Garden mulch refers to anything that can be placed over your soil. I however delineate this further by categorizing some items as mulch liners, that is things that go beneath your mulch but above your soil, and then anything that sits on top is just “mulch.” People typically use mulch for a variety of reasons. It looks nice, it moderates soil temperature, it keeps weeds down and moisture in, and it can add organic matter to the soil as it decomposes.
So, among mulches you’ll have both inorganic things such as stones and organic things like leaves, woodchips, straw, pine needles, etc. Then, among mulch liners, you’ll have the familiar black plastic and landscape fabric, but also such things as cardboard and newspaper.
Now mulching might seem like the simplest task in the world, just throw some chips down on your soil and you’re good right? Well, no, you aren’t good. It really is a complicated topic and by doing it right to begin with you’ll save yourself labor down the road.
As far as mulches go, inorganic things like stone look great and they do not decompose, buy them once and they’re good forever. They also stay in place and do not attract termites. Many building inspectors will recommend stone mulch for beds that touch your foundation for this very reason. The downside of stone mulch is that it does nothing for soil quality, no organic matter is added because the rocks don’t decompose.
With organic mulches such as wood chips or leaves you get all sorts of organic matter added to the soil. Unfortunately this is from decomposition, which means you’ll need to add new mulch annually to keep enough on your beds. Decomposing wood also attracts more bugs and rodents than stone, and so it can cause pest issues.
With mulch liners, well black plastic is useless because it doesn’t let water through, so don’t use it. Nice landscape fabric is great for keeping weeds down, but if you’re growing perennials that multiply each year you’ll have a hard time digging them up for division, and the fabric can stop them from growing outward if the holes you cut aren’t big enough. Cardboard & newspaper are both great for keeping weeds down, but then decompose within 90 days. This is enough to kill preexisting weeds, but not enough for ongoing weed control.
So, with all those issues in mind, here is my guide for mulching your beds.
There are two main types of landscaping beds. The first is a bed with trees or bushes, permanent plantings, sometimes around your foundation. With this type of bed I recommend using rock mulch and landscape fabric. Bushes & trees extend from the ground at one constant point, so you can safely bring the fabric up to the trunk/stem of the plant and it isn’t going to hurt it. This means stellar weed control.
So, fabric on bottom, stones on top. Why stones? Two reasons, one, termites, you don’t want a lot of rotting wood near your foundation. The second reason is that I never never never never put organic mulch on top of landscape fabric. The reasons are many, for one the fabric keeps the organic matter from reaching the soil, so it doesn’t add organic matter to the soil at all (one benefit down). Instead what it does it add organic matter to the fabric, which means that eventually weeds will germinate on top of the fabric and some weed roots are even strong enough to dig through the fabric on their way down (another benefit gone). Finally considering you’re not getting the weed protection benefit, and you’re not getting the organic matter benefit, you might as well use something you don’t have to replace every year (aka stones).
The second type of bed is a perennial bed, or a bed with plants that you dig up often or that expand outward and multiple. With this type of bed I recommend putting down a thick layer of newspaper or cardboard first, and then wood mulch on top. This way you’ll kill any preexisting weeds, which really are the tough ones, and as the cardboard decomposes it’ll add organic matter to the soil (yum). When it comes time to dig up or move plants you won’t be fussing with fabric. Finally the woodchips will decompose, adding organic matter directly to the soil and attracting all sorts of earth worms.
Sure, once the cardboard is gone you’ll have to weed manually, but its not that big of a deal. Get them while they’re young and you’re all right. I recommend a weeding tool you can use while fully erect, a good hoe is as simple as it’ll get. Once or twice a week as you stroll through your garden use the hoe to uproot all those weed seedlings. Also by keeping your wood mulch thick, say 2 inches or so, it is a rare weed that’ll germinate to begin with.
The only time I use fabric in a perennial bed is for edging when it borders grass. If your beds are not raised, and simply have a border, grass can easily send runners under your border and into your bed. By playing fabric down under my brick borders and extending 6-8 inches into the bed I keep some of this grass running activity down.
Now of course, some beds can contain both trees and shrubs and perennials, and while mixing mulches in the same bed can look funny, you can mix underlayments without issue. If it comes down to it you can simply use fabric around trees & shrubs, and cardboard around perennials, even when they’re in the same bed.
If you’ve read this far you probably think there isn’t anymore I can say on the topic of mulch. Well you’d be wrong. There is one more piece of knowledge I wish to impart. You can change soil characteristics by using different types of mulches.
For instance, organic mulches will decompose and as they do so they'll add organic matter to the soil that will improve drainage as well as attract earth worms which will improve overall soil quality. Also when using organic mulches you can get desired affects by switching between them. For instance pine-based mulches will acidify the soil more than other mulches. So if you are mulching around plants that need acidic soil, like blueberries or rhododendrons, you should use pine, but if it is something like a clematis that likes alkaline soil, you shouldn’t.