The time to apply pre-emergent herbicides to lawns to prevent winter annual weeds likely slipped by because we went from too early right into continual rains. There is nothing you can do about that now. Know that you will have many fall weeds germinating in your lawns. Do not make the mistake of waiting until early spring to attempt controls.
An excellent time to apply post emergent lawn herbicides is in fall when the trees are coloring and dropping. At that time, the weeds will still be young and actively growing and the sap is going back down in the trees, minimizing the risk of picking up herbicides. Be sure you carefully follow the label regarding days to rain and temperatures. Rain in the forecast risks moving the herbicides off-site into our groundwater! Usually late Oct-early Nov is ideal. If you wait until spring, the weeds will be at the end of their life cycle, close to throwing yet more seed, and not receptive to the chemicals.
If we must use chemicals, we want to use as little as possible in as small an area as possible at a time it will be most effective. I recommend that if you choose to use these products you use a liquid form that can be spot treated in the worst weedy areas, avoiding overall applications. Again, our goal is to limit the use of herbicides in favor of a healthier environment. For those of you who are growing organically, the options are to fertilize regularly, maintain an optimum pH and mow high to out-compete the weeds. While organic pre-emergents exist, there are not organic selective herbicides.
If you are seeding your lawn this fall, you will want to forego the use of herbicides and let the new grass get a good start. Again, mowing high to outcompete weeds is a good idea. Time your aeration and sowing to minimize compaction. If the lawn has been recently wet, stay off it until it has had a chance to drain. That may mean a slightly later sowing date than you had hoped for this year, but the benefits of avoiding compacting the soil outweigh the down side of a late sowing.
Consider soil compaction when planting trees, shrubs and perennials. Digging when the soil is too wet will only make a smooth-sided hole like a glazed pot that roots will have difficulty breaking out of into native earth. The biggest danger to planting perennials late is frost heaving and rotting. Because their roots generally are fine, they are more prone to rot. A tree or shrub will grow roots well until close to Christmas so its chances of getting through the winter are good.
Late in fall, a thin layer of mulch should be applied. A deep layer of mulch will encourage rot, allow insufficient oxygen, and invite voles to damage young bark and roots. If your soil does not dry out enough to transplant perennials in time to re-establish this fall you will have a second opportunity in early spring. Most perennials will take a spring and fall transplant equally well. Consider waiting if your soil is still soggy past mid-Oct.
Another fall chore is the destruction of woody weed and vine pests. Just as leaves are coloring is an excellent time to cut them and do an immediate stump application of an herbicide that will translocate directly through the plant tissues and go very deep into the plant, increasing effectiveness. This method eliminates the concern of overspray to adjacent non-target plants and movement through soil into our groundwater. Look for brush killers that contain directions for stump applications.