I remember reading a blurb somewhere about the difference between growing something in the ground versus in a container. For example, a plant which normally could survive 30 degree F. temperatures in the ground, may not be able to survive if grown at the same temperature in a container. Does anyone know what the temperature variance is between the two?
There's a rule of thumb that being in a container knocks about a zone off the hardiness (10 degrees F) because the pot doesn't insulate the roots nearly as well as being planted in the garden would. So in your case, things that would be hardy at least to zone 7 should be OK in pots, but things meant for zone 8 may not. You can get around this by sinking your pots in the ground for the winter if you have some things that are borderline. Of course rules of thumb are never 100% accurate, I've had things that are only supposed to be hardy to 9b survive here in containers just fine (without sinking them in the ground). But you're always taking more of a risk if you have a plant that's borderline hardy in a container.
Thanks for your imput...I suspected it might be 10 degrees, just wasn't sure about it. I appreciate your input!
It's really not the insulative value of the container that affects cold-hardiness. I'll offer an illustration: Place a Dixie cup full of water in a chest freezer set at 0* F. In an hour or two, the temperature of the ice will be near 0*, if not 0*. Now, place a foam cup, a much better insulator, inside of a cooler and set it in the same freezer. The cooler, we know, is a very good insulator; and the dead air space inside the cooler surrounding the cup is also one of the best insulators we know of, but after a day or so, the temperature of the ice will also be near 0*, if not 0*. Since it is actually roots being exposed to a killing low temperature that kills entire plants, we can see that the insulative value of the container provides no protection - unless the container provides substantial insulation AND exposure to cold was very brief so the 'buffering effect' of the container comes into play.
Burying or mulching containers is effective simply because it takes advantage of natural geothermal heat. It is rather unusual for 6" ground temperatures to be less than 25* F., even in zone 4. The 'one zone rule' is quite handy, but as Ecrane points out - only a guideline. An illustration: If you had a plant that is borderline hardy to your zone and in a container, WHERE the plant is overwintered can have a profound effect on whether or not it survives. If you bury the container in the ground and mulch well, the odds are it will. If you set the plant ON the ground, it will still receive some advantage from the earth's heat, but perhaps not enough. The small surface area of only the bottom and the additional surface of the container's sides exposed to colder air may leave the plant in trouble because the heat from the earth is quickly transfered from the pot to the colder air. Finally, if you set the plant on a piece of wood or a cement block where it receives virtually NO heat from the earth, it's unlikely the plant would survive. The above examples are to illustrate temperature issues only, and don't take into account the additional desiccation occurring when containers are exposed.
Cold-hardiness is an interesting part of plant physiology, and there are many cultural (as well as genetic) variables from plant to plant.
Al
