Brrrr, baby, it’s cold outside.

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.

A close call regarding how cold the centre of the root system got and how much damage will have been done. It was recently potted from a 5 into a 10L pot, so there are no roots in the outer 2-3cm of compost. ‘Snowdrop’ is not the hardiest of varieties to begin with.

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.

Smaller pots up off the ground will have been colder still, frozen solid, but it’s unlikely to bother these Viburnums.

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.

This pot, 50cm wide at the top, retained quite a bit of heat at the centre, compared to near the sides.

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?

9 thoughts on “Brrrr, baby, it’s cold outside.

  1. I never knew anyone would take the temperature of a plant!! I hope most of your plants survive and thrive. I watched a neighbor’s daffodils bow down low yesterday, but they perked up today. We’ve had cold weather for several days in the mountains of North Carolina.

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    1. Freezing roots had passed me by completely as a concern until we lost nearly a thousand one year at the nursery, at £10 a plant. When you lose the odd plant it’s probably the last thing you’d think to blame.

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  2. I was surprised to see ice on the top of one of my watering cans the other morning. I have a lot of pots outside including a camellia, I guess I will find out what’s dead and alive before long.

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    1. I get increasingly nervous about the damage frost can do as we move towards spring. My guess is that most people, when their camellia started to look poorly in May, would not think to blame the frost they had in January.

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  3. I had always believed that camellias were not too sensitive to frost but rather to snow on flower buds. Mine have been in the ground for 15+ years and they don’t seem to have suffered at -7 ° this winter. It may depend on the varieties of camellias maybe… I’m rather afraid for your agave and your echeverias which may have had some damage.

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    1. In the ground most Camellias are pretty safe. What slightly surprised me was how even a fair sized pot, 10L, loses most of its heat on a cold night, with only the very centre staying much warmer than the air temperature. Our Agave montana suffered some damage for the first time in the 10 years or so we’ve had it. Might be a bit further out from the house wall than in previous years, stayed a bit wetter and just a bit colder perhaps. around zero, one degree can be the difference between life and death.

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