Move citrus outside for summer now the threat of a late frost is receding: they'll be delighted by a summer holiday outside on a sunny patio and will relish the higher light levels and extra rainfall outside.
Citrus trees are straightforward to care for and rewarding to grow, with sweetly-scented blossom in late autumn into winter which then takes a year to ripen into fruit - so the same tree can be flowering and fruiting at the same time.
Lemons are a good place to start if you've never grown citrus before, especially the variety 'Meyer' which is relatively hardy and compact. In our garden centre here in Hereford you'll also find 'Kaffir' limes with aromatic foliage and 'Calamondin' oranges, whose small, slightly acidic fruits are delicious in marmalade.
All need a frost-free greenhouse or conservatory over winter, but can spend summer outdoors. The golden rule is never to move them outside suddenly.
Start with an hour or two a day at first, then over the next fortnight leave them out longer and longer. Finally choose a mild night and leave them out all night. You may have to bring them in again briefly if it turns suddenly chilly, but by June they should be settled outdoors for the summer.
