I use Preen in my paths to keep the annual weeds down. Without it I'd be spending half my summer pulling weeds. It will not work on perennial weeds or things that are already growing, as stated above. It does last a good 3 months or more, so timing has to be right to 1) stop the annual weeds before they germinate in the spring; and 2) have the longest lasting effect as possible. It's possible on zone 6 you will have to reapply it in midsummer. I can usually get by with one application per growing season, but by the end of the summer I can tell the effect is wearing off, esp. if we have had a lot of rain.

I whole-heartedly recommend using Preen. A very small amount is effective and often you don't even need to reapply it! My lot is 80% garden and 20% lawn. I use no chemicals other then Preen, totally organic-- even the lawn food-- but I just had to make the exception for Preen. Yes, there are quite a number of perennials which reseed themselves, and as you observe your garden you'll figure out which ones these are. Foxgloves, hellebores, violets, Johnny Jump Up, melampodium (little yellow daisies), blanket flower (gaillardia), orange butterfly weed, and some coneflowers come to mind. So you sure asked the right question! You can just skip the areas immediately around these so they can reseed. That still leaves areas around the shrubs, azaleas, roses, ferns and other perennials; only treat the areas where there's a problem. I find edges of beds are the most important area to treat. Any area of the bed that borders on the lawn can easily be invaded if you have an organic lawn with lots of little "wildflowers" (e.g., weeds!). I rake back the mulch around those edges, apply Preen to the bare soil, water it in and replace the mulch. I could not garden at my age (70) without Preen. It saves so much time at home that I have time to maintain a huge organic rose garden at the zoo, PLUS two smaller rose gardens --one antique and one modern-- at the arboretum. Corn gluten is great for small patches of lawn or the vegetable garden where you don't want any chemicals but it does not remain effective very long. You really need 50# of the stuff; it is a component of livestock feed so you have to go to a feed store to get it. A side benefit of corn gluten is that it has a high nitrogen content and so it feeds the plants. I used it on my organic lawn back in Texas and it worked great. (It would work here too I just can't lift the bags anymore!) Bless you Babs for bringing up cardboard; that would work equally well with edges and the mulch will hide it. Lasts longer than newspaper and breaks down completely except for the scotch tape. I like to use cardboard under paths and between rows in the vegetable garden.

What will they think of next? Flavored water in a plastic bottle? Phones that play music? Wait, I shouldn't put ideas in their heads. Stop the world, I wanna get off.

Preen (trifluralin or Treflan) interferes with cell division in seedling roots and shoots. The result is short, thick roots that can't absorb nutrients and don't grow so the seedling dies quickly. Seed that does not germinate will not be affected and can germinate and grow at a later date. Corn is semi-tolerant of Preen and if you want a visible example of the effects, plant some corn where you have treated. Those that come up will give you a much magnified example of what happens to a weed seedling. So--it is always best to prevent the weeds from going to seed in the first place, but if you have a heavy seed bank in the soil, using Preen will help control the weeds and decrease the future seed supply. Even 1 year of use will noticeably reduce the weed pressure the next year, but it takes years of eliminating all weed seed production to really clean things up. I can't think of a specific reason why you would be limited to using Preen 3 years running without a break, although in general you don't want to use the same herbicide without rotating to others as sooner or later a variant seed will germinate that is tolerant of the herbicide and you will then select for superweeds if you continue using the same herbicide, removing all the susceptible ones and leaving the tolerant ones. In addition, Preen may not fully break down in the soil over a 12 month period, depending on application rates and conditions, so continual use **could** result in a buildup of herbicide levels that could affect the plants you are trying to protect. Skipping a year will allow any residual to fully degrade.