Welcome to the Saruyama Blog, intermittent and generally off topic. Occasionally you might see some trees...and weird ones at that.

Tuesday 27 September 2011

Winter Protection

After the devastation of the last few winters, it is essential that bonsai enthusiasts in the UK take precautionary steps to protect their trees from the harsh winter that will be coming. It may snow in October and we are likely to see some very low temperatures. Lots of factors are causing climate change, man made and natural, however what is certain is that if we have another harsh winter and trees are unprotected then we will have a lot of dead trees on our hands. I will admit that over the last two years I have lost close to £10,000 worth of trees. I cannot let that happen again. This happened because I didn't prepare for both the weather and my absence from the country during it.

Having learnt my lesson the hard way it seems perverse to me that many people are unwilling to invest a little time, effort and money in creating winter protection when they may have spent lots of it on their trees. Bonsai is a continual investment, if that is lost at any point, then all the previous work and effort has been wasted. Just for the want of a little extra spending, everything goes to waste. The same is true for soil, pots, fertiliser etc.

Bonsai are more susceptible to the cold as the roots in a pot will freeze much easier than those in the ground. The earth retains a lot of heat and only the surface layers of soil are seriously affected by the frost. In a pot, the entire root ball can freeze and cause fatal damage to the tree. This must be avoided....but how?

Trees must be protected from cold soil temperatures, over watering, and wind. To this end, for the few trees I own, I have constructed a small poly tunnel, in which there is a heating bed to maintain soil temperature above freezing, a fan heater to keep the air above freezing and hopefully settling on the tunnel itself. All delicate trees will be buried within the heating bed which contains a heating element attached to a thermostat which will activate the heating once the soil temperature drops below 3 degrees C. The idea is not to keep the trees growing but rather to stop any damage. The lack of air flow through the poly tunnel will also keep moisture loss to a minimum and so I cannot imagine that any water will be needed over the winter.

Poly tunnel frame, ground prepared for the bed etc. My mum wasn't overly pleased.

Constructing and choosing the materials was relatively easy. A little though was required to find the most cost effective way but carefully searching the internet and recalling some long forgotten physics helped.

The poly tunnel was purchased from ebay for £90, 4m x 2m, with careful packing it can fit a good sized collection. The heating element and thermostat were also purchased from the internet at a price of £80. A cubic metre of perlite and expanded clay was delivered to my door for £120. We had some old fence boards at home which helped but timber to build a bed can be acquired for £30. A fan heater £20, plus RCD breaker and extension cable for £10. The electricity used over the winter will be minimised through setting the thermostat as low as possible. In total, around £400 spent to safeguard the current financial investment and six, seven years of work and some future masterpiece trees....even if I do say do myself.

The bed was constructed and the path of the 25 m cable planned out. It was just as planned. The walls of the bed are old fence panels fixed together and held in by posts. It isn't the most permanent of structures but it will last the winter

A very thin layer of perlite was spread out over the base of the bed and the heating wire set out. On top of this, about 15 to 20 cm depth of perlite. If the temperatures were much higher, then over heating would be an issue, however we are aiming at minimal temperatures, just keeping it above freezing

I chose Perlite as the medium for the heating bed as it came out on top in several important factors; specific heat capacity, thermal conductivity and price per volume. It requires less energy to heat up, it retains heat and moisture incredibly well and is cheap. I will reuse it for my potting mix for freshly collected trees, whips, sick trees etc. I am a firm believer in Perlite for certain situations and having a cubic metre next spring is not a problem, I will keep the bed for problem trees I think. I topped off the perlite with a layer of expanded clay, slightly more expensive but heavier and larger particle sized, offering another layer of insulation and stopping the perlite from blowing away either from the wind or the hosepipe. The only issue with Perlite is the dust that comes with it, breathing it in is not a good idea, so I opened a hole in the bag, stuck the hosepipe in and made it damp. The hosepipe was used to stop any more dust from flying up when pouring out as well. A dust mask is advised.

All the trees in, some are buried in the bed pots and all, other stronger trees are left unprotected but will be wrapped when the weather turns.

You may notice that the deciduous trees are still in leaf and it is incredibly early to be preparing, however I am away almost all of October and there have been long range forecasts which predict snow soon. Better safe than sorry.

Heater and grow lamp. Heater is not pointing towards trees, on the lowest frost guard setting and on 1KW rather than the max 2KW setting. Lamp is above pines, junipers, yew and rosemary trees Still a considerable distance though, at most 1.5 m, closest is 1m

One other feature I have included which I have used every year in the smaller greenhouse is the grow light. A 250 W system which gives artificial sunlight, as close to the normal spread of wavelengths from the sun as possible. Why do this? All deciduous tree which lose their leaves do not need sunlight whatsoever and could be kept in a pitch black garage until close to spring time. Those evergreen trees which still have leaves are slightly different and require a little sun to see them happily through the winter. The idea is not to keep the growing throughout the winter, but just to keep the engine at idle and not stall. Once spring begins and we have natural sunlight, the trees start off quicker and grow much better. Mediterranean species and Japanese pines and junipers are not used to the grim Northern winters and so a touch of sunlight is essential for happy trees. It is not essential to keep them alive, but just to keep them happy I would advise investing in one. Prices start from £80 for a small system. I run mine on a timer, giving around 3 hours of sunlight a day, from November through until end of February. At around 1 Kwh of electricty per day that is roughly 15 p a day, lets say tops £20 a winter. The bulb has a tendency to blow or need to be replaced every four months, so thats another £15 a year. I'm still well under the £10k and years of work I threw away over the last few years.

Protect the electrics...the big white box is the ballast for the light. The other box is the thermostat. They are splash proof but use a safety RCD at the main socket just in case. With a 250 W heating cable, a 250 W light and a 1 KW heating fan, there is a lot of potential power going through there.

In conclusion...the basic rules for winter protection are to:

  1. Keep the roots from freezing, expanding and dying. This is achieved by insulation and minimal watering. Regulate water intake, bury or wrap trees in something...even snow is better than exposure to wind.

  2. Shelter from winds, especially the cold north winds

  3. Allow the trees to go dormant. Do not over heat the greenhouse and keep the trees from going dormant.

  4. Get as much natural sunlight as possible. Ideally supplement with grow lights.

Please give more consideration to the winter...even if it is just burying the trees in the ground, you don't have to go too far...just act now before it is too late.


  1. Thank you for this post, it is wonderful information, and gratefully read.