With the tree planting season in full swing, now may be a time to figure out what varieties of deciduous fruit trees to plant in your yard.
Variety selection is one of the most important decisions a grower must make prior to planting deciduous fruit trees. A plant variety is simply a plant that is different from its close relatives because of a consistent genetic trait. The ‘Anna’ apple, for example is a variety of apple that grows well in the desert, while the ‘Red Delicious’ does not. Knowing which variety to select for good growth and production is not difficult if we understand the basic botanical principles that govern variety selection.
A tree variety that cannot cope with the environmental conditions of the desert will almost certainly fail to produce a quality crop when it matures. Once the tree is in the ground, it is difficult to correct mistakes. For that reason, it is important to make the right selection before planting.
In general, the best deciduous fruit and nut varieties for the desert areas of Southern Arizona are those that demonstrate good pollination characteristics and have a low chill requirement. The deciduous fruits are those that lose their leaves during winter. They include peaches, apricots, apples, plums and pecans. This discussion will not apply to citrus because all types are ‘self’ fruitful’ and do not have a chill requirement.
Cold chilling, chilling requirement and chill units are terms that are used to describe a botanical principle that describes why some trees require a particular number of accumulated hours each year between the temperatures of 32 and 55 degrees Fahrenheit in order for flower and leaf buds to develop normally. The buds of plants with this requirement have the genetic ability to keep track of the total number of hours that accumulate between these two temperature limits each year.
Cold chilling works this way. When the ambient air temperatures reach 55 and continue to descend, the buds of each tree begin to recognize the lapsing time. When the temperature reaches 32 they stop accumulating chill units. As the air warms to reach 32 once again, the buds begin recognizing the passing time until the temperatures reach 55 when the accumulation stops. The eventual accumulation of total hours within the range fulfills one of the requirements that tells the plant each spring to begin its growth and reproduction cycle.
There is also a down side to the process. The tree also recognizes when the air temperatures exceed 60, and subtracts the accumulation of time above that temperature from that accumulated within the desirable range. Warm winters can cause significant problems for growers wanting or needing a crop.
Trees that have not received the necessary number of chill units during the dormant period will show symptoms of delayed and extended bloom, delayed budding of leaves, reduced fruit set and reduced fruit quality. These symptoms are dead giveaways that the tree has a chill unit problem.
The approximate number of hours needed for normal development varies depending on the variety. Some varieties of trees may require a thousand hours or more of chill units before they can be productive. In the desert, it is rare that we would ever see an accumulation of chill units of more than six hundred units.
In most years, the low desert areas accumulate an average number of chill units of between three to five hundred, well below the required hours of the more popular varieties of fruit, such as the red delicious apple, the Bartlett pear and Elberta peach. In the desert, varieties that have a low chill requirement are best.
Fruit varieties that require the least amount of cold temperatures, or low chill varieties that grow well in Southern Arizona include the Anna and Dorsett golden apple; the desert gold or Flordaprince peach; the Orient, Floridahome or LeConte pear; the Katy, gold kist, or Castlebrite apricot, and the Santa Rosa or Satsuma Japanese plum.
Another critical issue in selecting fruit tree varieties is pollination. For fruit to form, pollen from the male portions of a flower must be transferred to the female portion so that fertilization can occur.
Some fruit trees are self-fruitful, which means that the pollen produced by the tree’s flowers will successfully fertilize the female portion of the flower. Apricots, for example, are self-fruitful, so only one tree needs to be planted to produce fruit.
Other fruit trees do not do a good job of pollinating themselves, if at all, and are termed self-unfruitful. In this case, it is important to plant another tree of the same type but of a different variety. The pollen from one tree will then pollinate the flowers of the other tree. The cross-pollination that occurs between trees allows each tree to successfully set fruit each year.
The pecan tree is a good example of a tree that needs to have two different varieties planted in close proximity to yield up to its full potential, although the western Schley will do fairly well by itself if there is room at the planting site for only one tree.
Most apples and Japanese plums are also generally self-unfruitful. The low chill apple variety ‘Anna’, however, will set fairly well by itself. Even still, it does seem to do best when it is cross-pollinated by another variety, such as the ‘Golden Dorsett’. The ‘Santa Rosa’ plum also sets fruit fairly well without cross-pollination.
It is no secret that good tree fruit and pecans can be grown here in the desert. The key to success lies in selecting varieties of trees that have a low chilling requirement and good pollination characteristics.
If you have questions, you can reach one of the Master Gardeners at the Cooperative Extension office, 820 E. Cottonwood Lane, Building C, in Casa Grande. The telephone is (520) 836-5221, extension 204.
Rick Gibson is an agricultural extension agent and the director of the Cooperative Extension in Pinal County.