The killing frost

The fact that the last many winters have been mild in Denmark has made me act with presumption, ignoring several basic rules for growing cacti in freezing conditions. The week before Christmas Denmark was hit by an early cold spell, bringing hard frost that made many of my coldhouse grown plants suffer the consequences of my folly.


Frozen Lophophora williamsii (SB 854; Starr Co, Texas)

The family spent the Christmas holidays in the summerhouse arriving late December 21, the day of the winter solstice. Of course I had to check up on my plants in the coldhouse first thing, but was met by a disheartening sight: The beautiful turgid, dark jade green Lophophora williamsii plants pictured above are frozen solid.


Lophophora williamsii draped in horticultural fleece

The greenhouse walls are covered with curtains and the plants are draped in horticultural fleece to protect them from the cold, still the temperature was measured to slightly less than -7 C (~ 19 F) just next to the plants. The temperature alone shouldn't be a problem as these plants have endured conditions in the coldhouse since 2004 (actually these very plants started my experiments in Lophophora cold hardiness so I'm double sad to loose them), but freezing conditions combined with plants that haven't had the time to go flaccid before subjected to the frost is a deadly cocktail. I'm sad to admit that I watered the above Lophophora williamsii plants along with a couple of Gymnocalyciums and two large pots of 3 year old L. williamsii seedlings very late in the season (late September). To make things worse October and November came with a perpetual overcast sky and constant, high humidity levels, leaving the plants no chance to dry out thoroughly. Finally, to top things off, December brought very early and double digit freezing temperatures (measured in celsius) killing off the plants still bloated with water. I have learned my lesson and will never water any of my coldhouse plants later than the end of August (the exception being very young seedlings that might otherwise succumb to drought).


Thawed Lophophora williamsii (SB 854; Starr Co, Texas)

Come Boxing Day the frost went away, leaving me with thawing plants and the possibility to asses the damage – unfortunately I had to leave for home the day after, so the exact casualty figures will follow in a later post.

The thawed Lophophora williamsii plants pictured above felt soft and slightly mushy to the touch but I didn't have the heart to squish them to see if they were fluid inside.


Thawed Lophophora williamsii seedlings

The two large pots of 3 year old Lophophora williamsii seedlings are gone for sure. Upon thawing the seedlings first felt like small bags of slush ice before the interior turned completely fluid. Subjected to a light pressure the epidermis broke and liquid oozed out, indicating that the cell walls are lacerated by the frost.


Frost killed Lophophora diffusa

Another mistake I made was to move new plants into the coldhouse without leaving them sufficient time to acclimatize properly for the winter – the plants were moved to the coldhouse in late September/early October. On that account I lost 3 large Lophophora diffusa like the one pictured above.

As is evident from the flaccid (and very much alive) L. diffusa pictured below, these plants are able to survive the cold conditions if treated properly. The two plants are from the same batch, are growing under the same conditions (and the pictures were taken the same day). The only difference being that the survivor was given proper time to adjust to its harsh environment.


Alive Lophophora diffusa

I also recently introduced a handful of new Ariocarpus plants to the coldhouse – they had me somewhat worried but seem to cope.


Frost damaged Penis Cactus

My last mistake was to leave a collection of columnar cacti out on the terrace until mid October, exposing them to precipitation and moisture for much longer than what's healthy for plants that must winter in freezing conditions. This has taken its toll on my formerly so erect Penis Cactus - I'm not sure if the plant is completely killed off by the frost but it is certainly damaged. I'm worried for the rest of my Trichocereus collection also; many of the plants felt completely frozen to the touch but are not showing any evident signs of damage yet – time will tell, though.


The lesson I have learned from this experience is to always expect the worst from the weather and prepare accordingly (which should have been evident in the first place!). Even if the last many winters have been mild, I should have known that frost can set in suddenly and quite early in Denmark – instead of being lulled into the assumption that most of the winter would be mild with hard frost arriving late, as the situation has been the last few years. I won't be that careless in the future.

After a couple of mild days the weather reports again predict a period of hard, day- and nighttime, frost extending into the new year.

Update, April 11, 2010
Read more about the casualties and survivors of the frost.