Answer: The USDA winter hardiness zones are based on average low winter temperatures. As you noticed, there can be many other variables besides plain cold that determine the harshness and average length of a winter season -- including altitude, exposure to wind, proximity to water, and how deep the frost line is. Also, there are many variables that determine the type of summer and number of growing days an area will have, even if the min/max temperature ranges seem similar. Humidity and night time temperatures can have a strong influence on plant growth, as will the number of days over about 85 degrees.
There is also a new plant heat index from the American Horticulture Society that you may be interested in. That system is new and many plants are still being measured according to it, but in your area it will be useful in deciding which plants will do best in your type of summer climate as more plants are rated. You can check your rating at http://www.ahs.org/garden/zipform.asp
Most cannas and some varieties of elephant ears should be hardy in a sheltered location in zone 7; a deep winter mulch can be helpful in their survival -- remove it in the spring. Planting them deep can also be helpful in insulating them from the cold, but it may delay their appearance in the spring while they wait for the ground to warm up again and start growing.
In your area, most roses would not need excessive winter protection such as cones or wrapping. Hybrid teas might be trimmed back to keep them from being whipped around by wind, and you could use a generous layer of mulch over the root zone and graft (applied very late in the fall when the weather has settled into cold) to protect the graft from temperature swings. Shrub roses and ground cover roses would not need extra protection and would not be pruned back in the fall, they should be fine with the usual two to three inch year round layer of mulch over the root zone.
I hope this answers your questions.
Q&A Library Searching Tips