Fall is the best time to plant many things. Root growth can be very rapid in the fall even though top growth slows. The reason it is so good to plant many things in the fall is that the plant has a chance to get established (grow more roots) before having to grow lots of top growth and being stressed by summer heat.

Fall is THE time to plant! (When do you think the garden centers might catch on to this?) I buy things all summer (lots on clearance sales) and keep them in pots and plant in the fall. Easier on the plants and easier on me! There is so much garden work to do in the spring, especially if you plant vegtables or (bite my tongue) a-a-annuals. Also if you are as disorganized as I am you can't remember what is coming up where in the spring, so by digging to plant something new, you risk killing plants that are late to come up. Technically brandon7 said it all in his informative post. But sometimes my enthusiasm gets the best of me and I can't resist putting my 2 cents in.
;-)

Thanks for the re-assurance -- I will definitely plant in the fall, with this encouragement. It's my first year in a new place, so it will be exciting to think about my new plantings over the winter. Thanks again!
Rachel