Daylength. As noted, the plant seems to require short days for pollination. However, this is not certain because satisfactory fruit set has been noted in south Florida at any time of year.13
Rainfall. Naranjilla requires considerable moisture. It is commercially grown in the Andean areas where annual rainfall is 1,500–3,750 mm. The lower moisture limits are uncertain, but even moderately dry conditions check its growth.
Altitude. This is not a restriction. Samples have been collected near 2,000 m elevation in Ecuador, and the plant grows near sea level in New Zealand and California.
Low Temperature. Below 10°C the plant's growth is severely checked. It is frost sensitive.
High Temperature. Above 30°C the plant grows poorly. It does not set fruit in areas with high night temperature—a possible reason why it has failed in some lowland tropical and subtropical areas.
Soil Type. In Ecuador, the naranjilla grows best on fertile, well-drained slopes. It requires soils that hold moisture but that drain well enough to avoid waterlogging. It seems particularly sensitive to salt.14
Related Species. Naranjilla has several relatives that produce desirable fruits and deserve more attention. They could be useful in their own right as well as perhaps for genetically improving naranjilla.15
Solanum pectinatum This plant, although undomesticated, produces high-quality fruits, almost as good as those of the naranjilla. Compared with the naranjilla, the fruits are slightly smaller, but their hairs rub off more readily, and they taste somewhat sweeter. This is a lowland species, widely distributed from Peru to southern Mexico, and from sea level to 1500 m. Throughout the region, people gather the wild fruits for making juice. It, too, deserves wider appreciation; a little horticultural investigation might produce a new crop as popular as the naranjilla, but suitable for cultivation in areas too warm for naranjilla.
Solanum vestissimum Native to Colombia and Venezuela, this is another wild species with pleasantly flavored fruits. It grows at higher altitudes than naranjilla. Known as “lulo de la tierra fria” (naranjilla of the cold lands), toronja, or tumo, it is a small tree that bears fruit about the size of duck eggs. The chief objection is that the fruit's hairs are quite bristly and the juice is difficult to extract. For all that, however, it has an excellent flavor and deserves research attention.