The best time to apply mulch is in early spring, about the same time you remove winter protection. In areas with warmer winters, apply mulch just before your roses start to leaf out and before weeds start to sprout. You can apply mulch anytime, and you usually need to replenish it every two to three months.
If possible use organic mulch (grass, compost, wood chips, and so on) for roses because, as the mulch breaks down, it adds organic matter to the soil, improving its texture and sometimes adding nutrients. Note that if you use grass clippings, mix it with another type of organic mulch for aeration because the clippings tend to pack down and decay into slime.
Mulch also gives the ground in your garden a cleaner, more orderly appearance. Many organic mulches break down fairly rapidly, so you need to replenish them often. For effective mulching, apply a good, thick layer (at least three to four inches) of mulch in spring before the weeds start to grow. Spread it evenly under the roses, over an area slightly wider than the diameter of the plant. Or spread it over the entire rose bed. Add a fresh layer whenever the first one starts to deteriorate.
Composted manures make particularly good mulches, looking neat while adding nutrients to the soil as they break down. Just make sure that the manure is fully composted and that you don’t add too much. You can buy bags of composted manure in most nurseries and garden centers. Fresh manures contain salts that damage the plant and make its leaves look as if they’ve been burned by a blow torch. Horse manure is generally safest and chicken manure the most dangerous. Mix manure 50/50 with some other organic mulch. That way, you won’t burn your roses, but they still get some nitrogen.
Organic mulch has some downsides, so think about the materials you use and what they may do to your garden:
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Keep a close eye on soil pH and correct it accordingly if you use bark mulches, such as pine, which are quite acidic.
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Make sure that you haven’t used weed killer on your lawn if you intend to use grass clippings. The residue of weed killer can damage or kill your roses.
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Add supplemental nitrogen if you use the organic mulches. Fresh sawdust, for example, needs extra nitrogen to break down properly.
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Avoid peat moss. It can get hard and crusty when exposed to weather. Water may not penetrate it, so the water runs off instead of soaking through to the roots. At the very least, mix it with something else, such as compost.
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Avoid lightweight mulches, such as straw, if you live in a windy area. They can blow around, making a mess and leaving your roses unmulched.
Generally, though, inorganic mulches, particularly plastic, are hard to handle, especially on roses, where you need continual access to the soil for fertilizing, watering, and so on. So unless you need to heat up your garden, or like the look of plastic, steer clear of these mulches.