Adding mulch to the garden should be a staple on every gardener’s to-do list. It’s a fantastic way to beat weeds and retain moisture. But, being an organic material, mulch does break down and needs to be replaced over the years. As the mulch decomposes, you need to add more to keep an adequate layer on your garden beds.
As you prepare to replenish the mulch in your garden, you might wonder if you should first remove the old much before replacing it with new.
Not quite, so save yourself some work.
There is no need to remove old mulch before adding new. Leave the old mulch in place to continue decomposing, which adds healthy organic matter to the soil. New mulch can be placed directly on top of the old mulch to reach the optimal depth of 2-3 inches.
In the process of adding new mulch to the garden, you might not have more of the same type of mulch you used before. It’s just fine to mix and match mulch types in order to get enough coverage.
If you aren’t sure what other types of mulch you can use, I’ve got a breakdown of the best organic mulches for your garden, along with some pros and cons of each in this episode of my podcast, Organic Gardening For Beginners.
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Leave the old mulch in place for added benefit
Leaving old much on the garden bed from year to year will almost always benefit your garden. The exception is if there seems to be disease in your mulch, but we’ll get to that in a moment.
Healthy mulch contributes to the amount of organic matter in your garden, and organic matter is one of the most essential components of soil. Basically, it’s any plant matter that has broken down and composted in place, such as plant leaves, roots, or in this case, mulch.
The organic matter adds nutrients, retains water, and builds fluffy soil that is easy for plant roots to push through. As mulch breaks down over time, it creates more of this beneficial organic matter. If you remove old mulch, you’re missing a shortcut to building new, healthy soil.
Leaving old mulch in place also prevents having bare soil in the garden. Bare soil is exposed to all the weather elements, from the sun drying out and hardening the surface to the rain washing away precious garden soil and leaching nutrients.
If you aren’t yet ready to place down fresh mulch, the old layer is a great placeholder to cover and protect the soil.
How can you tell if mulch went bad?
If you start to see mold or fungus growing in your mulch, your first thought might be that it has gone bad and needs to be removed and replaced. Not to worry, though, as both mold and fungus are natural parts of the decomposition process.
A common type of mold that develops on organic mulches such as wood chips is called slime mold. It forms a big bubble on the surface and is orange or yellow. It may look gross, but it isn’t harmful if you touch it accidentally or if it’s touching your plants. In a few days, it will dry out and slowly break down and disappear.
Mushrooms popping up in your mulch are a sure sign of fungus making itself at home in your garden. This, too, is beneficial to the garden as it introduces more plant diversity, nutrient-producing mycelium (sort of like mushroom roots), and more organic matter.
If you want to learn more about fungus and mold in mulch, Penn State has a helpful guide that you can reference.
If you happen to have bags of mulch sitting around, this is one instance where the mulch might have gone bad. If you open the bag and the mulch is slimy or sour or sulfuric smelling, then there is most likely too much moisture trapped inside the bag as the compost decomposes.
This mulch can still be used, but it needs to dry before putting it on your garden beds to avoid harming your plants. To do so, spread the mulch out in a thin layer on top of a tarp or piece of plastic. This will make it easy to drag over to your garden beds when it’s ready.
Leave the mulch spread out for a few days. If it gets rained on, that’s okay since it’s more or less rinsing the mulch for you. Let it continue drying out until it no longer smells sour, then apply it to your garden bed as usual.
When should I add more mulch over the old layer?
Add more mulch to your garden when the current level has broken down or thinned out too much to be effective. If the mulch is too decomposed or too thin a layer, it won’t be efficient at blocking weeds or retaining moisture. Add the new layer over the old one until the mulch is 2-3 inches deep.
Spring: Spring is a great time to add more mulch to the garden. It will help settle in your plants by providing insulation to the new roots, holding in water, and preventing weeds right from the start.
If you have any overwintered plants or have already done your spring planting, be sure to leave some space between the mulch and the stems of your plants. You don’t want to snug the mulch right up to the stems because it could lead to rot and hold in too much moisture directly in the root zone.
Leave a two-inch diameter circle around each plant, and they will be all set.
Fall: Another great time to add mulch is in the fall. This can be a perfect time to reapply mulch if you use leaves as your material. With the abundance of leaves falling, you’ll have a virtually limitless amount of mulch.
If you have access to more leaves than you currently need, try storing some in plastic garbage bags for spring application. Just make sure the leaves are not wet when you bag them, or they will likely start to rot and get slimy over the winter.
They can also get compacted by sticking together. Dumping the clumped leaves onto your garden beds can prevent water from penetrating through to the soil. Save yourself the headache and make sure the leaves are dry before storing them.
There are more garden chores along with replenishing your mulch that will keep your garden healthy and productive all season long. For ideas, check out this post, 6 Summer Chores To Keep Your Cut Flower Garden Performing.