There are two parts to this answer. Winter temperatures and Winter dormancy.
Winter Temperatures
Cold winters not a problem!
Hosta is hardy in zone 3 through 9.
Average minimum temperatures of -40 to +20 F ( -40 to -6.7 C )
Hostas don't mind the cold. They'll start the dormancy process in the fall with your first frost. The top growth changes colors, withers, and dies - much like the foliage of a deciduous tree. Everything slows down... Because the top growth has withered away, cold winter air does not effect hosta. The roots are sitting out the winter underground. We do recommend a top layer of mulch and we talk about this later on. (page down)
Certainly, but never try to dig out hosta when the ground is frozen!
Winter Dormancy
A natural, necessary resting time for hosta. Several weeks ( minimum
6 ) of cold weather are needed so hosta can start and maintain
dormancy. Temperatures must drop low enough to cool the ground
and stay cold. For many of us there are variations to winter temperatures.
Here is an example of acceptable temperature fluctuations:
Several weeks with temperatures 30 to 35 F. ( -1 to +1.6 C ).
Variations in temperature from 20 to 25 F ( -6.7 to -3.9 C ) may
occur, with some temperatures 40 to 45 F ( +4.4 to +7.2 C ) and
averaging 30 to 35 F ( -1 to +1.6 C )
Every day does not have to be extremely cold but you do need temperatures
low enough to keep the ground cool and hosta dormant. Warm days
followed by cold nights will keep the ground cool.
Exceptions can be southern exposures in early spring. The ground
may warm here too early for hosta if your winters are short with
marginal temperatures.
The reverse example is northern exposures. These are usually the
cooler areas of the garden...and the last area to warm in the
spring. Hostas planted on northern exposures may experience a
longer dormancy...as much as 2 to 3 weeks.
Winter warm spots
That's what we call them. Locations in the South and South West
with very warm winters. They're charted as zone 9 but in reality
are warm micro climates. Elevation, warm winds from Southern waters,
and other geographical influences can result in a semi tropical
or very warm winter with little or no freezing temperatures. Sometimes
hosta grows well in these areas if marginal temperature lows are
met. Before you purchase hosta consider your average winter. Hostas
must have enough cold weather for winter dormancy to thrive. Please
read Winter Dormancy section above.
Hosta is not a Tropical plant
Warm tropical winters ( temperatures never reaching 32 to 40 F
or 0 to 4 C ) are not cold enough for hosta dormancy.
Summer Temperatures
Hostas in the South / Southwest US
With proper care, hostas can grow well in areas with mid-summer highs of 90 to 100+ F ( 32 to 37 C ). They'll need more water and shade. Mulching will help keep the roots moist. Growth will slow down during the mid-summer heat wave. In extreme heat, they just sit and wait it out...like we do. Not a good time to divide and move your plants. Reserve these projects for Spring and Fall.
Extensive hybridizing has brought many new varieties to the market that work great in the warmer areas. In our plant descriptions, we highlight those that are heat tolerant and maintain good color even in heat.
Garden micro climates
All gardens have temperature variance. Gardeners in the South
and Southwest US that experience mid-summer temperatures of 90
to 100+ F ( 32 to 37 C ) seek the cool spots for their hosta
plants. Cooler spots are under shade trees, north side exposures,
near creeks, shaded ponds, elevated spots with good air movement,
and with shade to morning sun only.
And then there is that area where the cool spots transition to
warmer sunnier spots. A good place for heat tolerant or sun tolerant
hostas. Look for these terms in our descriptions. But don't forget...these
transition spots will need more water!
A good practice for all areas.
Keeps roots and crowns warm and protected from freezing winds
in the winter.
Your root system can become exposed due to:
Helps retain moisture during the growing season. A well balanced moisture level will: