A short summary
In 2017 we have moved majority of plants from Vienna, Austria to Ven, island of Sweden with a normal range of temperatures from -15C winter/+35C summer  in Vienna to -5/+25 on Ven.
Knowing how the plants were "behaving" in  Vienna we can  observe and compare the growing conditions. The peaches and apricots do not give fruits (the minus/plus fluctuations continue until May) despite milder climate on Ven. The prunus plants are often sick. On the other side  Oleander survives in the protected area, also Fig and Eriobotrya. Taking into account formal categories of the hardiness scales the conditions are better on Ven hence we try to figure out what factors are included in the hardiness definition.
the climate and soil condition on the island of Ven.
The island of Ven is situated between Sweden and Denmark,  20 km north from Copenhagen and north-west of the town of Landskrona. The island is a flat plateau (imagine a cake) about 30-40 m above the sea level. There are very few trees and the only areas protected from wind are shore areas and the protection is only possible if the wind is coming from the opposite directions. The winds are mostly from the South-West directions but in the winter time, a cold dry wind may come from the East.
The climate general information
Ven has maritime, relatively mild winters (mostly around 0 C) but during our 3 years on the island one night the temperature fell to -11C. The summer time  temperature is around 18-22 without any hot tops. The photo on the  side shows vegetables and budleja with some leaves in Febuary.
Comparing to Vienna/Austria where the winter time the temperature was -5 to -10 with down to -20 night time at night but we have experience +40 C one summer. It is important to know our reference location as we in 2017, took many plants from Vienna and planted them on Ven. The fall is prolonged due to influence of the see around  but spring is slow to enter Ven, due to the same reason.
Sesame plants survived winter, very very strange. They were planted randomly, we bought dozen of various spices and spread in the garden, just to see what will come up.
Local climate on Ven, some specific observations.
We have lived also on the cost in Italy, north of Rome and of course in Vienna. We have then noticed some particularities/anomalies in the climate on Ven. During winter there are frequent oscillation of temperature around 0C. it will go up to +5 during day and fall to -3 at night. It happens that that temperature in February may go up during the day to +10 C and stay on the plus side duringDSC02529 the night.  On the picture we can see a cydonia plant which developed  leaves too early, this is 12 March, the cold nights are "burning" the tops, the plants is protecting itself from strong sun buy coloring the leaves brown. In 2020 the peaches started to go in bloom already at the end of February, apricots in March. (of course all fruits were lost and the leave buds “restarted” again. ) Then there is a risk that the frost may come as late as in
 May and the sun radiation in May is very strong and long. Ven is an island and there are long periods of very sunny weather, one has an impression that the clouds avoid the island.
The clorosis is extreme during the spring,
All leaves are either yellow or
Flowers of mipilus at the beginning of June. The leaves are burned by earlier frosts, but the late flowering ended with a lot of fruits.
Apricots tree, April, very strong sun, the flowers develop near the thick braches which get heated but  get cooled during night, get dry and die. I think i have seen similar conditions in Korea with cold winds from Siberia in Spring.
The soil
According to an official classification the soil is absolutely at the top  quality in Sweden. The humus layer is about 30-50 cm resting on a clay base. The soil is very alkaline.
Precipitation and water conditions.
We have been only 3 years on island and do not have any long interval for making conclusions. Apparently the last 3 summers 2018, 2019, 2020 were exceptional. We have observed extremely dry weather, with a very uneven reinfall  distribution. The late winter mid-January and February were very rainy then 3 month the precipitation is minimal. The annual participation is around 500 mm if one interpolates from the towns located in the vicinity, in Sweden and Denmark.
The water retantion on the island is a difficult question. It seems that the clay is interwoven with a net of mash structure of clay sand and gravel (at approcimatelly 2-5 m depth) so the surface water disappears relatively quickly and is not forming permanent ponds. The astronomer Tycho Brahe in 16 th century created a chain of connected ponds in order to collect water for his paper mill. We assume this way he was preventing loss all water if he would dig one big artificial lake. By testing water retention in small ponds he was able to choose location without drainage to the underlying porous structures and outflow to the sea, see the picture
We have created several ponds but unfortuanatelly they loose water after 2-3 month.