Better idea is to start raking the seeds off the garden this fall. Then next spring use a knife to cut any seedlings off under ground. Some will still come up but not as many. I have the problem with oak seedlings. If I cut them off above the surface they can send up a new shoot from the cut portion.

Theoretically this stuff is effective for up to 4 months so applying it in the fall in the hopes it would work in the spring is a dream that will not come true. If these seedlings are germinating in your garden soil they can be easily controlled by properly mulching. If these seedlings are germinating in the mulch they can be easily controlled by stirring the mulch a bit to disrupt the roots attachment. There is no really good reason to spend you money on some product to control "weed" growth since there are other, more effective methods.

Preen may work--you could try it. It may last through the winter, but you will get better results if you apply it in the spring. An early fall application is better if you are trying to prevent fall annuals from starting in the fall. Just apply after the frost goes out of the ground in the spring and lightly rake into the soil or water with about 1/4" of water to carry it in. You want to apply it before the seeds start to germinate, so if you know when they first start appearing, apply it a couple weeks earlier.

Preen inhibits germination and root growth, but will have no effect if they are already growing. Also, some species may not be affected by it and the label doesn't list tree seedling control, so you will have to try a little to see if it works on the tree seeds you are dealing with.

If you also plant flower seeds in these beds, the Preen is likely to prevent them from growing, so it is best used where you have established perennials or plant transplants.

This is from the manufacturers web site, "One easy application of Preen Garden Weed Preventer keeps over 200 broadleaf and grassy weeds from sprouting for up to three months." Therefore applying this product in the fall will not prevent seed germination in the spring.

Here is a link that might be useful: Preen