Mulch is anything that you use to cover the soil. It can be organic (meaning will biodegrade) or non-organic in nature. Both have their advantages and disadvantages, but keep in mind that you do not want to put anything near your food plants that will possibly contaminate them with unwanted chemicals. Just use your good sense and you'll do fine. :)
Layer your mulch 2-3 inches thick for both water preservation and weed prevention. About 3 inches of mulch will prevent light from feeding any weed seedlings and those that do make it through are stressed and easy to pull up. Be sure to keep the mulch back out 3-4 inches from the stem of your plants. This will help keep out any critters that decide to make the mulch their home and allow for good air circulation around the plant so it is less likely have any disease issues during wet periods.
When you have a good layer of mulch in your garden be sure to check the soil under the mulch before you decide whether or not to water. It can be easy to over water and under water because you can't see how moist the soil is or is not.
The beauty of an organic mulch is that as it breaks down it adds valuable nutrients and bulk to the soil. The downfall is that as it breaks down it requires some input of nitrogen and it will take it from the soil (and therefore your plants) if it needs to. You can compensate for this by mixing nitrogen rich options like compost with heavy nitrogen users like wood chips. Fine material like lawn clippings and leaves should be placed with care as they can compress with time and become almost impermeable to water if over 2-3 inches thick.
lawn clippings (NOT treated w/weed killer)
Many of these are a great option for non-food producing areas like flower beds or around trees because they do not decay and need replacing on a regular basis. Frequently some of these (plastic or landscaping fabric) are used under a more cosmetically appealing option (rocks or wood chips) to hold back weeds because of their durability.
As I continue to think about watering, I look forward to sharing with you the value of having a rainwater collection system in your watering arsenal and how simple it can be.
Happy Gardening and Watering! :)
Other Post in this Series:
Garden Watering Basics
Capturing Rainwater for Drier Days
Thinking Through Making Your Own Rain Barrel