When spraying herbicides over an area, it may be a good idea to include some form of dye. The dye will mark the area you have already sprayed, saving you time and money by not spraying over already-finished land.
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Lawn dyes come in just about any color and can be mixed with herbicide or fertilizer to indicate what areas you have already sprayed and if the chemical is getting anywhere that it shouldn't.
You want to make sure to use a herbicide that does not leach. Ask your supplier about finding the right chemical to use with your herbicide and dye mixture.
The dye may stain concrete and other porous materials. If it does stain, clean the area with bleach and water or household ammonia. Also, the dye will make it easy to see if you are getting herbicide on yourself. Make sure to wear the proper protective materials.