How often should you remove and replace mulch? The short answer is rarely. But you may need to replenish organic mulches because they break down over time. And you may need to remove non-organic mulches like stones, marble chips or lava rock to landscape or excavate — or because your dog is eating your mulch. But actually raking up mulch and replacing it for any other reason? Not necessary.
That said, you may need to replenish or reinvigorate organic mulches from time to time.
As mentioned, organic mulches break down over time. This is actually good, as it feeds the soil and helps it hold moisture and nutrients. It also encourages beneficial soil organisms. But it does require replenishing every three or four years.
Mulch also needs to be replenished when some of it washes away. This is more common when mulching a slope or an area near a drainpipe. Lightweight bark chips are especially prone to washing away, whereas shredded mulch tends to hold its ground better.
If you’re mulching with shredded leaves, they will break down faster than wood chips and will need to be replenished annually in most cases. (Oak leaves are slower to decompose and you may get a couple of years out of them.) Fortunately, there are usually plenty of leaves available for that task.
Organic mulches fade over time, turning grayish. This is not a problem for most people, but some prefer fresh mulch because it’s more colorful. Others use dyed mulches, which also fade but not to the same degree. One caveat with dyed mulches: They can leach chemicals into the soil and chase away earthworms. When in doubt, go with natural.
Cedar and cypress are rot-resistant, break down more slowly than other organic mulches and are less attractive to insects. Many municipalities offer free wood chip mulch, but its quality varies and some have debris from foliage and pine needles. It’s biodegradable and not usually a problem, but some people think it mars the clean look of the mulch.
The most common mulching mistake: Putting down too much. An overly thick layer of mulch can prevent rainwater from reaching plants, especially during lighter rainfalls. And thickly applied organic mulches can stick together, become moldy and decompose anaerobically — without oxygen — releasing chemicals harmful to plants.
Avoid problems by laying down only about two inches of mulch at a time. Keep it away from plant stems because it can cause rot and invite insects. It’s a good idea to leave a shallow saucer of mulch to collect irrigation water and direct it to the roots.
One other point: Fresh wood chips can cause a temporary shortage of nitrogen, which is more noticeable when you’re mulching annuals and perennials than woody plants. Consider applying a high-nitrogen fertilizer to compensate.
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