Why put down a layer of mulch?
1. Insulates the soil and roots.
2. Aids in water retention.
3. Makes weeding easier.
4. Improves the soil.
5. "Fixes" soil pH.
6. Aids in disease control.
Mulch helps to insulate the soil, reducing temperature extremes. During July and August, in the heat of the day the soil just below the mulch, where the tender feeder roots are, can be as much as 10 to 15 cooler. Come fall, an unexpected freeze will not kill the mulch protected feeder roots.
A layer of mulch helps to reduce evaporation of the soil's moisture. This is a big deal here in West TN from late May through mid-September when the temperature is scorching hot, rains are infrequent, and watering can take so much time.
Any weeds that sprout will have to reach all the way through the mulch to get a firm purchase on the soil, leaving plenty of time for you to notice the weed and oh so easily pluck it from the mulch before it's roots even get close to the soil.
As organic mulch decays nutrients are released into the soil. More importantly, organic mulch provides food for worms and microorganisms that release more nutrients and loosen the soil, helping to turn our clay and the mulch into rich black soil.
Organic mulch can "fix" the soil's pH so when fertilizers are added or depleted, the pH will not fluctuate as much.
Splashing water can spread many rose diseases such as blackspot and powdery mildew. When diseased leaves fall to the ground and are hit by falling water, there is a splash that can carry disease spores up onto the plant where the spores will germinate and infect. Mulch helps to absorb and disperse the impact of falling water, reducing the opportunity for the spores to re-infect the bush.
What to use as mulch?
1. Stable droppings and manures.
2. Pine bark.
3. Compost.
4. Leaves.
5. Pine straw.
6. Rocks.
7. Lawn clippings.
Stable droppings and manures are excellent mulches (especially horse manure), but may contain weed seeds if not properly composted. Most rosarians who use horse manure take it straight from the stable to the rose bed, I know of very few who allow it to compost, although composting is the best way. One suggestion I've heard uses a preemergent herbicide to prevent seeds from germinating. To do this, spread the preemergent herbicide over the bed after spreading horse manure, or after removing winter protection and putting down any organic material in the spring.* I use the stable droppings from a nearby horse farm and get very few weeds, more than acceptable for the quantity and price I pay - LOTS & FREE! Music to a rose lover's ears. The one drawback is sore arms and back from all the shoveling.
*NOTE: A preemergent herbicide prevents seed germination - all of them. If you plan to allow annuals or perennials to reseed do not use the preemergent.
Pine bark comes in several forms: Ground, Shredded, and Chunks. Most nurseries, garden centers, and landscapers have it for sale.
1. The ground pine bark makes a very good soil conditioner (it is used in most soil-less mixtures so that tells us something). When first put down, it has a tendency to wash away during heavy rain, but after a few light to moderate waterings seems to lock together nicely. Further rains are handled just fine.
2. Shredded pine mulch is almost as good, but the strands can make shoveling and other chores a bit more difficult because the mulch tends to lock together. The first heavy rain may cause some washing away, but then it locks together and requires a large thunderstorm's downpour to move the stuff.
3. Pine bark chunks are more decorative (to my eyes anyway), but may wash away. The chunks seem to be too large to lock together and prevent washing.
Compost is the result of organic materials decaying (composting) and is a wonderful soil additive and mulch. The heat generated by proper composting kills weed seeds. If you have the room, compost can be made at home and the materials may be free.
Unless leaves are shredded, they either blow right off the rose bed or compact into a slimy mat that is nearly impenetrable. Once shredded and watered onto the bed (to lock them into place) they seem to work quite well as a mulch, decomposing into leaf mold and making a great soil amendment. Did I mention FREE!? How about no bagging?
Pine straw is the mulch of preference at the American Rose Society's Headquarters (the fact that it is located on 118 acres of pine forest may have something to do with this). Pine straw doesn't pack easily, is slow to decay, allows water and air to move through it, and can usually be acquired for FREE! Personally I prefer pine straw for winter protection only.
Rocks are inorganic so don't decay. They add nothing to improve the soil structure or nutrient availability. Weeds still grow between them, now you have to fight the rocks to get at the weeds roots; which are firmly entrenched in firmly packed soil (probably clay around here). The only benefit is you won't have to add more rocks each year. Oh, you think rocks are cheap? Go price before deciding to go this route!
Lawn or grass clippings are readily available, but have several disadvantages. The clippings will matt if spread too thickly and may become so dense that water can't penetrate. If weed and feed or other herbicide was used on the lawn then you might poison your roses.
Where to put the mulch and how deep should it be?
When laying the mulch down, leave the graft union uncovered to allow sun, air, and special fertilizers to reach the graft an encourage basal breaks (big new canes).
For individual bushes, a ring of mulch should extend at least to the drip line of the leaves and be 2-4 inches deep.
For rose beds, the mulch should cover the entire bed to a depth of 2-4 inches.