You will never get a hold of your weed problem until you deal with the seed bank. There is only one method that offers partial relief: lots of mulch, never till. If you have to till, till once at the beginning, incorporate whatever manure you can, then end it. The broken down mulch will form a top layer of relatively seed free soil. You need one foot of mulch for one inch of top soil. Dandelion seeds will still blow in, but the existing ones will be buried. They will resurface in small amounts as wildlife (earthworms mostly) churn the soil, making the problem more manageable. I even seed my own weeds (arugula, corn salad, purslane and miners lettuce). If you have existing perennial or biennial weeds, the method is the same with cardboard below the mulch in the first year. Use straight Roundup applied with a small paintbrush on those weeds that manage to push through (mostly near the cardboard holes where your veggies are). If your mulch is harsh for plants (wood chips), balance it with urea and wood ash, and use veggies that are more tolerant of those conditions until it becomes soil. Dead leaf mulch turns into soil in one year, which is generally mild and friendly to all vegetables, although at my site it tends to have a few maple seeds in it.

From the Extoxnet link, "In August, 1979, trifluralin was brought under Special Review by the EPA because of the presence of a N-nitrosamine contaminant which had been shown to cause tumors and to have mutagenic effects in animals. The principle manufacturer of trifluralin had already instituted manufacturing methods to reduce N-nitrosamine contaminant levels. The Special Review was concluded in 1982, with the requirement that N-nitrosamine contaminant levels in trifluralin not exceed 0.5 ppm, a level which EPA believes will have no toxic effects (12, 13)." There is a requirement that even at the 0.5 ppm level there be on the label a cautionary statement. And that should be reason enough that this is not a good rpoduct for an organic gardener to use.

Hello DebbieSue! Preen will not work on grass. Grass is propogated via root systems...Preen is a pre-emergent - it will keep seeds from germinating, but it will not kill or hamper any of the grass, weeds or clover that spread via roots. Irises should be weed free with plenty of light and air. Use Roundup before you plant to kill all weeds through their roots, mow with a mulching or vaccuum mower to avoid tossing seeds in your irises, find a wand or sponge to 'pat' roundup on the weeds closest to your irises - but do not drip, smear, or pat roundup on the iris fan or rhizome...it can kill irises. Once you have your iris bed weed free, keep it that way with regular applications of Preen for seeds and Roundup for weeds. If you have wild onions, mix some dish-soap with the roundup and make sure you lavish special attention on their green stalks. Old fashioned pulling weeds and hoeing out the beds is also effective. s

Sassy... since you haven't used it, it may help to know how it works. Preen is a pre-emergent... it tell seeds not to germinate. It is pretty effective. We have more garden than we can really take care of, particularly as we spend a lot of time in spring getting ready for our annual sale and can't stay on top of all the weeding. I am careful using it around youngish plants... it is not supposed to affect growing plants, but some have noted a tendency for it to retard the growth of a young plant. it will also keep your hosta seeds from germinating - it wasn't until we didn't get around to using it for a couple of years that I realized just how easily hostas reseed. I'm told that weed seeds will pile up and when you quit using the Preen, you will have a big weed bloom. Don't know if this is true. We've had good luck combining the Preen with Pine Bark Nugget mulch. We lay down newspaper, then mulch... and then toss Preen into the mulch to retard the growth of the weed seeds that fall into the spaces in the mulch.