Nerine bowdenii - and bulbs in pots in winter

John Grimshaw
Sun, 07 Oct 2012 01:44:52 PDT
Returning to this thread after a busy week...

In my experience any amaryllid that gets frozen solid in a pot (especially
free-standing) in winter is a dead amaryllid. This is as true for Galanthus
and Narcissus as it is for Nerine, Hippeastrum and Ismene. At best, the
roots are lost and the shoot is able to make a small bulb, but if the basal
plate is killed the plant is dead. I write as one who has had the
responsibility of bringing a commercially very important crop of potted
Galanthus through many winters, and have seen a lot of casualties in some.
[The response of other bulbous taxa varies - some, such as tulips, seem more
tolerant, but in general my comments are widely applicable.]

Why hardy taxa such as Galanthus and Narcissus, and indeed N. bowdenii,
which can withstand freezing in the ground, including freezing of the soil
to the depth of at least their rooting zone, are so damaged in pots is a
subject that interests me greatly. There is a clear difference in the degree
of insulation between the ground and a pot; the  cold penetrates an
uninsulated pot much more quickly. Perhaps too, the water present in a pot
is more confined as it freezes, creating greater pressure on plant parts.
The well-known maxim of keeping plants on the dry side in winter certainly
helps in minimising damage.  Having seen innumerable cases of snowdrop bulbs
lying on the surface and growing both roots and shoots perfectly happily
through the recent hard winters (by UK standards), it is clearly not low
temperatures alone that 'do in' such bulbs when potted.

Free-standing pots are worst affected: in plunges they are much more secure,
whether under cover or not - so long as the plunge is in contact with the
ground. Many UK growers of bulbous plants in unheated alpine houses suffered
huge losses in December 2010 when elevated plunge beds on staging froze
solid. Even when frozen through the rooting zone the ground always seem to
act as a buffer to the worst of the effects of freezing (in locally hardy
plants: if tissue is frost sensitive it will be killed if it is frozen
beyond its tolerance level).

It's a complex issue, clearly, and an experimental approach would be useful
- but if in doubt, don't let pots of bulbous plants freeze solid.

John Grimshaw

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Dr John  M. Grimshaw
1 Kirkhill Farm
North Yorkshire
YO17 8NT

Tel. 01944 768494

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