Caladiums are indigenous to Brazil and neighbouring areas of South- and Central- America where they grow in open areas of forests and along banks of rivers. They have been in cultivation in Europe since the late 18th century.
Plant outdoors only after the soil and night time temperatures have reliably warmed up to above 60F/15C, preferably even higher. These very tender bulbs, which are grown in the subtropics of Florida and Brazil, love and require warmth above anything else. To get a headstart they can be grown indoors in flats or individual pots filled with damp peatmoss or vermiculite. Cover the top of the bulbs (look for the 'eyes' on the rounded side) with 1"/3cm of soil. Again make sure to give them warmth (65F/18C or higher) and moisture/humidity to get them going. Then transplant them in your garden or containers in early Summer. Water well and frequently. In areas with lots of bright intense sunshine definitely plant them in full or partial shade (or the leaves will get burnt). In cooler climates with more overcast days they can be planted in full sun but do keep them watered very well during periods of bright sun and heat.