Perennial growing zone for containers?

[Greenhorn]

It seems with most perennials the temperatures Zone can determine whether many plants work out as annuals or perennials. As far as container growing, I've heard other members comment that their container perennials died out over winter despite the fact that the plant and growing zone should have resulted in the plant being a perennial, but since the plant was in a container it acted as an annual ( apparently they die off over winter because the roots or other underground or low ground parts of the dormant plant were colder and froze harder/deeper because containers are more exposed.

I was wondering if there is a chart or a general rule of thumb for determining the winter heartiness of determining whether a plant will behave as a perennial or an annual when it is in a container outside over winter?

Perhaps as a general rule of thumb for gauging whether a plant will behave as a perennial or an annual in a container, that may be one should assume that container winter heartiness should use one or two zones colder?

(I hope you understand what I'm trying to say. I'm not as gifted or as smart with using language as many of you.)

(I'm sorry if this question has been asked before and answered somewhere else.)

[Kisal]

According to what I have read, if you're going to grow a perennial in a container, it is best to chose a variety that is rated for at least 1 zone colder than your location. In other words, if you're in zone 5, choose plant varieties that are hardy in zone 4 or 3.

In addition, it's a good idea to move the container (and its plant, of course) to a somewhat protected spot on your property during the winter, even if it's just up against the wall of your house or in a corner where it gets some protection from the worst of the cold winds and weather.

[applestar]

What Kisal said.
Also, mulching/burying -- container and all -- with a pile of leaves helps, don't let the soil dry out completely (it should be located where natural rain/snow will provide moisture). Personally, I feel that the container bottom should be in direct contact with soil. But pay attention to the ground conditions and make sure it's NOT in a location where it'll sit in puddled water for more than part of a day at time, especially during spring thaw.

[hendi_alex]

Growing zones vs. cold hardiness don't seem to be the same thing to me. I love growing a variety of salvias and most are rated for my area, zone 8 through zone 10. Well, to me a rating of zone 8 means that the plant will be cold hardy in an average year. It also means that every couple of years or every few years, that plant is going to get killed. It seems to me that the hardiness zones are based upon average temperatures, average lows. Whereas the cold hardiness is based upon the minimum temperature that the plants can normally tolerate. While our average low temperature is somewhere in the mid to upper twenties, about every third year, the temperatures dip down to anywhere between 5 and 18 degrees. Those normally cold hardy plants in our area, rarely make it through such drops.

So IMO if you really want a safe plant, whether in a container or not, you will chose one that is cold hardy at least a zone lower than where you live.

I do move all but my hardiest container perennials off of the deck and onto the ground, where they get covered with the natural mulch of the falling oak leaves. That seems to help give them far more potection than get on the above ground deck.