Some sources on the action of pre emergents will say they do not allow the seeds to germinate while others will tell you they allow the seed to germinate but keep the seed from rooting which would kill that seed. A far better means of "weed" control is a good mulch that covers any "weed" seeds and keeps them from germinating at all, or if they do prevents the new plant from getting the sunlight it needs to grow so the plant dies.

I attended a gardeners convention several years ago and a representative of the company that makes Preen stated that you should use the product no more than three years. Although pressed she would not explain why. This is not on the label but according to the rep was to be put on next year. Since this was about five years ago and no change was made I will leave it up to each person as to what to believe.

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.