In message .com, Dave
Hill writes
It has always struck me as strange that some young plants can take
frost, snow etc when only just emerged from the ground, whilst when
they are mature the first frost will kill them,
I am thinking of plants like sweet peas.

David Hill
Abacus Nurseries


If you'll pardon an "evolutionary just-so story", this seems quite
readily explainable. If we consider annual plants, if they grow in an
environment where frosts are regular occurrences then seedlings which
are not frost tolerant are going to be strongly selected against. At the
other end of the season, frost sensitivity is not so strongly selected
against (say a loss of 10% potential seed production, rather than 100%).
As frost resistance has metabolic costs it may well be that frost
resistance is beneficial to seedlings, but not to adult plants, as
frost-sensitive plants may outcompete by producing a heavier crop of
seeds before the frosts arrive.

I've noticed that with some pauciennial plants that they are more frost
hardy in their first winter than in their second winter, sometimes
behaving a biennials (except for a light seed crop in the first year),
or that their hardiness in their first winter depends on whether they
flower in their first summer.
--
Stewart Robert Hinsley