Douglas Alpenstock wrote:Perhaps the UK and EU have different systems also?
Well, yes and no. Mathematically EU and US are mostly identical, but in practical gardening, they are worlds apart. Here is a quick breakdown of what you need to know (I will use Poland as example).
The Similarity: The Thermometer Formula
On paper, the baseline is exactly the same. Both US Zone 6 and Polish Zone 6 are defined by the average annual minimum winter temperature:
• Zone 6a: -23.3°C to -20.6°C (-10°F to -5°F)
• Zone 6b: -20.6°C to -17.8°C (-5°F to 0°F)
If you are growing hardy herbaceous perennials (like Echinacea, Hostas, or Daylilies), they behave identically. As long as their roots don’t freeze past their genetic limit, they will pop back up in the spring whether they are in Ohio or Masovia, Poland, EU.
The Major Differences: Summer Heat & Latitude
The thermometer only measures the single coldest night of the year. It completely ignores what happens during the other 364 days.
"Wood Ripening" (The Summer Heat Factor): Summers in the US Zone 6 (Midwest/Northeast) are hot, humid, and consistently sit between 30°C and 35°C. This intense thermal energy cooks the plants, allowing new tree branches to "ripen"—meaning the soft green growth turns into hard, woody bark before winter. Central European summers are much milder. Without that intense summer baking, plant stems stay soft and green, making them highly vulnerable to freezing when winter hits.
The Latitude Illusion: Poland sits around 52°N latitude (parallel to central Canada). The US Zone 6 sits much further south, around 40°N latitude (parallel to Madrid, Spain). Because the US is further south, it gets much higher light intensity and stronger solar energy year-round, even though its winters drop to the same freezing temperatures as ours.
Real-World Examples
• Figs (e.g., Chicago Hardy): In the US Zone 6, this variety can grow into a large, productive woody shrub because the blazing American summer hardens the wood to withstand the winter. In Poland's Zone 6, that exact same fig tree will almost certainly die back to the ground every winter unless heavily wrapped, because our milder summers don't ripen the wood enough.
• Peaches and Grapes: US Zone 6 gardeners grow these with massive success. In Poland, even if the plant survives the winter cold, our lack of intense summer heatwaves can lead to poor sugar development in fruit, and sudden European spring frosts are notorious for wiping out buds much more aggressively. Only a few areas of Poland with specific microclimate are good for grapes.
So, we use US-adapted zones but we do not trust them blindly for woody shrubs, fruit trees, or Mediterranean plants.
A plant rated for "US Zone 6" might need a Zone 7 or 8 microclimate in Central Europe (like a south-facing brick wall or a greenhouse) to make up for the lack of summer heat.
As to the UK, (as far as I know) they have their own RHS system of zones, from H1 to H7.
• H1–H2: Tropical to frost-sensitive
• H3–H4: Half-hardy to hardy (H4 roughly matches USDA Zone 8)
• H5–H7: Very hardy down to continental European winters (H7 roughly matches USDA Zone 5 and below).
