In your Outdoor Living Spaces and flower beds, your soil is your bank. You deposit in your bank water and nutrients to invest in your plantings.
As in business, you have to protect your ‘seed’ capital and invest it wisely in investments that will produce the desired return. Some investments are higher returning but also higher risks. Others are lower risk, but also lower return.
Either way, you need to make sure the bank in which you keep your ‘seed’ capital is protected properly, or else you run the risk of losing the water and nutrients you plan to invest in your plantings.
There are several potential thieves for your bank. The first thief does a pretty good job blending in initially with your plantings. It is very sneaky and slowly deprives your soil of the water and nutrients. 
Suddenly, one day you look and notice the nasty little perpetrators have become quite large and are difficult to eliminate. These thieves are weeds.
Weeds are not just unsightly, but they also steal water, nutrients and sunlight from your plantings. Controlling weeds, therefore, is not merely an aesthetic thing, but also is very important for the health of your plants.
A second thief to watch out for is much more obvious than the weeds. This thief is very flamboyant and makes the fact that it is robbing your soil no secret at all. This thief spends all day long robbing your soil, every day. He robs from your soil, but he also invests in your plantings and weeds, in fact his contributions are absolutely necessary to the success of your plantings. This thief is the sun.
The sun is obviously necessary to the health of your plantings, and perhaps is inappropriately referred to as a ‘thief.’ The sun does nevertheless take water and nutrients from the soil. 
As the sun beats down on the bare soil, water moves slowly upward to the surface and then evaporates and leaves the ground. As the water moves up to the surface, it takes with it several soluble nutrients. When the water hits the surface and evaporates, it leaves behind those nutrients which then react with other nearby things and turn to salts. These salts can not be used by the plants, so the sun takes both water and nutrients from your plantings.
Obviously, you can eliminate weeds through many methods–chemicals, pulling, burning, etc. The sun, however is more difficult to control. There is an excellent way to control both thieves at the same time and that is by covering all bare ground. This can be done with plantings, especially invasive ones like grass, or with mulch.
Mulch is a great tool for creating a sustainable landscape and conserving water. Three to four inches of mulch on all bare ground will lower the temperature of the soil surface preventing rapid evaporation, and prevent most weed seeds from germinating and beginning to grow.
Additionally, an organic mulch like compost or shredded bark, 
will break down and improve the soil structure by creating larger ‘holes’ in your soil for water and nutrients to penetrate.
Many different types of materials have been used as mulch. Some of the most common are bark chips, compost, rocks, plastic, weed fabric and, more recently, shredded rubber. I recommend using an organic mulch when it can be fit into your design scheme.
One other note I would make here is that you do not need to used weed fabric and another type of mulch on top of the fabric. Three to four inches of mulch will be just as effective at preventing weeds and holding in water as using weed fabric underneath. Not to mention, it requires a lot less effort to install and maintain. So in case you didn’t catch it here the first time, I do not use weed fabric!
As I mentioned earlier, a healthy ground cover is one of the best types of mulch. So follow my tips on keeping your lawn healthy to develop an ideal mulch and prevent weeds. A healthy lawn will have no weeds in it.
Whatever you use for mulch, the mulch will protect your bank and make sure that the water and nutrients you place in it will go toward the health of your plants and make your landscape beautiful and healthy.