Pre-emergent control of weeds (in other words, weed control that prevents weeds from sprouting) can be accomplished in three basic ways: organic pre-emergent herbicide (corn gluten), synthetic pre-emergent herbicides (trifluralin and other chemicals), and blocking the weeds preemptively using mulch or landscape weed barrier. Which solution is best depends on your gardening philosophy, proximity to waterways and budget.
Synthetic Pre-Emergent Herbicide
The most commonly available synthetic pre-emergent herbicide available to homeowners contains the active ingredient trifluralin, which works by inhibiting cell division on seedlings. This means that you should not use it around plants smaller than 3 inches tall, such as in immature annual displays.
This is the cheapest method, costing about $3 per year as of 2010 to apply as directed to 100 square feet of garden bed. However, it is extremely toxic to fish and marine invertebrates, so if you live near a waterway or have drains near your property that empty into a waterway, this is not an appropriate product to use. It is not safe to use around vegetables and food crops.
Organic Pre-Emergent Herbicide
Corn gluten meal is commonly used as an organic pre-emergent weed control; it works by creating a film on the soil surface, which discourages seedlings from sprouting. It can inhibit growth of plants smaller than 3 inches tall, but because it is organic, it is safe to use around vegetable starts and food crops over 3 inches tall.
This is a medium-cost way of reducing weeds. As of 2010, applying corn gluten as directed costs about $25 per year for 100 square feet. However, an added benefit of corn gluten is that it also contains nitrogen, an important fertilizer, so you may save money on fertilizer if applying corn gluten as a pre-emergent weed control.
Weed fabric, or barrier, is a long-lasting way of reducing the emergence of weed sprouts. It costs about $53 per year (as of 2010) to apply professional-grade weed barrier to 100 square feet of garden bed; but the benefit is that, if properly maintained, weed barrier can last up to 10 years without a reduction in effectiveness.
Wood chip mulch is another option. Applying wood chip mulch has a higher initial cost than applying pre-emergent herbicides, about $50 per year as of 2010 for the 3-inch layer needed to cover 100 square feet adequately for pre-emergent weed control. But the added benefits of using wood chips in this way are numerous.
Wood chip mulch helps hold moisture in the soil, so that you will need to water less; it contributes organic matter to the soil as it breaks down; and helps prevent soil compaction from foot traffic or driving rains in winter. One application can last three years or more, so after the initial labor of applying mulch, it is less labor-intensive than herbicidal pre-emergent controls.