Weeds are typically grouped into 2 categories based on their plant structure. Broadleaf weeds usually have flat leaves, netlike veins, and grow by means of a taproot or coarse root system. When they first germinate, two leaves emerge from the seed. Grassy weeds, in contrast, grow narrow, upright leaves and have parallel veins. They sprout only one leaf from the seed pod and develop finer, more fibrous root structures.

Weeds are also classified by their life cycle. Just like flowers and other desirable plants, they can be annual, biennial or perennial. Annuals complete their life cycle within one year or less, and can be further divided into summer and winter species - depending on when they begin to grow. Biennials take two years to germinate, bloom and die. They are the least common type of weed, and will germinate in any growing season. Perennial weeds live at least two years, and have the potential to reproduce indefinitely. If you don't pull out all their roots, they'll just grow back to plague you again and again.

Perennials can also be sorted into three subgroups based on their life cycle - simple, creeping and bulbous. Dandelion, dock and plantain are classified as simple perennials. They reproduce from seed and grow either a taproot or fibrous root mass. Creeping perennials, such as quackgrass and johnsongrass, are the most difficult to control. They may reproduce by seeds, rhizomes (thick below-ground horizontal stems) or stolons (above-ground horizontal stems). Finally there are bulbous perennials, like wild onion and wild garlic, which can repopulate from seed and/or bulbets.

Weeds deprive desirable plants of needed water, light and soil nutrients. Often unsightly, they can also scratch and irritate skin, aggravate allergies and even harbor insects and disease. Without the use of weed control products, economic losses in the U.S. would reach 20 billion dollars each year.

Weed seeds are especially adapted to spread. Their seed pods have special structures, allowing them to either cling or float. They are widely distributed by animals, insects, hay, mulch, and topsoil. Once they find their way to your lawn and garden, they can remain there in what's known as a weed seed bank. While most weed seeds only exist in the soil a few years, there is a small percentage that can remain dormant for decades, waiting for the right growing conditions to occur.

Most plants only produce several hundred seeds, but weeds are especially prolific. One single weed can produce anywhere from 10,000 to over 100,000 seeds. With these odds, it's easy to see how weeds can quickly take over your garden or lawn if left untreated.

Chemicals that prevent or kill existing weeds are known as herbicides. Their mode of action can vary dramatically - from inhibiting weed seed germination, to preventing cell division and growth, to destroying cell membranes and disrupting photosynthesis.

While all herbicides control weeds, the key is knowing which one is most effective for your weed problem. It's important to first properly identify the weed you wish to eliminate and understand its growth cycle. This will enable you to choose the appropriate herbicide product and correctly time its application to achieve optimum weed control.

Herbicide products can be classified several ways. One is by their activity - selective or non-selective. Generally most herbicides are selective, meaning they will only control specific weed species without damage to nearby grasses and ornamentals. When controlling weeds in lawns, it is also wise to know your turfgrass variety, since their tolerance to herbicides do vary.

Non-selective herbicides will kill or damage all vegetation, so it is critical to apply them precisely to avoid injuring desirable plants and grasses. They are commonly used in landscaping for lawn renovation and for controlling weeds and brush along highways and railroad beds.

Another classification for herbicides is based on the time they are applied in the weed's life cycle. Preemergence herbicides are most effective when applied a few weeks prior to the germination and emergence of weed seeds. They will not control established weeds. These herbicides create a weed control zone on the soil surface, preventing seeds from germinating. Because most preemergents have a long residual, be aware that they can affect newly seeded turfgrass. Annual weeds such as crabgrass, poa annua (annual bluegrass) and goosegrass are typically controlled by preemergence herbicides.

Postmergence herbicides are applied after the weed is established and actively growing. While a few products work through soil application, the majority of postemergents work on contact. They are absorbed through the surface of leaves and translocated throughout the plant. It is recommended that applications be made during a dry, eight-hour period, since rain can wash off the material and reduce control. Most broadleaf weeds are controlled by postemergent herbicides.

In the past, herbicide products simply controlled one type of weed - annual weeds or broadleaves, and offered either preemergent or postemergent control. Today, new chemistries have been developed that cover a wider range of weed species, and can supply both pre and postemergence coverage.

Before you buy and apply any herbicide, carefully read the entire label for exact weed species controlled and sites where it can be safely used. Always review precautionary information and restrictions, follow directions for use, and store and dispose of empty containers as indicated on the label.

The easiest way to stop weeds from invading your lawn and garden is preventing them in the first place. Proper cultural practices can be instrumental in discouraging weed establishment. Here are some helpful tips to achieve a beautiful, weed-free lawn and garden.

• Buy only high quality, certified grass seed. Cheaper brands contain more weed seeds. Read and compare grass seed labels and check guarantees for purity and the percentage of weed seed.

• Select a top-rated seed variety. Choose the one best suited for the amount of sunlight and traffic expected in the area, as well as its ability to withstand drought, insects and disease. An independent organization (NTEP) tests and evaluates hundreds of grass seed varieties each year. To learn which brands rank highest for your growing situation, visit www.ntep.org.

• Avoid light, frequent waterings or overwatering. Plants that receive deep, infrequent watering, generate extensive root systems. Strong roots foster thick, hearty plants and lawns that withstand stress, preventing invasions from pests, weeds and disease.

• Fertilize your lawns and gardens on a regular basis. Be sure to first test your soil to determine its pH and any soil amendments necessary to ensure ideal growing conditions. Determine which nutrients your plants need and the frequency required for proper feeding. If desired, contact your local county agricultural extension office or nearby garden center for advice specific to your area.

• Always cut lawns at the proper mowing height. Never cut off more than 1/3 of the grass blade at one time. Keep mower blades sharp to avoid tearing plant leaves. Scalping or mowing too closely will stress your lawn, while weeds thrive under these conditions. A dense, healthy, vigorous stand of grass will resist the intrusion of weeds.

• Properly cultivate lawns and gardens. By routinely tilling flower beds and aerifying lawns, you reduce compaction and thatch. This allows air, water and nutrients to flow freely through the soil and become more available to plants.

• Densely plant and generously mulch flowers, trees and ornamentals. By eliminating space and sunlight, weeds won't have the needed room or nutrients to gain a foothold in beds and gardens. Always be careful mulch does not come in contact with stems and trunks to avoid moisture and disease damage.

• Plant ground cover or landscape hard to grow areas. If weeds are a persistent problem and you have difficulty growing grass in certain areas of your yard, consider alternative plants or decorative material such as landscape rock. Check with your local garden center or landscaper for suggestions on what may work best to eliminate trouble spots.

• Stop weeds before they can seed or develop extensive root structures. Remove existing weeds by pulling or hoeing them or use an all-purpose weed killer of your choice. Then apply a preemergent control to prevent new weeds from germinating.