One question I do have is: honestly how cold can a rose take, say,
overnight? Can they do down to the 20s or lower if it warms up in the
daytime? I think I took mine in too soon last fall, right after the
second or third frost, and they had a hard time going dormant.

Kim I'll answer your question seperately.

First a roses cold tolerance depends
on the variety. Some old roses such
as Alba's or Gallica's can take upto
-30F before they give up.
JP's tend to be Hybrid Teas.

These tend to have fatal difficulties
w/ Zero degree tempretures. Very tender.

No rose that I know can tolerate a Zone 1
unprotected. The rose w/ the greatest cold
tolerance is the species rose Rosa Acicularis
which grows at the poles and
even it is Zone 2 rated.

On the other hand your micro-climate might
be milder. If so Suzzane is a rose much recommended
for extremely cold area's. Also the Rugosa roses
are known to tolerate -35F tempretures without
protection.

Second When night time tempretures
dip to 20 F most roses tend to shut down
most of their processes. It takes them atleast
a week of warm weather to recover. Some
Hybrid Teas will loose substantial parts of
their branches as they are not cold acclimatized.
So if such cold threatens you might move your
roses into the garage.

Thirdly keeping roses dormant inside heated/unheated
garages or basements is a common problem.
It is not a function of how long you kept it in frost,
as it is the garage tempreture. You indicated in another
post that you garage gets up to 50F. This is way
too warm for a dormant rose. the tempreture needs to be
@ about 25-30F to keep the rose dormant.

If you can turn down the heat to the garage
get one of those cushioned play mats from toys'r us
and place the roses one that. This will ensure the
pots don't freeze from contact w/ the ground.

Alternately you could withhold water and keep
the soil just moist. This too helps keep a rose
in dormancy. DO NOT let it go dry.

--
Theo in Zone 5
Kansas City