What does "hardiness zone" mean?

Question

Several questions here refer to (hardiness) zone 6a, 7, 5-8, and others.

What is a hardiness zone exactly? Is it determined only by the temperature (maximum, minimum) or are there other factors?

Answer 1

I posted this answer to a related question on meta. Quoting the relevant portion here:

USDA Hardiness Zones are based on the average minimum annual temperature. I live in "Zone 5" which means I can expect temps of -10F during the winter. You can look up the temps associated with the various zones in about a million places on the web, including (of course): wikipedia, which also has references to hardiness zones in a number of places around the world.

While annual minimum temperature is sometimes a useful bit of information, it is often useless:

• A question about the best time to plant cabbages in the spring doesn't care about how cold it's going to be in January. This will be localized, requiring fairly specific information about when the really hard frosts have tapered off. (We get frosts a fair bit later than people who live just 25 miles down the road.)
• A question about caring for plants in a warm area may care about how hot it gets in the summer time. Maybe it's ok to start lettuce in July in Fort Kent, ME, but you certainly wouldn't want to do it if you live in Dallas, TX.
• And rainfall: Phoenix and Tampa are both zone 9. But Tampa gets 45 inches of rain a year while Phoenix gets 8. Answers to the question about fast growing shade trees will be radically different -- who cares how cold it's going to get!
• And length of growing season. We have a short growing season (defined as time between frosts): roughly June 12 - Sep 7 (115 days). Last year we had snow on Mothers Day and again in mid-September! I put out my basil a week ago because it looked like it would stay warm and then we got a 38F night. There are annual vegetables that we can't grow outdoors because they won't mature in time. Topeka, Kansas is also in Zone 5, and they have a 175 day growing season -- that's an extra 60 days to ripen watermelon!
• And soil type (clay, loam, sandy) and pH (acid, neutral, alkaline), urban vs suburban vs rural, pests (e.g. we have deer but not pocket gophers) and countless other considerations.

The Zones are useful for figuring out if perennials are likely to survive the winter where you live. Otherwise I'm not sure it has that much value.

Since you live outside the US, the maps on this wikipedia page may be useful, though I don't see specific mention of Hungary on that page.

Answer 2

According to this, hardiness zones are only determined by "average annual minimum temperatures." This means that hardiness zones only take into account average winter weather. They do not account for maximum or extreme minimum temperatures, rainfall, humidity or any other factor.

The above linked page lists several additional factors that should be kept in mind

Stress Factors. We became aware of additional stresses to plants during the 1970's. Acid rain, gaseous and particulate pollution, security lighting, and toxic wastes, among many other stress factors, have significantly increased the potential for unsatisfactory performance of landscape plants. We need to document the tolerances of plants to these factors.

New Plant Management Systems. New techniques of planting, transplanting, watering, fertilizing, and providing pest control measures have done much to increase the vigor of landscape plants. But used unwisely, these same measures can reduce plant hardiness.

Artificial Environments. We have pushed the use of plants into totally artificial environments such as expressways, malls, elevated decks, and buildings where plant roots are totally removed from the ground and its warming influence. The assortment of plants that can adapt to such environments is proving to be very restricted. Hardiness ratings alone are inadequate to guide landscapers in selecting the most successful plants.