Annual weeds reproduce by seed within one year, then they die. Annual weeds generally produce thousands of seeds per plant- this is how they take over an area so quickly. 

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The ecological purpose of annuals weeds is to quickly protect the ground. They grow best in disturbed areas where disturbance has taken place. These disturbed areas offer the perfect biological and chemical makeup for annuals to thrive. 

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The seeds of many annual weeds need light to germinate. An effective control is mulching an area before seeds begin to germinate, or scraping the soil surface as the seedlings are beginning to germinate because they don't have any energy reserves in the roots for a full recovery.

 

If an area is already infested with annual weeds the best control is to ensure the flowers do not go to seed by mowing, cutting or using a string trimmer. Causing a major disturbance in the soil to remove them will only make room for more annuals to germinate.

Winter annual weeds follow all the characteristics of annual weeds except that they germinate in the late fall or early winter. Because of their early start, they are often producing seed in February and March even though those seeds will not germinate until the following fall or winter.

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The seeds of many winter annual weeds need light to germinate. An effective control is mulching an area before seeds begin to germinate, or scraping the soil surface as the seedlings are beginning to germinate because they don't have any energy reserves in the roots for a full recovery.

 

If an area is already infested with annual weeds the best control is to ensure the flowers do not go to seed by mowing, cutting or using a string trimmer. Causing a major disturbance in the soil to remove them will only make room for more annuals to germinate.

Biennial weeds reproduce by seed within two years, then they die. The first year plants are only vegetative, but flower in the second year. Biennial weeds generally produce hundreds of thousands or even millions of seeds per plant. 

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The ecological purpose of biennial weeds is to mine nutrients from deep in the the ground. Many of them have taproots that grow down deep during the first year of growth. They grow best in areas where disturbance has taken place and there is a mineral imbalance, or existing plant coverage is thin. These unhealthy areas offer the perfect biological and chemical makeup for biennials to thrive. 

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Biennials take more work for effective control. Controlling them immediately after they germinate is best, but once the seedlings are removed or killed something should be seeded or planted in that area immediately in order to fill in the empty space. 

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If a population of biennial weeds is already established, it's important to prevent the flowers from going to seed. If the area is full of first year growth the first year rosettes can be removed by cutting a few inches below the soil surface to ensure all below-ground buds are removed. This is easiest by "stabbing the soil" with a shovel. As long as there are no seeds, you can leave the leaves to decompose on the soil surface.

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Herbicide can be used with great care. Many weeds have become resistant too many types of herbicides. Make sure you read the label completely as there are certain herbicides that will kill non-target plants (trees and shrubs), and not all herbicides can be used in all environments. 

Perennial weeds can reproduce by seed, but for many of the worst weeds, their primary form of reproduction is through extensive root systems. . 

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The ecological purpose of perennial weeds is to restore soil health and protect unstable ground from erosion. Many of them form colonies with their rhizomatous root systems that fully knit together the soil below the surface. They grow best in areas where disturbance has taken place and there is a mineral imbalance, or existing plant coverage is thin. These unhealthy areas offer the perfect biological and chemical makeup for biennials to thrive. 

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Perennials take much more work for effective control. Controlling them immediately after they germinate is best, but once the seedlings are removed or killed something should be seeded or planted in that area immediately in order to fill in the empty space. Turning the soil can cause a worse infestation if root fragments aren't carefully removed from the soil. Once an area is cleared of mature plants, it should be reseeded immediately.

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Perennial weeds are often indicators of poor soil conditions. Some only grow in soils that are high in heavy metals, some in compacted soils with low oxygen content and others in soils with high levels of salts that remain from heavy fertilizer and herbicide applications.

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Herbicide can be used with great care. Many weeds have become resistant too many types of herbicides. Make sure you read the label completely as there are certain herbicides that will kill non-target plants (trees and shrubs), and not all herbicides can be used in all environments.