So which is it? Unfortunately the answer is both, and knowing what is right for your specific plant can vary, but here is a general rule to help you as you get your green thumb back this fall.
It turns out that fertilizing too early in the fall can bring on new growth that is then stifled by winter frosts. This damages the plants and inhibits them from growing properly in spring. On the other hand, fertilizing in late autumn—when the colored leaves are falling off the trees—actually stimulates plants’ root systems. The roots absorb the nutrients in the soil, prepping them all winter for the spring thaw. In fact, fertilizing in late fall will likely be sufficient for your plants so that you will not need to re-fertilize them in early spring.
This being the case, be sure not to over fertilize. Not only will it waste fertilizer, but it can also damage your plants or cause them to produce a bad crop.
All in all, fall fertilization is less about immediate growth than it is looking to future growth. Better roots now will mean better foliage later. So fertilize in late summer; then wait to do it again until November.
Note: This is a general rule. Certain plants, shrubs, and trees will require different fertilization methods. Double check the needs of your specific plants depending on whether or not they are perennials, summer annuals, or winter annuals.