The following article was written by CPS employee Mike Frank:
Lets take a look at using fertilizers with herbicides included. These can be a great time saver when used properly. Let’s start by making sure to calibrate your spreaders and apply according to the label. Follow those directions to a fault and you should be in good shape, Below I will go over some basics.
There are two kinds of herbicides mixed with fertilizer. One is a pre-emergent. The pre-emergent is intended to stop the production of spouts once the seed has germinated. That is it. It will not take care of weed that got a foot hold last season. Dandelion (as well as others) are famous for germinating in the fall, putting down root and then going dormant when the weather gets cool. These little buggers will come up past your spring pre-emergent application and smile at you. The pre-emergent action will NOT take care of these late season squatters from last fall.
The second type of combination product is one with a broad leaf herbicide in it. This one works differently from a pre-emergent. It is a post-emergent intended to control broadleaf weeds, including those that over wintered. The application efficacy is optimal when the plant leaf is wet. The herbicide needs to be in contact with the target plant to achieve maximum translocation. The granules simply stick better to wet plants. A happy weed is easier to kill than a stressed weed! So if you have been experiencing drought conditions prior to application you may want to give them some irrigation a day or two before the application to make them happy again and then proceed with applying the fertilizer/herbicide. Be careful watering the product in too soon, you want the herbicide time to translocate into the target plant. Six hours is a basic rule of thumb.
Weed control is considered good at about an 80+% kill rate. Therefore diligence on your part to spot treat those few survivors will go a long way for total control. Be aware that control in areas that are under heavy weed pressure becomes a process that may take a few seasons to achieve maximum results.
Oh…one more thing. Per Colorado State Department of Ag, application of combination products requires the proper licensing if work is done on a commercial basis. Just had to say that.