Sunlight and Temperatures: Finland versus Europe, part II

In the previous post the amount of sunlight in Finland was compared against some cities in Europe.

The sunlight itself is not enough for the plant to thrive. Combination of temperatures which stimulate photosynthetic ability and activity is what the tree makes grow. Also has to be kept in mind that the amount of sunlight shown in the previous post represents exactly that: amount of sunlight but not the light which is absorbed by the plant! The continuation tries to address that issue.


From the same site as earlier the average high temperatures are extracted being shown in the graph below:


The light blue thick line is Helsinki data. The high average temperatures are used because they represent the day temperatures, not the night ones when the trees metabolism slows down.

That together with previously documented graph of sunshine…

Sunny hours

… served as the basis for the next step.

The rate of photosynthesis

In order to asses the ability of the tree to grow it is necessary to estimate the trees photosynthesis rate which depends on the temperature.

The photosynthetic rate is extracted from the documented table. The simplified curve (triangular shape) was created for temperate coniferous trees with initial/boundary points as below:

Temperature Photosynthesis rate
-3 0
25 1
35 0

Multiplying photosynthetic rate (as a function of temperature) with the available sunlight amount makes the next graph:


The bell curve of Helsinki is very strong in summer, especially July with sudden drop in August and very low values for September.

It is also visible that too high temperatures are not favorable for the tree to grow. I consider the drops (very strong for Athens, less for other cities) as a clear sign of the summer dormancy. Does that itself defines it is another story but this is nearer the reality than the sunlight or temperature alone can show.

Annual growth

Comparing the curves form the latest graph is not simple. The simplest way is to create an integral on them to estimate the surface area which would represent the annual “growth ability” (the latest is purely my term :D).

Those values normalized around the Helsinki value (which now equals to 1) makes the data for the next graph showing the relative growth ability (there must be a better name but right now my brain is stuck with this!). Now the comparison is much simpler and it gives also more value than just “better” or “worse”.


Growth period extension

On all graphs above I included the line/bar with title “Helsinki (extended), Finland“. On line graphs it is the dashed light blue line which is mostly overlapping with the basic Helsinki curve.

The “extended” values are the same as basic Helsinki set of data except that the average high temperatures for March and April are increased by 5 degrees C. The idea was to compare the difference in other curves/comparisons if the spring would be more favorable and what I kept in mind was the greenhouse ability to keep the temperatures even just by 5 degrees above the normal ones (in average) over those 2 months.

The latest graph shows that Helsinki jumps to be in par with Berlin and right after the rest all way more south European cities. The rest of north and west was left behind.

Update: Growth period extension 2

After Janis comment I added the next step. “Extending” growth season even more for September and October by also 5 degrees C. The values are marked by “Helsinki (extended2), Finland”.

The results show that Helsinki jumped over Berlin, though not really coming much closer to Istanbul which is very much understandable.

Conclusion of mine

This is plain theory! Yes, yes and absolutely yes.

Do you need this calculation to know that warmer spring helps in growth?! No, of course not but in order to play with concrete values, to be able to compare them one to another some data manipulation is needed. Before this “research” I would not consider stating that growing a bonsai in Finland might be easier than in UK/Central Europe after figuring out how to avoid the winter dangers of course.

But now I can. It is just my statement but as such it suffices to me 🙂

Understanding the potential could lead to other implications on the way how to grow bonsai here in the north. Some are already mentioned in Janis blog and for sure will come more when practical actions/procedures prove them.

Also to be kept in mind is the following:

  • the used values are average per month over longer period of years
  • the “temperate trees” is very generic and should be considered as such

The data and all calculation is available here.

Sunlight: Finland versus Europe

Idea of my “research” was to compare the conditions to grow bonsai between Finland and the rest of Europe.

Potential of solar energy in Finland, the name tells it all.

2 pictures from the above title, giving reliable data:


Finish cities






Excerpt from above title:

The amount of solar energy is about the same in Finland as in Central Europe, but most of the radiation (1170 kWh/m² per year) is generated in the southern part of Finland during May to August. (VTT, 43, 2015) In Finland, there is more diffuse radiation than direct radiation. Diffuse radiation is more effecting, because in southern Finland half of the radiation is diffuse radiation. Diffuse radiation means that the sunlight has been scattered by molecules and particles at the atmosphere, but has still made it down to the surface of the earth. Solar irradiation is lower in northern Europe than in central or southern Europe. The average daily irradiation in Finland is about 900 kWh/m².

The same can be seen on the map of European solar irradiation. More data per country is also available.

Finland (south) is sunnier than Sweden/Norway, part of Be-Ne-Lux, UK and Ireland!

Average sunshine in Europe shows Helsinki compared to other cities, 1780 hours versus:

  • 1546 in Brussels/Belgium,
  • 1504 in Cologne/Germany
  • 1364 in Birmingham/UK

More accurate date per month to see when is the most light and how it compared with other months/cities can be done via solar calculator.

Not to loose the link to this interesting material:

The long days of a temperate-Arctic growing season provide an integrated fluence of photosynthetically active radiation (PAR, λ = 400–700 nm) similar to mid-latitudes (Jagels and Day 2004). Compared with mid-latitudes, however, the Arctic light regime provides irradiances below the photosynthetic saturation point of many tree species for a greater amount of time (higher proportion of “useful” radiation). In addition, irradiance in the Arctic lies for a greater proportion of time within the more linear portion of the photosynthetic light response curve, where photosynthetic efficiency (mol CO2 fixed per mol incident quanta) is maximized.


Graph of the sun light per city during the year shows the relation between the cities (Thanks Jani!):

Sunny hours

It is easilly visible that south of Finland is not at all having low level of sun light. All UK cities (from the available statistics) are below. It does show slightly narrower bell curve than some other cities but all of them are way more south.

My 5 cents
  • The lack of sun is not an excuse for not growing bonsai or to complain of slow/insufficient growth.
  • The slightly longer autumn is actually even better for plants to prepare for overwintering.
  • I have not noticed summer dormancy in my trees. They grow all the time during the summer. Maybe the speed slows down but to me not noticeably.

What does create difficulties over here is the severe winter. Nevertheless, with correct tools/structures that can be overcome. Any garden suffices, balconies are slightly bigger problem but not at all impossible!

So, only I can say is, take your tools and take the challenge 😀

Next: Relating monthly temperatures with the sun light.

Cold frame – Upgrade II


I needed more space so I decided to redesign the cold frame. The walls should be upright, not anymore under angle so more trees can fit if I arrange some kind of shelf for smaller trees.

Idea was good…

Shape redesign

The walls are up now:

End result

The inside:











Nevertheless all trees still didn’t fit into it 🙁 Instead of stopping gathering new ones I figured out I can make another cold frame in front of my house 😀 The “sickness” is incurable! Something to do next autumn.

Increasing transparency

20171105_085213The roof at the moment is nontransparent insulation which I will change later to a 2 layer greenhouse walls so that light can enter also from above. Sides do give lots of light but more is always more 😀

I made 2 sheet fiberglass “window” instead of the half south wall so more light can enter and I can peak into the inside without opening anything and check how the fans work etc.


The cold frame showed to be fairly warm and it should offer protection to more sensitive species while Finish spring is not yet stable with temperatures on the plus side. With such protection extending the growing season by starting the spring earlier should be possible.

When trees start to transpire there will be more humidity inside and closing hermetically is not an option so ventilation is needed. I decided to try active ventilation on the basis of already measured temperature and humidity controlling when the fan would be turned on.

20171105_085228The fan is installed in the bottom right side wall, blowing the air out:20171105_085402






The right photo shows how vent is opened when the fan is switched on.

20171105_085500The fresh air would enter on the left side wall, from the top (photo on left).

Both air holes have one direction vents, simple ones, around 1€/piece from Bauhaus. They close the hole by gravity pulling the cover.

I set the new air to come from above and leaving the cold frame from the bottom to avoid natural ventilation effect. If the vents let air flow opposite then when air is warmer inside it would rise up and could leave the space sucking in the cold air through the bottom vent which I wanted to avoid not to lose precious warmth. This way the vents will be (should) closed as long as the fan doesn’t blow. First trials with the fan showed it does work, when fan is on the vent opens. I am not yet sure how will it work with the other one because I assume there will be some air entering through imperfect closure holes but it doesn’t really mater through what place it comes in. If it is not enough the other vent will open.

The fan is connected to TP-link wifi controlled power outlet. The decision to turn it ON/OFF comes from KumoApps which is a feature of wirelesstags products. A simple javascript can be installed to the cloud account which on the basis of temperature/humidity values decides what to do and sends the command to the power outlet thus turning the fan ON/OFF.

At the moment the script does the following:

  • temperature<0°C = fan is OFF
  • temperature between 0°C – 15°C and humidity between 80%-100% = fan is ON
  • temperature above 15°C = fan is ON

The values can be set without changing the script by mobile app remotely so this will be very handy. I am planning to modify the script to do the decision depending on  the outside temperature too (example, if outside is -5°C and inside +1°C -> don’t blow air) but won’t do it before I see how this behaves. Improvements will be very simple and easy to do.

The drawback of this solution is that if wifi/internet is down I will have the fan blow or not blow until it comes back and the new trigger event happens. This could be done more complex and safe but those I will do on the run. It is all in software. The net is relatively stable so more than 20 mins without it twice a year is risk worth taking.

I did try making the same with standalone, no-internet depending device. I found a 14€ piece which didn’t work out of the box. It was so unstable that I through it into garbage at once.  Similar more trustable got so expensive that I went with internet dependent solution.

Crazy ground “heating” idea


I dag a ditch 2 m long and around 20 cm deep into which I put metal pipe. On both sides are bendable plastic pipes. The idea is to push the air through to bring warmer air to the space above. The ground thermal capacity is much greater than of air and there is constant influx of heat from below and there is no danger it would cool down significantly the space underneath the pots.

One one side of it I installed a small CPU fan which would be turned on when needed to bring more warmer air.

The first trials were not very positive and I am not sure if I will even use this. The pipe needs to be buried deeper, around 50 cm to get more significant effect of warmth. Like this I do get 2-3 degrees warmer air than outside but its volume at the moment seems to be negligible.

This can be solved much easier and with the greater effect with just a simple heating cable.


I planned some plant lights to start increasing daylight earlier but at the moment am not sure if I do anything like that. There is tome until the spring and the trees are at the moment so cramped with some under the shelf that I most probably will postpone this until next year.