It is vital to distinguish between deciduous and evergreen agapanthus.
As far as I know, they are better known and therefore more popular than the deciduous agapanthus. Containers holding a plant with many large flowers swaying gently on long stems make a striking display for every beholder, plantlover or not, and its sheer elegance is stunning. Even when the plant is not yet in flower, the luscious green foliage has its own decorative value. Also the spent flower can be very decorative.
So many agapanthus lovers face their first troubles with their beautiful plants in the wintertime. These elegant plants are not easily satisfied. The large, heavy container has to be moved to an ideal place to overwinter it. Enough light and a temperature between 5 - 10 degrees are necessary. These conditions are difficult to find in our centrally heated houses. Cold greenhouses are a solution. In larger houses, there are cool stairwells and landings, which are good spots unless they are draughty. Many nurseries charge a small fee for overwintering.
In general, they are smaller than the evergreens, both in foliage and flowers. When comparing them, the evergreens may seem more striking than the deciduous ones.
But for people who cannot provide ideal overwintering conditions for the evergreen may still have space for the deciduous agapanthus.
As they do not need any light in the wintertime, a cool, dark room is ideal for them. A garage without windows which does not freeze is just as good as a cellar. Although light is not necessary, the temperature should remain between 1 - 10 degrees. If you have received an agapanthus as a present or have bought one without proper identification, there is the question how to find out if it's an evergreen or a decidous plant. An English gardener mentioned this rule of thumb: the wider the foliage and the larger the flowers are, the less hardy is the agapanthus.
In general, a night with light frost does no damage to the evergreens. If the plant is taken inside and moved to its overwintering spot right afterwards, it should not suffer. Foliage that turns yellow should be removed, as it could start rotting.
Deciduous agapanthus can withstand frost a bit better.
Only in very few German regions can agapanthus be left planted in the garden without mulch. These are places where vines are grown or which are in the vicinity of a lake. But even there, one cannot be careful enough.
If you know someone who lives in a climate where agapanthus can be left outside, this person may be ready to give away a shoot.
In all other regions in Germany, mulching is a necessity. A thick layer of dry leaves offers good protection. To keep it from getting scattered by birds or the wind, the leaves can be kept in place by pine branches or wire-netting.
Deciduous agapanthus which have been bred from seeds should be overwintered frostless in their first winter. They can be transplanted in the garden before their second summer.
Deciduous agapanthus in containers must be overwintered in frostless surroundings. Otherwise, the whole container could freeze through and the roots could get damaged. In the worst case, the whole plant could get killed.