Apple trees are among the most cold hardy of fruit trees for temperate climates, as they are native to cold winter regions of North America, Europe and Asia. North Carolina State University notes there are several factors needed for successful production of apples on outdoor trees; more issues must be addressed if apples are to grow indoors in a greenhouse. If the basic greenhouse cultural requirements are met, drought or heat stresses are avoided, and pollination is made, apple trees might grow in an indoor environment with considerable effort. Shane Smith, author of "Greenhouse Gardener's Companion" still thinks growing apples under glass is "nearly impossible."
Apple trees need four basic needs in order to have any chance of successful culture in a greenhouse. According to the "Sunset Western Garden Book," apples must have winter chill (900 to 1,200 hours of temperatures below 45 degrees Fahrenheit), abundant sunlight, moist and well-drained fertile soil, and a means for pollination if fruit is desired.
Apple tree varieties are loosely grouped by their mature sizes: standard, semi-dwarf and dwarf. Standard apple trees grow quite large, 20 to 30 feet tall and wide. Semi-dwarf and dwarf trees are grafted upon root stocks that lessen the mature growing size of the plants. In a greenhouse, dwarf trees would be most practical, as they will best remain within the size constraints of soil beds, containers or roof and wall dimensions of the structure.
Vernalization is the process in which apple trees are exposed to sufficient winter cold to initiate formation of flower buds. If the threshold or chilling hours requirement of an apple tree variety is not met, no flower will occur (or subsequent fruit production). The Estate Fruit House in the West Conservatory complex at Longwood Gardens in Kennett Square, Pennsylvania grows nectarines under glass. Here the protective glass keeps winter temperatures slightly warmer than outdoor temps. The windows are vented for temperatures to drop low enough to guarantee vernalization. Even in the depth of winter, the greenhouse is warm, providing spring-like conditions.
Logically, a recommendation listed in Sunset's Western Garden Book is to utilize low-chill apple varieties. Such varieties only need 100 to 400 hours of chilling below 45 degrees Fahrenheit to produce flowers. They are often grown in Southern California or along the Gulf Coast--cultivars such as 'Anna,' 'Pettingill,' 'Tropic Sweet' and 'Dorsett Golden.' The latitude of the greenhouse and presence of a cooling system will play a role in what chilling hours can be maintained.
If a greenhouse-raised apple tree flowers, pollination is needed to yield fruits. Although many apple trees are partially self-fruitful (their own flowers can pollinate each other), a general recommendation is that two or more apple tree varieties are grown near each other that bloom at the same time. Moreover, pollination is best facilitated by insects like honeybees or bumblebees. While the apple trees bloom, these insects must have access to the greenhouse to ensure pollination of flowers.
Sunset's Western Garden Book mentions you can overcome the issue of needing more than one apple tree by grafting a branch of another tree onto your apple tree or placing a bouquet of apple blossoms from another apple tree variety in a vase of water at the base of the tree in the greenhouse.
Additional concerns arise when attempting to grow apple trees in a greenhouse. Insect pests and diseases are two issues. Ambient temperatures in the summertime and proper watering in the intense indoor heat in summer are two other concerns to overcome. Fruit development is impaired if trees do not have good soil moisture or nutrition. Many apple fruits need bright sunshine and chilly fall temperatures to develop the sweetest flesh quality.