What is Mulch?

Mulch is a general term for any material placed on the surface of soil. In a typical garden you will see this material be mainly wood chips, rocks, or black sheets. At times it might even be a combination of several different materials to make-up a mulch as a whole. Now, for many we really only think of mulch as needed if you are doing fancy landscaping and no as an added element that can help with a garden used for eating. However, mulch has many benefits to it that an eating garden might want, such as reduce evaporation or suppress weeds. So, we are going to go over some of the basics of mulch and adding it into your garden routine.

Why Mulch Your Garden? 

There are numerous benefits to using mulch in your garden! Barren, exposed soil is prone to erosion and runoff of water and/or nutrients. It is also an easy target for fast evaporation. In contrast, applying a nice layer of mulch on top of bare soil will simultaneously enhance the soil’s ability to retain moisture, protect plants, and nurture the living soil food web below. 

Benefits of mulch:

  • A well-mulched garden will need less water, both in frequency and volume. That saves time and money, too!
  • Natural mulch materials like compost, straw, or fine bark slowly break down and eventually become incorporated into the soil, which provides fresh organic matter and nutrients. 
  • This cycling of nutrients combined with improved moisture content directly contributes to enhanced beneficial microorganism and worm activity. That leads to overall improved soil fertility! 
  • Mulching can suppress the growth of weeds – sparing your back and time. It also reduces dust and erosion. 
  • Fresh mulch adds a tidy, appealing look to a garden space.
  • Mulch insulates the soil, protecting plant roots from temperature swings. Plants are less stressed when they have steady temperatures and even moisture levels, and this helps them grow more vigorously.
  • Repurposing natural organic materials found around your yard as mulch can reduce waste. If you have a large property with many trees to prune, a wood-chipper would be an awesome investment! Even some man-made products can be up-cycled into mulch, such as using cardboard to smother weeds in garden pathways. 
  • Finally, mulch layers create habitat for beneficial insects. Take ladybugs for example. These awesome aphid predators like to overwinter in leaf piles or similar debris. However, pest insects may also hide in mulch as well. So, this is both a benefit as well as a potential drawback.
A diagram by Heidelberg Farms showing what the Soil Food Web looks like below ground. There are tree roots with compost and micro arthropods on the soil surface, with bacteria and fungi, mycorrhizae, and nematodes and protozoa below the soil surface, in and around the tree roots.
Image Courtesy of Heidelberg Farms via Pinterest

How to Apply Mulch: Best Practices

The general rule of thumb is to spread mulch about two to four inches thick over the soil surface. When mulch is too thick, it can be counterproductive and prevent water from reaching the soil. On the flip side, too thin a layer of mulch may not adequately stop weed growth or prevent evaporation as intended. After mulching, you could water it in to help it settle in place (though it isn’t required). Keep in mind the depth recommendation can vary slightly – depending on the type of mulch used, the area that it is being applied to, and what type of plants are growing there. For instance, some folks use even deeper layers of straw mulch since it is very light, starts out quite fluffy, and compacts over time.

Preparing the surface

You may need to do some groundwork before mulching (though not always). If weed suppression is your goal and weeds are already present, it is best to thoroughly weed the area before applying mulch. Or, if you’re dealing with incredibly invasive and persistent weeds in open areas such as pathways, consider laying down a layer of cardboard or longer-lasting synthetic material like geo-textile weed barrier fabric first. Then, spread a more natural and attractive-looking mulch material on top. 

When to avoid mulch (or go light on it) 

Avoid piling mulch right up against the stems of plants or tree trunks. Constant contact with moist mulch could cause tender plant stalks to rot. It may also make plants more susceptible to attack by slugs or other pests hiding in the mulch. This is especially true for freshly planted young seedlings. Therefore, we usually keep our garden mulch pulled back by at least a couple inches around the base of plants in our raised garden beds (or at least until they mature and become more hardy). For trees, it is best to maintain about a 1-foot open ring of space around the tree trunk, or 4 to 6 inches from each side. 

If you’re growing bulbs or perennials that die back in the winter, lightly pull back any deep mulch layers in the springtime to promote faster growth. Also, just as mulch does a great job at smothering emerging weeds, keep in mind that it can do the same to your sprouting seeds. Therefore, you would not want to direct-sow a bunch of veggie seeds in a garden bed and then heavily mulch over it. Wait to apply thick mulch until after everything has had a chance to sprout and grow a few inches tall. 

When to Apply Mulch

The best time of year to apply mulch is in early to mid-spring, and again in the fall. 

Mulching the garden in spring gives everything a fresh look and helps prepare the soil for the growing season ahead. Getting ahead of the emerging spring weeds is ideal, thwarting their efforts to pop up as the soil warms. If you also like to add slow-release granular fertilizer to your garden in the spring, I suggest sprinkling that over the soil surface first. Then, spread your new mulch and water everything in. In a veggie bed, wait to heavily mulch until after any directly-sown seeds have sprouted. However, a thin layer of mulch can help prevent the top soil from drying out and aid in seed germination. 

In the fall, a fresh layer of mulch prepares your landscape for the cold winter months ahead. A thick, warm, fuzzy blanket of mulch will insulate the soil and protect plant roots from freezing cold and snow. It will also prevent soil from washing away with excessive rain. As the organic mulch materials sit, mellow, and decompose over winter, they release nutrients and nourish the soil food web in preparation for spring.

Not all mulch will need to be refreshed that often. Mulch that decomposes slower may only need to be replenished once per year, or less. Or, if it has slightly compacted or only partially broken down, consider adding only a light 1-inch layer instead of a full new 2 or 3 inches.

Types of Garden Mulch

As you look around our garden, there is virtually no exposed soil to be found. Every “open” area is covered with mulch – be it gravel, bark, compost, or even living mulch provided by sprawling plants. Mulch can (and should!) be applied to a wide variety of garden spaces. For example, in vegetable beds, established perennial areas, herb or flower beds, pathways, around shrubs or under trees, and even in potted plants. However, you may want to use different types of mulch for distinct areas! 

See a list of popular garden mulch options below.

Common materials used for mulch:

  • Straw or hay. Opt for seed-free straw or hay. Otherwise, it can sprout and create even more “weeds” in your garden. Note that some straw or hay may be sprayed with herbicides. Inquire to your local supplier about their cultivation practices, or purchase organic straw. 
  • Bark, including chunky bark, fine bark, or shredded bark. You can buy bagged bark products, or order bulk bark from a landscape supply company. Another option is to call up your local tree-trimmer to see if they have any wood chips to spare! I highly suggest using natural bark, not treated or dyed bark products. Note: There is a common misconception that bark or other wood products ‘rob’ nitrogen from the soil. While it is true that it does draw up some nitrogen, it is small and temporary. I am not personally concerned about it.  As long as you’re routinely replenishing soil with mild fertilizer and compost (especially for hungry vegetable garden beds), the mulch eventually breaks down to feed the soil anyways.  
  • Compost (and manure) – either homemade compost, bagged or bulk compost, or composted manures. The fine texture and nutrient density of compost makes it a wonderful mulch option for vegetable garden beds, though not quite as long-lasting for broader areas. Keep in mind that fresh animal manure is not the same as well-aged composted manures. Fresh manure can be too rich in nitrogen and potentially harm plants.
  • Leaves or shredded leaves (leaf mold). Dense leaf layers do an excellent job at smothering weeds. However, full size leaves may create a mat over the soil that doesn’t allow sufficient water and air exchange to the soil. Therefore, it is best to use shredded leaves as mulch. An easy way to shred leaves is to go over them with your lawn mower. Then, it’s all collected and chopped up into the perfect small pieces for mulch.
  • Rock or Gravel. Pea gravel, river rock, crushed granite… There are many choices when it comes to landscaping with rock. Rock isn’t a great option inside garden beds. However, it is awesome for other open spaces, pathways, between garden beds, or in xeriscaping! It’s durable and looks sharp. 
  • Other green or dried plant matter. Plant matter such as cut cover crops, pine needles, or dry grass clippings also make excellent garden mulch. We don’t have access to pine needles or grass, but love to collect various plant materials from around the garden for mulch. Especially nutrient-rich plants like comfrey, yarrow, borage leaves, and fava bean stalks or leaves. Chop them into smaller pieces before spreading over the soil surface.
  • Paper products. Cardboard and newspaper can create an effective ‘sheet mulch’ layer, spread on the soil surface alone or with other natural mulch materials piled on top.  Cardboard is ideal for large spaces, under new raised beds, or in pathways. You can use newspaper directly in garden beds around plants – ideally covered in a layer of topsoil or other material to keep it damp and from blowing away.
  • Synthetic ground cover.  There are several types of man-made mulch products available, such as black plastic sheeting or woven geotextile weed barrier fabric. While they will not break down and contribute to soil fertility like organic garden mulch, synthetic products are more durable – and sometimes more effective. That is, if you choose high quality ones that don’t readily rip and tear, like some do! Also seek out materials that are readily permeable to water. Otherwise, plastic sheeting can cause pooling or drainage issues. We use commercial-duty weed barrier fabric below our raised beds and gravel pathway areas. It does a great job at preventing the noxious crabgrass and weeds that were once present from invading. 

What type of garden mulch should I use?

The kind of mulch material you choose depends on your individual needs, space, style, budget, and the availability of materials in your area. Furthermore, every type of mulch comes with a unique set of pros and cons.

In general, chunky durable mulch materials like gravel or large bark are best suited for general landscaping around shrubs and trees or pathways. On the other hand, finer organic materials such as compost, straw, shredded leaves, and dainty bark or wood chips make ideal mulch for vegetable garden beds. There, plants are rotated frequently, and you can replenish the mulch once or twice per year as it breaks down and becomes part of the soil biomass. If you use thicker mulch materials in veggie beds, simply plan to push it aside and or work around it when replanting the beds.

Source: Homestead and Chill: Mulching 101


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