There is much written about mulch, and there are many differing opinions. I don’t think there is definitive right or wrong. A lot of it comes down to personal choice. Some people like compost, some like chunky bark, others like fine bark, some like stone, some like shredded rubber mulch (please don’t use that!) Some use NO mulch on ornamental beds because they hate ‘bark’, and think that all mulch is bark. There are many types of mulch. Let’s find out why mulch is a good thing.
What Is Mulch?
It is really anything that covers the bare ground. It can be inorganic like plastic mulch that is used to warm up veggie beds sooner in the spring, and stones or rocks. Or it can be organic like bark, grass clippings, leaves, pine needles, straw, newspapers, nut shells, etc.
Mulch has 3 main functions: inhibit weed growth, retain moisture (or aid in drainage), moderate soil temperature.
Over a period of time all organic mulches break down to form compost which helps the soil retain moisture or drain better, improve the soil’s texture and improves fertility.
Most all organic and inorganic mulches can do the 3 main mulch functions (to greater or lesser degrees), and it’s personal preference as to what looks best. But because most inorganic mulches can’t eventually enrich the soil in a meaningful capacity, and because over time in our climate stones and gravel become weed-ridden, I reject them for most garden beds. I reserve them for special effects like dry streams, water features, edging, decorative elements, paths, and rockeries. (Note: stones & gravel get hot and transfer that heat into the soil. During a hot, dry period, this heat can kill plants in the soil below.)
To be good at its first main function, mulch should be high in carbon content. The items in the second paragraph fit that bill. Since seeds (weed seeds included) require water and nitrogen to germinate, any mulch that is high in carbon and not yet decomposed (composted) has it’s nitrogen content locked up and unavailable to seeds and plant roots. The nitrogen is released through the composting process and that happens first where the mulch layer touches the bare ground (or more compost). There is where the soil microbes go to work converting the high-carbon organic matter (that doesn’t grow stuff well) into high-nitrogen organic matter (that DOES grow stuff well).
Important note: Tilling fresh woody (high-carbon) substances into the soil robs the soil of nitrogen because nitrogen is required to make the composting process happen. Only fully composted mulch should be tilled into the soil, and if it is fully composted, it is no longer called mulch, it is called compost. That is not an absolute, but for the beginner ornamental gardener, it’s ALMOST always a rule. So I just like to teach that it’s an absolute. Don’t till un-composted woody mulch into the soil.
What Is Compost?
Compost is decomposed organic matter. Simple as that. Over time organic matter (mulch) turns into compost.
Adding compost to clay soil improves drainage. Adding compost to sandy soils improves water retention. Adding compost to most soils provides nutrients, increases soil microbial activity, and encourages earthworms to do their helpful thing. Compost is one of nature’s miracles! The end result of the composting process is called humus. It is the smallest particle size left at the end of composting. Humus can hold many times it’s weight of both water and plant nutrients. So you want it in the soil in healthy amounts because it keeps your plants watered and fed. This is why we want compost to the soil.
Use Fall Leaves as Mulch
Autumn leaves make great mulch (with the exception of Black Walnut (Juglans nigra L.) and Butternut (Juglans cinerea L.) — see OSU Extension’s fact sheet on why at: http://ohioline.osu.edu/hyg-fact/1000/1148.html). Also, diseased leaves should not be composted. And some other leaves are so dense or hard they take a very long time to compost. If free of disease, these larger or larger or evergreen leaves can be shredded to make fine mulch: cherry, oak, magnolia, big-leaf-maple, rhododendron, plane tree, etc. If leaves are diseased, leaves should be recycled in your yard waste toter or taken to a yard waste recycling facility where they compost on an industrial scale and are sure to kill disease pathogens.
An interesting aside: Fall leaves have the correct balance of ‘greens’ and ‘browns’ in them naturally to make perfect compost. So if you’ve been afraid to make your own compost, start in the fall with leaves and nothing else. It will show you the color, texture and aroma that compost is supposed to have. Great science project!
Lazy gardeners, like me, just leave the fallen leaves where they are in our gardens beds. They provide a year of weed-preventing, moisture-retaining, temperature-moderating mulch and then become compost as they decompose, feeding my plants along the way. In open areas, they do tend to blow away, so just leaving them lay might not be practical for everyone. Though, if you shred your dried leaves and spread them on the garden, they will stay in place. [Here is some info to get you started shredding your own leaves: http://www.leafmulcher.com/, the same product with Craftsman® name and higher price tag is at Sears.]
Here’s another way to shred leaves: Spread the leaves on your lawn when the grass is not too tall. Mow. Dump what fills in the bag directly onto your ornamental beds. This method adds some additional ‘green’ to the already perfect leaves increasing the nitrogen content, but not enough to make an offensive odor. Don’t till it in because it’s not yet composted.
Keep ALL mulch and compost from touching the trunks of trees and shrubs to avoid rot. Over the winter, the worms and soil microbes will compost the leaves for you, right in place on your garden beds. It just doesn’t get easier than that.
When To Use Compost. When To Use Mulch.
I’m not going to go in to how to make compost here (that is a whole other lecture), but let’s clear up what might cause a bit of confusion. The words compost and mulch are sometimes, but not always, interchangeable terms. Compost can almost always be a mulch (though I don’t know why one would really want to do that), but a lot of types of mulch are absolutely NOT compost. No inorganic mulches are compost. Cedar chips are mulch, not compost. ‘Fresh’ bark is mulch, not compost. Partially-composted hemlock bark is mulch that is ALMOST composted.
Compost is used to immediately feed the soil. And in our climate, it does not inhibit weed growth as fresh bark (woody) mulch would. It is good, rich soil ready to grow stuff. Put down a layer of compost 1/2-1” deep if your soil needs nutrients. Then cover that with 2-3” of a carbon-based, woody mulch (like bark chips or fall leaves, etc). Since the mulch has not composted, it doesn’t provide a favorable place for weeds to germinate. The mulch covers up the compost and helps it hold moisture while the microbes are mixing the compost into the soil below, thus feeding the plants in the garden.
Over time the mulch composts. If you start to notice you are getting weeds, or you can see some bare soil, it’s PAST time for more mulch. If your plants aren’t doing well or you can see bare ground (no black compost, you could apply another layer of compost before topping off with mulch. But once you have a stable garden environment, you shouldn’t really need to add more compost. Just keep topping off the existing layer of mulch (or compost if the mulch layer has fully composted) with more mulch. However, don’t apply an overly thick layer of mulch in garden beds. That can damage plants. A total of about 4″ is good in most situations.
Remember: You can cover compost with mulch, but never cover mulch with compost. Doing so robs nitrogen from the soil making it unavailable to the plants. If you MUST cover mulch with compost, then be prepared to apply some nitrogen fertilizer to your garden for a year or more.
There is debate in gardening circles about which is the better season for applying compost and mulch. I say fall because that is when we have a ready supply of freshly fallen leaves. Mother Nature’s mulch free for the taking. All you have to do is plant enough deciduous plants on your property to get enough leaves never to have to buy bark mulch again.
If your mulch layer is too thin, it might mean you didn’t have a thick enough layer of fall leaves down. Take that into consideration the next fall. And if the mulch layer is too thin, add some partially-composted hemlock bark. It will closest resemble what is already mostly composted, and break down quickly enough to be covered by the next crop of fall leaves. Remember, the mulch helps retain moisture, so it’s best to keep a good layer before the soil dries out.
Types of Bark Mulch
If I lived in an area where I had unlimited supply of fall leaves, I’d never buy bark mulch. But because bark is what most people around here choose to use, and it’s readily available, let’s talk about different types of mulch.
Bark comes in a range of colors and chunk sizes. It makes good mulch in the Pacific Northwest. The larger the chunk size, the slower it will break down. So if you need to enrich the soil quickly, use bark ‘dust’ or partially-composted bark. If you already have a good layer of compost on top of the bare soil, then you can use a larger ‘nugget’ bark.
Cedar or other wood chips are fine, and take a long time to break down. I’d only use them if I already had good soil below. Cedar chips are great for dog runs, hiking trails and walkways, and kids’ play areas (also the flooring around your raised veggie beds).
In the PNW, we generally find 2 types of bark mulch available for purchase. Fir bark and hemlock bark. Fir bark is reddish colored bark in a range of nugget sizes. The very-dark-brown stuff that is very fine-textured is usually partially-composted hemlock bark, though it may be a blend of yard and/or forest products. I’ve also seen ‘bright’ hemlock recently, more the color of fir bark.
The beautiful, dark color of partially-composted hemlock bark (which is sliverless) is what shows you that it’s partially composted. So it will break down completely into compost quickly and need to be replenished more often than nugget bark.
Yes, there are other organic mulches (pine needles, cocoa hulls, etc.), but most aren’t found in abundance around here, and they are expensive. So that makes the local choices between fir bark, hemlock bark. I like partially-composted hemlock bark because it is ‘sliverless’. Though no wood is completely sliver-free, hemlock bark has so many fewer slivers than fir bark that you can almost spread it with bare hands. Almost all hemlock bark available in our Portland, OR / Vancouver, WA area is partially composted. This composting is why it’s such a rich dark color. And I like the color because it’s very natural looking and the same color as compost making it a good choice if you didn’t get enough leaves down and are just applying a bit of it to hold you over until the next fall.
None of the other ‘fresh’ barks are this same dark color, so you KNOW they aren’t started on their way to being compost.
How Much Is Needed
The smaller the mulch’s particles, the faster it composts. With smaller particles, you need less mulch depth to keep weeds from getting to the compost or bare earth below. Because the mulch particles settle more closely against each other, they create smaller and fewer holes for seeds to fall through, thus making a better cover. Larger chunk bark lays more loosely causing large holes. It takes a thicker cover of chunk bark to keep weeds at bay. Also, apply thicker if your garden area is in the sun; thinner if it’s in the shade.
As a general rule, you need about 4” depth of larger chunk bark; or about 2” depth of finely textured bark will do the same coverage job. So you will use less by volume of the partially-composted bark to get the job done. And if you use fall leaves, you’ll likely need more like 6″ to get you started. If some leaves are still there the next fall, maybe you don’t need so many that year. But remember Mother Nature’s example: the forest. No one cleans up the fallen leaves in the forest and there are never too many. :)
When to Spread Mulch
It really doesn’t matter when you lay mulch. I mentioned before that some gardeners spread it either spring or fall. I like fall. But by all means, if you need more cover right now, put some down! Bare ground or compost is prime growing medium for weeds, so the sooner you can get it covered with mulch, the sooner you can reduce your weeding chores. (We know a gardener is never REALLY free from weeding, but that’s another lecture.) For an established garden, I touch up the mulch coverage in late spring or summer, or after I’ve renovated quite a bit.
If I want to plant something and mulch is already down, I scrape the mulch away from the planting area, then dig the hole, plant, then re-spread the mulch back into place at the end of the job. Be sure to take care that the mulch doesn’t touch the bark if it’s a woody plant.
DO NOT put fresh bark or mulch into the planting hole and mix it with the dirt. Only put finished compost into the hole. Partially-composted bark mulch is ALMOST compost, but it’s not close enough. You can tell when compost is finished, it is when you can not identify anything in the compost as it’s original form. No leaves, no pieces of wood, etc. And it will not smell like rot or at all like the original ingredients. If you can ID pieces and smell wood or bark or grass, etc, it’s not finished. It can go on top of the soil, but not be incorporated into the soil at planting time.
To Wrap Up
I am a lazy gardener. But I also want good soil and healthy plants. So I use employ a constant layer of woody, carbon-based mulch because it helps prevent weeds, helps conserve water, and helps moderate the soil’s temperature. When I start a new garden bed, I am committed to starting with a thin layer of finished compost covered with a thick layer of mulch. And my mulch of choice is fall leaves.