Agapanthus have fleshy roots and leaves and this can make them prone to frost damage. The deciduous varieties are hardiest, dying down in winter. Once they are established, they should all withstand most conditions.
Evergreen types are more tender and their leaves can be damaged by frosts. Therefore, a mulch of straw or fleece is advisable when young plants are establishing or extreme cold (below -5ºC) is forecast. Established clumps of evergreen Agapanthus can withstand -10ºC to -15ºC if the ground is well drained, but the number of flowers maybe reduced the following summer. Planting in beds against house walls can reduce the likeliness of frost damage.
Agapanthus doesn't like to be potted into pots that are too spacious as this will encourage leaf growth rather than flower production. Ideally, they perform best where root development is restricted but the plants are well watered and fed.
Feed weekly or fortnightly with a balanced liquid feed during the growing season until flowers begin to show colour.
Although plants flower well in pots if the roots are constricted, they should not be allowed to become completely root bound and should be split and replanted in fresh compost if the roots become too congested, otherwise flowering will suffer.
Good information !