Dry fertilizers can be spread by hand, by a shaker or a lawn spreader. Drop spreaders are more accurate than broadcast spreaders. Liquid fertilizers require a sprayer. Almost all sprayers have plastic parts that will absorb the herbicides, and when used later with insecticides or fungicides, may cause damage to your plants. There are dangers with all chemicals to pets as well as people, but if you use them according to directions at recommended rates, you can use them safely.
Some herbicides are very residual, affecting plant growth for years.
Others break down quickly and cannot even be detected. Read your labels carefully. They always list any environmental hazards. As with all lawn and garden chemicals, herbicides should only be used if a problem exists and only on the areas that need treatment. Routine applications without a specific need can be costly and ineffective.