It doesn't make a lot of sense to plant certain types of plants at all at this time of year. Read labels, ask questions, and be a smart consumer.
Early fall is a great time to plant containerized perennials or balled and burlapped shrubs and trees. Remember that it isn't just our cold winters that challenge them, but also wind, openness to the sun and soil acidity. Plants that require acid soil, like rhododendron and bearberry, also known as kinnikinick, are not adapted to most areas. Check out the individual plant's requirements, either on the label or by asking garden center employees. If in doubt, pass that plant by for one that you know is adapted in your neighborhood. If you choose to plant something that nobody in your neighborhood grows, there just might be a good reason why you haven't seen it there. If you need help selecting the right trees and shrubs, get a copy of Extension Bulletin 123, "Tree & Shrub Selection Guide - A resource for Montana landscaping projects" from your local MSU Extension office or online through Extension Publications' website, http://www.montana.edu/publications.
It's too late for annuals, unless you're just planning a garden party and need color for one or two days. Otherwise, planting annuals at this time of year is a waste of money. Many counties have had freezing temperatures already, and the rest will have them very shortly.
On the other hand, fall is a great time to plant spring flowering bulbs, just be sure they are adapted to your conditions. Not all bulbs overwinter successfully. To avoid disappointment, do your homework. In our part of the world, crocus, narcissus, tulips, and grape hyacinths are always good choices. And get your irises in the ground right now! If you order from a catalog, pay particular attention to when the bulbs are shipped. Mid-October shipping dates are too late for much of our area, since you should plant to get the bulbs into the ground by about the first week in October. Skip the bone meal. It doesn't do much good in most Montana soils. A good resource to help select bulbs is also available through Extension Publications. MontGuide 199903AG, "Choosing Biennials and Perennials for Montana Gardens." It is available for free as an Adobe Acrobat file at: http://www.montana.edu/wwwpb/pubs/mt9903.pdf . Or you can request it from your local MSU Extension agent.
In much of this area, garlic is best planted mid-September to mid-October. Select the largest cloves and be sure to plant them root-end down. In locations with minimal snow cover, mulch your garlic to moderate the soil temperatures. Early fall is also a great time to divide and transplant rhubarb. Try seeding your spinach in October for an early spring treat.
In general, phrases like "easy to grow," "new and improved," "bigger and better" are nice phrases that don't give you solid information. "USDA Hardiness Zone 5," "Acid-loving" or "Low pH required" can all serve as real red flags to growers in much of this area. If it says "grows ten feet in a season," ask, "Where does it grow ten feet in a season? In Montana? South Carolina? Where?"
It's a good idea to err one USDA hardiness zone cooler than your location. If in doubt, contact your local MSU Extension Office.
A sound bite to go with this story is available on the web.
MSU Extension Horticulturist Cheryl Moore-Gough says to take care when buying plants (:18) http://www.montana.edu/msunews/radio/Cheryl2.mp3
Contact: Cheryl Moore-Gough (406) 994-6523 or firstname.lastname@example.org