Fertilizer and Weed Control

If a plant (that we want to grow) can be killed by giving it too much fertilizer, why doesn't it work that way with weeds?

You make a great observation on weeds. As you know weeds are plants out of place and plants that seem to withstand adverse conditions including excess fertilizer. Fertilizer burn is the result of salt (the nutrients in the fertilizer) levels being much higher in the soil than the roots. This causes water to leave the roots (water moves from areas of high concentration to areas of lower concentration) resulting in drought stress and often death of the plant. Weeds are often more drought tolerant and better able to withstand fertilizer burn than the plants we are trying to grow.

Using fertilizer to purposely kill plants would have a greater negative effect on the environment and your budget than some of the alternatives. Pulling and hoeing have long been used to remove weeds. Mulch helps suppress weeds. Recent research found that mulching the soil was as effective as pre-emergent herbicides in preventing weed sprouting in gardens. Corn gluten meal is an environmentally friendly pre-emergent that controls annual weeds in the lawn and garden. Post emergent eco-friendly weed killers use soaps, plant oil, and vinegar to burn the leaves and stems of plants. Several applications are needed to kill perennial weeds. Traditional herbicides use various active ingredients to kill the leaves and roots of plants. No matter what product you select be sure to read and follow label directions carefully.