Southview asked me in the previous post:
"I bought a couple of lilac bushes (just stems with a couple of leaves) and they just arrived. Should I stick them in the ground now or pot them till spring then plant them?"
Here's my quick advice:
Stick the lilacs (or any plant you have) in the ground now. Preferably in a spot somewhat sheltered from the wind, but that isn't essential with most lilacs, as they are very hardy (mostly to zone 3 or zone 4; we are in zone 5; there are more tender lilac species, but I don't think anyone around here would be selling them).
The notion of a fall planting season isn't only a marketing gimmick, although in a colder climate like ours, the spring season is a slightly safer one for marginally hardy plants. The heat of summer does provide a real gap between these seasons, even in the Berkshires. But it's late enough in the year for this area that heat stress is no longer a factor.
Certainly by now we're in the fall season here (and the first official day of fall is September 23 this year), and the earlier in the fall you put your plants in, the better established they will be when the ground freezes hard (roots grow even after leaves fall, but not once the soil nears freezing).
Last year I don't believe the ground really froze until about January 10, so even November plantings of Zone 5 plants should have done fine, but you can't count on such a mild early winter. Basically, we have a couple more weeks of likely good planting season, although I might consider an October shrub purchase if the price is radically cheap (like 60% off a fair initial price).
Lilacs are not houseplants. Keeping them in the house in pots is actually more risky than planting them out, perhaps even if you have a cold greenhouse. And keeping plants in pots outside would be the one way cold could kill them. Effectively, being in pots tends to reduce hardiness by 2 zones. I believe that nurseries holding potted trees will bury even the hardiest of them in foot-deep piles of mulch.
I do recommend that, after some cold weather, when the top half-inch or so of ground is frozen, and rodents have found other places to spend the winter, you spread 2 inches of mulch around the new shrubs, keeping the mulch a couple inches from the trunk. The mulch layer is primarily meant to reduce sudden freeze-thaw cycles, but will also somewhat reduce the depth of hard freezing.
Most winter deaths for plants claimed to work in your zone, especially of new plantings, aren't due to absolute cold, but rather to dehydration, as frozen roots can't supply water to a dessicating plant. This is a bigger problem with evergreens, especially broad-leaved evergreens like Rhododendrons (I recommend Wilt Pruf or other antitranspirant spray for new plantings). You should note if there's a fall drought (which isn't as obvious as a summer drought) and if so keep the plants well-watered.
But ironically, good drainage actually helps prevent the frozen-dehydrated problem. Soggy ground freezes solid, while soil with proper drainage has air pores, freezes less deeply, and allows meltwater periodically to percolate in.
Naturally, I have not done extensive experimentation on all of this (i.e., with randomly selected plants and placebos), but am reporting the collected wisdom of botanists, arborists and other authors whose expertise I trust. I have seen Wilt Pruf-treated rhododendrons looking much happier than nontreated neighbor plants, and have noted the tendency of well-drained and amended beds to not freeze as hard as other spots.
[One last tip. Don't fertilize any plants other than annuals now, or after about mid-August for most zone 5 areas. Fertilizing will encourage soft new growth -- which will be killed off in cold weather -- and should be held off until next spring, generally when Forsythia blooms.]