On 8/20/2018 3:00 PM, Pilky wrote:
I'm new to gardening and I want to overwinter petunias, pelargoniums and
begonias in my conservatory. What is the minimum temperature they will
tolerate to survive please?

Annuals are called that because they generally live only one year. If
they are summer annuals, they are unlikely to survive the winter under
any conditions no matter how well protected. Perennials, however, live
more than one year.

Petunias are short-lived perennials. They are best treated as annuals.

Pelargoniums are perennials. Tender, they are treated as annuals in
climates that involve freezing winter temperatures. However, they might
indeed thrive under glass. Since I live in a mild-winter area, I have
no experience with such protection.

There are many forms of begonias -- over 1,800 species plus hybrids --
all of which are perennial. I have wax-leaf begonias in my garden,
blooming all year long. They generally survive my winters without any
protection, but they will not survive freezing. Tuberous begonias and
hardy Asian begonias go dormant in the winter; both should be protected
from freezing, either under glass or with a thick mulch. Etc, etc.
Since you are apparently in the U.K., check with a local garden center
that knows your climate about what variety of begonia you have and how
to keep them during the winter.

--
David E. Ross
Climate: California Mediterranean, see
http://www.rossde.com/garden/climate.html
Gardening diary at http://www.rossde.com/garden/diary