From this time on, two climbing plants have dominated Central Europe: ivy and grapevine. While ivy grew uninvited, the breeding of grapevines was a deliberate choice in facade greening. Introduced by the Romans in the first millennium after Christ, problems arose time and again with the ripening of the berries. But wine had to be produced- above all for the sacramental communion wine for their rituals- but also as a source of enjoyment and comfort for the toil and hardships that characterized such a time; when distilled, the fermented 'spirit' was used as a disinfectant and narcotic for medicine. But what was found in the barrels often resembled bitter vinegar instead of drinkable wine. Because the native vines coming from the south lacked warmth, and the German monks were anxious to cultivate particularly fast-ripening varieties, sour grapes were often the result.
When grapevines were put on protective monastery walls, vineyard walls, or on house walls, they warmed up, ripened better, and became sweeter and sweeter! Clearly, such fruits (or what children and sweet-toothers left behind) went immediately to the wine press. Wall greening became thus a very practical undertaking.
Grapevines are thus always suitable on buildings that were greened in the Middle Ages, because everywhere where such greened structures stand, viticulture (wine growing) was practiced! Today, however, modern, fungus-tolerant varieties are recommended.
The question of whether the ivy on medieval castles was always there is up for debate. Tourists don't question it; they simply find it "beautiful." In any case, ivy is always appropriate when large-scale greenings are desired on very old masonry.
The presence of honeysuckle, and especially roses, have also been detected in this period-- not our current cultivated varieties, but wild roses like Rosa Canina, the 1000-year-old (millennial) rose at the cathedral in Hildesheim. Espalier fruit and hops were also found. Other plants can look very nice now, but they are only an encore of our modern times, not Middle Age originals.