Selecting an orange variety that’s suitable to miniaturization is key to cultivating a successful bonsai orange plant. Calamondin (Citrus mitis) and Satsuma mandarin (Citrus reticulata) are two types that adapt well to bonsai treatment and are hardy from the Mediterranean climate. As with any citrus tree, they require full sun with plenty of bright light every day to thrive. If exterior temperatures require that the plant be grown indoors, the gardener will need to take particular care to set the orange plants in a well-lit window or greenhouse where they can get adequate light and warmth for best results.
The Calamondin (Citrus mitis) orange is hardy in U.S. Department of Agriculture plant hardiness zones 9 to 11. The Satsuma mandarin (Citrus reticulata) is hardy in zone 9. Move both indoors when overnight temperatures drop below 25 degrees Fahrenheit.
The University of Florida reports that dormant, older Satsuma mandarin trees have been known to withstand temperatures as low as 14 degrees Fahrenheit without harm. The Calamondin is the hardiest of this true citrus, withstanding temperatures as low as 20 degrees Fahrenheit.
If starting an orange tree bonsai from seed, monitor the soil temperature for optimal growing conditions. Because of citrus, orange seeds germinate best in a temperature selection of 64 to 72 degrees Fahrenheit. Maintain the seedlings moist but not waterlogged by spraying or dipping the pots in a water bath; this allows the soil to absorb water from the bottom up to prevent overwatering that prevents germination and suitable growth.
Orange plants grow best at temperatures between 70 and 90 degrees Fahrenheit and may be placed outdoors during warm, summer months. When overnight temperatures drop below 55 degrees, orange increase will slow or stop. Although both the Satsuma and the Calamondin orange plants can withstand lower temperatures, it’s wise not to stress young trees too much from the first few decades, but keep them warm enough to support vigorous growth and growth.