://www.ted.com/talks/suzanne_simard_how_trees_talk_to_each_other/transcript?language=en
To learn more about mycorrhizae, watch “The Earth’s Internet: How Fungi Help Plants Communicate.” https://www.youtube.com/watch?v=_tjt8WT5mRs
National Forest Foundation. 2021. Underground Networking: The Amazing Connections Beneath Your Feet. https://www.nationalforests.org/blog/underground-mycorrhizal-network
Miller, Robyn. (2021). “How Your Fallen Leaves Can Be Used to Fight Climate Change.” https://www.cbc.ca/news/canada/ottawa/how-fungal-dominant-compost-can-heal-soil-protect-environment-1.6237228
Sheet mulching can be a highly effective way to get the upper-hand on controlling invasive species at your site. It helps to smother weedy plants and weaken those that still manage to find their way to the surface.
Field Bindweed, Convolvulus arvensis, for example, can have roots that reach more than ten feet underground. It is resistant to many chemical herbicides and nearly impossible to remove mechanically. Even if you manage to dig down to extract all the roots (which would be incredibly difficult to do) any small broken pieces of rhizome will regrow anyway. Sheet mulching deprives undesirable species, like Bindweed, of light and energy, while allowing native plants a chance to establish a strong foothold with less competition.
Once invasive species do re-surface (yes, they always do), they are easier to control because they have likely used up a great amount of their reserved energy. After pulling the re-surfacing weeds a few times they will become very weak and may no longer be able to compete with the native plant community that has had a good chance to establish.
With a species like Garlic Mustard, Alliaria petiolata, the biennial life cycle of the plant will be interrupted by the solarization process and the new seed bank in the soil will not have a chance to germinate. It is possible to eradicate garlic mustard from a large area in this manner. For further reading about invasive plants in Ontario, please visit the Ontario Invasive Plant Council, especially the “Resources” section.
Fall and spring are the ideal times of year to start a sheet mulching project to shrink your lawn in favour of habitat for more productive (and beautiful!) native plants.
You can still sheet mulch in the summer as well, but try to work after a heavy rain or water the area deeply with a sprinkler before beginning. Moisture is a key component in the decomposition process and it will be hard for moisture to get down through all the layers after you finish.
Wetting the cardboard before you layer on top of it is an optional, but helpful, step.
Before beginning your sheet mulching layers, mow the existing vegetation as low as you can. Leave the clippings in place.
Try to identify the weeds that are present. Sheet mulching is a great way to get rid of Garlic Mustard! But, monitor for its reemergence if there are other Garlic Mustard plants nearby that are flowering. You don’t want to let it re-establish in your new garden of native plants!
Weeds like Creeping Charlie and Field Bindweed will continue to need control as they will likely re-surface even through many layers of sheet mulching.
You can get lots of cardboard very easily if you collect from grocery stores and recycling centres. Just remember to remove tape from the boxes as it won’t decompose.
Overlap the cardboard edges by at least 10 cm - there is no such thing as "left-over" cardboard when you are sheet mulching! Use it all!
If you want to directly seed a sheet mulched area, wait at least one full year before doing so.
If you are planting any large clumps of native plants, or have trees or shrubs to add, you can plant them first, before sheet mulching the area around them. Don’t pile up sheet mulch layers high around the base of a tree; limit the depth of the mulching layers near the trunk of the tree (the sheet mulch layers will shrink over time and be closer to the original ground level you began with).
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