Basically, the summer lilac (Buddleja) is an easy-care and undemanding plant. However, since the shrub is not really hardy in this country, you should provide some winter protection.
Older plants are usually hardy
Provide light winter protection in winter
This is especially true for younger plants up to four or five years old - these should be mulched with a thick layer of leaves and branches before the first frost to prevent damage to the roots. An underplanting with ground covers also provides a certain natural protection. If the above-ground shoots freeze during the winter, this will not harm the plant - it will sprout again from the roots anyway.
Overwinter potted plants properly
In the case of specimens cultivated in tubs, however, overwintering is not that easy: If they are not hardy species, you should bring them indoors during the cold season and place them in a cool and frost-free location. Hardy varieties of Buddleja davidii, on the other hand, can also remain outside in the pot - provided it is wrapped in fleece and the container is placed in a weather- and rain-protected place on a house wall or similar that gives off heat.
tips
The summer lilac must also be supplied with water during the winter months. However, water infrequently and moderately. Avoid waterlogging.