FYI: I grow mostly tomatoes, peppers, and herbs in my garden.

Personally, I wouldn't start to consider Preen in my veggie garden. Please refer to my "My Page." I also use Roundup but not anywhere near the soil in which I produce food that I will eat.
The way that Preen works is that it inhibits root development by interrupting mitosis in them. It becomes ineffective in wet soils, and may biodegrade into either carbon dioxide or a bunch of other not so sweet things.
My personal view is that nothing but a hoe should be used in my veggie garden. Yes, there are others that will dispute this, but I prefer to err on the side of "organic" if I am going to eat it.
Preen may be good for SOME flower and shrub beds, but even there it can hurt certain plants and ground covers (READ LABEL!)
hortster

Preen has a version for veggie gardens. Read the label to determine the time between treatment and planting.

A good mulch will do more to help control "weeds" then any of the asked about poisons, with the added benefit of helping conserve soil moisture, helping control soil temperature, and adding organic matter to the soil.

Preen generally will not stop weeds that are already growing. You will need to till those out, which can be done while incorporating the Preen. I assume you are looking at the version of Preen with trifluralin. Using it pre-plant in the garden, you won't be able to grow squashes/melons or beets from seed this year, and may not be able to grow corn from seed. Most of the other common garden vegetables--legumes, potatoes, onions, carrots, radishes, are not injured. Transplants such as tomatoes and peppers and cole plants will have no problem with it. Not sure about herbs and how they might react, but trifluralin generally will not hurt transplants if the soil has been pre-treated at the proper rate. Be certain to lightly rake it in or sprinkle/rain about 1/4" to 1/2" of moisture as soon as you can after applying or the sun will destroy it. Do a web search for Trifluralin and find the label. It will tell you which vegetables it is registered for use on.