It used to be that once September arrived we could count on cooler temperatures and increased rainfall. The cooler temperatures, especially at night help the apples color up nicely, but we are way behind on rain. We really need a good, long soaking.
So dont put your hoses away yet. If you need to ration your water, dont bother watering the annuals that will die with the killing frost anyway, but do water the plants you want to survive next year. Perennials and any trees or shrubs you have planted in the last two years should be your first priority.
Watch as you apply water to make sure its soaking into the ground instead of just running off the surface. We always say that mulches do a good job of conserving water, but once they get bone dry on top they actually repel water. A light rainfall or water from a sprinkler will often roll right off the dry mulch surface. You can avoid this problem by installing a soaker hose under the mulch layer. If you are watering from above, scratch up the surface of the mulch to loosen it which will allow the water to move through more easily.
I cant emphasize enough the importance of supplying enough water to your plants. If your plants are new, they need water. If they had an insect or disease problem this year, they need water. If backed into your tree and skinned off a section of bark, it needs water. If you want your plants to tolerate stress, weather and pests, the best thing you can do is make sure they have enough water.
We tend to have selective memories. We easily remember the rainy day that spoiled our outdoor plans but seldom is a rainy day or two enough to provide a good soaking. When I dig up a weed in my perennial garden and see the dusty, dry soil or when I walk across our field and feel the hard ground through the soles of my shoes, I realize any benefits from past rainfalls are long gone.