Did you know that autumn is a major root growth period for woody and herbaceous perennials? Roots will grow and absorb water and nutrients whenever soil temperatures remain above 5°C (conifer roots will grow at even lower temperatures).
The Ontario Ministry of Agriculture, Food and Rural Affairs (OMAFRA) says that up to half of the annual fertilizer requirement may be applied to landscape plants, after shoot growth ceases (mid-September to mid-October). In fact, they explain that by splitting annual fertilizer requirements into separate fall and spring applications, you can actually maximize plant uptake of important macro-nutrients such as nitrogen.
For full details, visit the OMAFRA blog here.