What you need to consider before preparing plants for winter: Clean plants before bringing them inside: Remove all fallen leaves, any withered or diseased foliage, and all dead wood. If there are pests on the plant, you need to treat it again. Never bring plants inside for winter with pests on them!
Choose a suitable location:
Light: Deciduous plants can be put away in a dark place for the winter, but evergreens need light. The warmer the location, the more light the plants need.
Temperature: There are three different categories: Plants which tolerate frost, plants which need to stay frost-free, and plants which need to stay warm over the winter. It is best to keep Mediterranean plants in a light place at temperatures of between 10-12 °C over the winter and to water them moderately so that they don't dry out. Keep tropical plants in a light place at a temperature of 20/22 °C and water appropriately so that they are not too wet but stand in moist soil.
And once again: The warmer the location, the more light the plants need!
Assistance: Inflatable greenhouses with frost montitors are now available and make suitable overwintering locations for Mediterranean pot plants if you don't have enough space for them elsewhere.
When should I bring plants inside for the winter?
The basic rule is as follows: Bring pot plants inside as late as possible and take them out again as early as possible. As a general rule, plants are better off outside than inside where possible since being inside is an abnormal situation for plants which causes them stress. Angel's trumpets, bougainvilleas, hibiscus and heliotropes, for example, must be protected from frost. Flowering maple, bottle-brush, cassia, citrus, tea-tree and leadwort can withstand freezing temperatures. Mild frost (to approx. -5°C) can be tolerated by hardy plants such as Japanese laurel, Norfolk Island pine, arbutus, Italian cypress, loquat, laurel, olive and Italian maple.
Things to consider about winter quarters for plants:
Ensure strict hygiene in the winter quarters. Clean the plants like you did before bringing them inside for the winter every month. Check for pests from the end of January since scale-insects may start to appear at this time and spider mites, mealy-bugs and aphids are also possible from the start of February. Make sure that the plants' winter quarters are properly climatised to maintain the correct overwintering temperature. It is just as important to install a frost monitor where necessary, since you will need to provide frost-free ventilation if the temperature is too high. Large glazed winter quarters in particular can get very hot from the the beginning of February. Overwintering plants should not, however, be exposed to large fluctuations in temperature or even be taken outside before time.
Information about individual varieties you have asked about:
Agapanthus: Overwinter all overwintering varieties with leaves in a bright, cool place at a temperature of 5-10 °C; Water sparingly - only as much as is needed to prevent them from drying out. Varieties which lose their leaves can be overwintered in a dark place and don't need to be watered. Repot every two years in the spring.
Thorn apple/Angel's trumpets: Take inside before the first frost: if the plant has been slightly damaged by frost and leaves are falling off but the stronger wood is not frost-damaged (brown and soft) it is still worth putting it inside for the winter. Prune as little as possible: The more you cut back, the later the plant will flower next year. Overwinter thorn apple in a bright, cool place at temperatures of 2-5 °C; If it is kept in a place which is too dark, it will shed its leaves and sprout comparatively late in the spring, which in turn means it will flower later. Water plants with leaves in moderation in the winter, just enough for them to survive with foliage; Water bare plants sparingly to keep them from drying out.
Fig tree: As yet unripe fruit for the following year now hangs on fig trees. Frost-resistant figs like the Bavarian Violetta fig in a pot can withstand outdoor temperatures down to -5 °C if water drainage is attended to and if you wrap the plant and its pot in fleece. The easiest thing to do, however, is to put the plant in the garage. There it can stand in the dark and also feel a little frost. Water it enough to ensure that the fruit doesn't shrivel up. A crucial factor to the overwintering of figs with their fruit: The plant should not be exposed to wild fluctuations in temperature (e.g. exposed to strong winter sun during the day and frost again at night), otherwise the fruit will shrivel up or fall off the tree.
Gingko: In a pot it is more exposed to frost hazards than in the ground. For this reason it is best to put in the garage. Or take it out of its pot over the winter and tuck it up in a protective position in the ground. If you have to keep in on a balcony over the winter, wrap the pot and the plant up well in fleece (not film!).
Herbs: Parsley, lovage, lemon thyme etc. can stay in their bed. Curry plants and lavender cotton are best kept in a bright, cool, frost-free place at a temperature of 5 to 10 °C
Daisy bush: When putting this plant away for winter, prune about a third of the plant. Overwinter in a bright place at a temperature of 10/12 °C and water in moderation so that it doesn't dry out. Put into fresh soil in the spring.
Oleander: As a strong, established plant it will tolerate frost between 0 and -5 °C, i.e. 'slight frost'. In its overwintering location it should be kept in a light to moderately light place at a temperature of 5-10 °C Water sparingly, but make sure that it doesn't dry out.
A comment on your pruning questions: We talked in detail about pruning oleander in the last GARDENA Newsletter, please refer to that newsletter for answers to your questions.
Passion flowers: In mild climates (wine growing climates) and if planted in a sheltered location, passion flowers ( Passiflora caerulea) can usually remain outdoors. There they will withstand temperatures down to -5 to -10 °C if they are well established. Otherwise, take them indoors and overwinter at a temperature of no more than 10 -12 °C if the outside temperature is between 0 and 5 ºC. Water sparingly to ensure that they don't dry out. Note that there are different varieties of passion flower, including ones which don't like heat. These, like tropical plants, need to be in as light a place as possible and at a temperature of 20 °C, and kept consistently moist.
Citronella plant: Overwinter in a light place at a temperature of approx. 10-15 °C and water occasionally in moderation.
A comprehensive guide to pot plants and tub plants including how to keep them over the winter was compiled by GARDENA gardening experts and published in 2003 by KOSMOS in Stuttgart.