Perceptions of Zones and Hardiness

Sat, 07 Feb 2004 10:06:40 PST
Jamie Vande    Cologne    Germany    Zone 8

As you can see from my zone, hardiness tests are not a major criteria for me, but environment adaptiveness is, we are wet and our cold periods are prolonged!  I do grow quite a few Kniphofia species and hybrids in the garden, finding their most delicate criteria is soil drainage.  They hate winter wet.(the ones I've tried, anyway). K. uvaria and her hybrids handle our wet pretty well, though!

In attempting to assess the possible sucessfulness of a plant in my climate, I find the zone is only a starting point.  This is particularly true of the South Africans, many of which come from higher elevations or inhabit niches within a zone.  Just because we have defined Zone 8 as having a minimum of -12°C, does not mean all plants that regularly survive a periodic dip to -12°C will survive in my Zone 8.  Most of the African Zone 8 plants are not exposed to prolonged freezes, which can occur in my Zone 8.  A typical Winter day in a SA Zone 8 may have a nightime temperature of -8°C followed by a daytime temperature of +18°C.  Prolonged freezes are very rare!  In my Zone 8, with the same nightime temperature, the following daytime temperature is typically about -2°C, still below freezing!  This may continue for a few weeks, stressing the plant to it's limits.

An example this Winter is Chasmanthe, which is winter green.  After a week of sub-freezing weather, the plants lost all their leaves.  In years with only occaisional/broken frosts, they are green right through to Spring.  Similarly Zantedeschia aethiopica can handle light frosts, but hard freezes take it to the ground (along with the developing flowers!)  Zone ratings are only the starting point in defining hardiness, in my opinion.

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