The temperature dropped to -2°C overnight, -3.5°C the night before and -4°C the night before that. That’s pretty cold for Cornwall and is what happens when you get a an easterly or north easterly air flow, low wind speeds and clear skies at this time of year. From all other directions the wind would be coming in from the sea and be less cold.
Most plants are still in winter hibernation so there are no consequences, but it being Cornwall there is a tendency for gardeners to push the limits of what they can grow and I am no exception. For plants like Leptospermum and Coprosma, damage only becomes apparent when the plants try to start into growth. Other things, the Agave americana that we had no room for under glass, the Echeverias and Asphodelines, show the damage as it happens, turning black and mushy.
Going outside this morning, my thoughts were on Camellias, which are for the most part reliably hardy outside in Cornwall. Outside, that is, provided they are planted in the ground. Growing in pots is a different matter. They can be completely killed in pots if the roots become frozen. Very frustratingly, I had potted on and moved some Camellias out of the glasshouse after the last cold spell we had and with a forecast on Saturday for 0°C or -1°C, thought that they would be OK when I remembered late in the evening that I hadn’t brought them back in. Sunday morning came with the shock of a thermometer reading of -3.9°C and the memory showing it had hit -4°C as an overnight minimum.
Shutting the stable door after the horse has bolted, I brought two of the three plants in on Sunday but the other one, a gangly thing in a ten litre pot, was left out for want of space. Out of curiosity, this morning, with the thermometer showing 1.5°C and an overnight minimum of -2°C, I checked the temperature of the compost in the pot. At the surface it was -0.4°C; 10cm down at the side of the pot it was -0.3°C and 10cm down in the middle of the pot it was +0.2°C. 15cm away from where the pot was standing I measured the soil surface at +0.3°C and 10 cm down at 2.7°C.
It would seem that even after three cold nights, the ground, while cold and even frozen right at the surface, remains above freezing just below the surface. Where the pot was standing, it would have been above freezing at the surface. That residual warmth permeates the pot so it may be that the centre of the pot didn’t freeze and while even a pot as large as 10L lost most of it’s warmth overnight, it may have stayed unfrozen in the centre. I hope so, I don’t want to lose the plant.
The other two camellias were in smaller pots, 5L, so more vulnerable, but they were only out for the first cold night and there may have been more residual heat still in the ground to save their bacon. I will find out in due course.
I’ve seen this effect before, we lost a lot of camellias at the nursery one year when it went very cold. They were spaced out on wooden staging about 90cm above the ground and the roots froze. In the tunnel next door, the rest of the batch, standing pot thick on the gravel covered ground, were undamaged.
I have several more Camellias in pots up at my allotment, both inside the tunnel and outside. All have their pots buried in the ground and I am unconcerned about their welfare.
Also left out that first night were some stragglers from my propagator, more camellia cuttings, that I had potted up just a couple of weeks earlier. They were under a clear plastic cover, but stood on a wooden shelf outside. To all intents they were completely surrounded by cold air and will have frozen solid, even on that first night. The tops look fine but my expectation is that the roots are dead and that the tops will betray that soon enough. I just clean forgot about them.
While I was out with my thermometer – a grill probe gadget, (not that we even have a grill, even the one on the cooker has packed up,) I poked it in a few more pots. A large glazed pot, 50cm across the top, even more round its belly, was -0.1°C at the surface, 3.7°C 10cm down in the middle, 1.4°C 4cm down on the sunny side and 0.4°C 10cm down on the north side. The sun was barely up, I assume that was because the south side had warmed more the day before.
A couple of small pots that I checked, one terracotta, the other plastic, both registered -0.3°C in the middle of the pot.
I don’t know how many plant species have roots that are much less frost hardy than the tops. I seem to recall losing a batch of Himalayan birch to frozen roots one year, with another birch species nearby being unaffected. I’ve never seen any data about which plants are susceptible or at what temperatures. You just get left to find out the hard way. I don’t think it affects very many, in that I’ve not lost many plants in pots where I’ve concluded that was the reason.
Most of the Camellia flowers around the garden are now brown and mushy. Interestingly, Annette Carol, which was looking as good as it ever has, still has some intact blooms, the ones not fully open. Perhaps the cytoplasmic content of the petals in part open flowers has a higher concentration of something acting as an antifreeze, or maybe the opening blooms are generating a little heat, just sufficient to protect themselves.
Other plants impress by their toughness. This morning my double hellebore was doubled over with it’s blooms face down on the ground. This afternoon it was back up, looking none the worse for the experience. Primroses were rime covered on one side, glistening wet as they thawed on the other. No sign of distress on the part of leaves or flowers was apparent.
The cold snap is over, we’re now back to wind and shortly rain. There is no frost in the forecast for the next two weeks, taking us to late March. It’s rare indeed for us to get serious cold after March but it has happened, I recall -7°C in May many years ago, but the later it gets, the more there is coming into growth, so less cold does more damage. I don’t know about ‘ne’er cast a clout till may be out’, give me a couple of warm dry days and I’m more than ready to start casting clouts. The winter may have been no longer than it ever was but has it ever passed so slowly?