Lawn weeds are a nuisance and something every homeowner has to deal with at one time or another. The general definition of a lawn weed is any plant that is unwanted and growing where it should not be growing.
Most plants that have been classified as weeds are ones that are hardy and/or ones that easily reproduce. Most weeds will compete with your lawn grass for nourishment. If left unattended, lawn weeds can completely take over your lawn.
Wind
Weed seeds are often carried onto your lawn by the wind, where they settle, germinate and grow. It isn’t just the fluffy dandelions that can be carried on the wind. Any weed that is growing in the vicinity of your home can end up growing in your lawn if there is a stiff breeze. Most plant seeds are dispersed in the wind. That’s just the way nature intended it to be.
Water
Weeds often end up growing in lawns because they are brought there by the water. When water flows over a piece of property, either through rain, a drainage system or flooding, it can collect weed seeds along the way and disperse them on your lawn. When the water subsides, weed seeds settle into the soil.
People
Whether you know it or not, you and your family may be contributing to the weed problem you have in your lawn. If your family has hiked or ridden bikes or four wheelers along trails, the chances are high that you have brought weed seeds onto your property. Other human factors that contribute to weeds growing in lawns include dumping the water from fish tanks onto the lawn, adding straw or hay to a new lawn and having bird seed feeders.
Prevention
The easiest way to deal with weeds is before they have the chance to grow in the lawn. Applying pre-emergent weed chemicals to the lawn is one of the more popular ways to keep weeds from growing in your lawn. A pre-emergent should be applied prior to germination, or early spring, and you should use protective wear such as eyeglasses and gloves when making the application. An additional measure to keep weeds from growing in your lawn is to allow your lawn grass to grow high, at least 3 inches. This will keep any weed seeds that do arrive in your lawn from getting the sunlight they need to sprout.
http://www.ehow.com/about_5045783_reasons-weeds-grow-lawn.html
