Hi Steve:
The hardiest species of Tropaeolum is speciosum: it has
naturalized (said to be a weed) in Scotland so should be hardy at least to
zone 7. it comes from the Southern Chilean cold rainforests. Immense
rainfall mainly in summer (up to 3,000 mm.) Tropaeolum tuberosum follows,
this is an alpine and a normal vegetable crop in the Andes with a number of
variants. Dry cold winter. Polyphyllum, incissum and sessilifolium spend
their winter dormancy very dry under snow. The zone would be U. S. 8, always
growing in scree. Andes of Argentina and Chile. Brachyceras, tricolor (not
tricolorum) and azureum come from foothills of central Chile and are best
grown as Cape bulbs (frost free). Very dry in summer. Pentaphyllum grows in
the gallery forests of the big rivers of Argentina. Subtropical climate and
moistly humid soil year round. Summer dormant.This is the monster in the
genus, a well grown plant easily reaching 5 m in stature with thousands of
flowers for a couple of months. Say zone 10 but acceptably well in a
protected spot in zone 9.
As you see, not easy to grow all species well in the same climate.
Regards
Alberto
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