I've read about a lot of people using Preen on "beds". Would most of you feel safe using it on a vegetable garden? I have some issues with my shoulder and lower back. This would be a godsend if it is really safe.
As a rule purchasing those kinds of products is a waste of your money since a good mulch will do the same thing, usually much cheaper, and a good mulch will, besides suppressing unwanted plant growth, aid in conserving soil moisture, aid in keeping the soil cooler, and can add needed organic matter to the soil improving it over time.
The only Preen product that I would even consider for my vegetable garden is their organic vegetable garden weed preventer, which is 100% corn gluten meal. DO NOT use regular Preen products around your veggies!
Here is a link that might be useful: Corn gluten meal
Never use Preen until your plants have come up if using seeds.
The active ingredient in Preen is trifluralin, which was sold originally under the brand name Treflan. It acts by killing weeds when they germinate by preventing the shoots and roots from elongating.
It is generally safe to use on legumes which are planted from seed (beans, peas), and onion sets and potatoes. It can be used on tomatoes, peppers, and members of the cabbage family that are transplanted into the garden. Theoretically, it should be safe on other garden plants as long as they have emerged and are growing well before the herbicide is applied to the ground, although I'm a bit leary of applying it to any members of the squash family, including melons, and to corn. It does a good job of preventing weeds in raspberries and asparagus.
Trifluralin does an excellent job of controlling kochia, lambsquarters, redroot pigweed, purslane, and annual grasses. You can look up the label to see what all crops it is registered for to get an idea of what you can use it on and what you can't.
It is degraded by sunlight, so should be shallowly worked into the soil as soon as possible after applying, or should be watered in with 1/4 to 1/2" of rainfall or sprinkler irrigation.
Under some conditions (particularly applying at too high a rate), it may last in the soil into the next year, which can cause problems if you plant sensitive crops in treated areas. This happened to me one year when I planted sweet corn where I had treated the prior year. Corn came up, then didn't grow and looked drouthy, and was easily pulled up to display 1" root stubs.
I like to use it in a small garden which gets all my tomatoes and peppers since I don't have to worry about planting anything sensitive there or chancing carryover. The larger garden with everything else I don't treat because I don't grow really large quantities of anything and don't want to risk carryover, although I will sometimes apply it in the large open spaces around squash and melons after they are starting to vine.
Preen has a product that's advertised as okay toa be used with veggies. So, what does the label say?
But as previously said, mulch is a less toxic choice even while enriching the soil as it decomposes.