Tree Selection

Selection

Selecting a quality tree and caring for it appropriately increases the chances for successful orcharding. This usually begins with a bareroot tree from a quality nursery. Most nurseries are retail nurseries and seasonally purchase their bareroot selections from major nurseries or propagators specializing in tree fruits. December to March is the bareroot season.

Trees are measured, sorted, and sold by caliper, a measurement in inches just above the bud union on the tree. Historically, orchardists have learned to avoid undersized (<3/8 inch) or oversized (>5/8 inch) caliper trees. Undersized trees may be inherently weak. Oversized trees may have tops largely imbalanced with the root system due to mechanical tree digging practices common to the industry. A large top and small root system taxes the tree’s ability to meet water and nutrient requirements of the top and sustain itself during periods of stress. A preponderance of large caliper trees can appear at chain and discount stores during the bareroot season.

Nurseries are required by law to label trees as to cultivar. In addition to the cultivar(s) selected, good planning also involves choice of rootstock. The backyard orchardist should know what the rootstock is (or rootstock choices) and should have that information before purchase. Nursery literature and catalogs, especially major propagators of fruit trees, are good sources of information on rootstock as well as cultivar characteristics.

When selecting bare root trees look for a well-healed graft union.

Due to generally poorer fruit quality, dwarf fruit trees are not recommended over standard or semi-dwarf ones. It is possible to dwarf standard trees by proper pruning and training.

Chill

The growth of deciduous fruit and nut trees follows an annual pattern that changes with the seasonal transitions in the surrounding environment. Typically, temperate fruit and nut trees grow rapidly during the spring and first half of the summer. Later in the season, the growth rate declines. In the fall, the growth of deciduous fruit and nut trees stops as length of day and temperatures decrease, and the trees drop their leaves. In a reaction to length of day and temperature, growth inhibitors (hormones) are produced in the tree that prevent it from growing. In the winter months, the tree’s internal processes are in a state of rest, known as dormancy, due to the presence of growth inhibitors. Growth will not occur even under ideal temperature conditions. This prevents the trees from beginning to grow during atypical periods of warm weather only to become damaged by normal freezing temperatures later on in the winter or early spring.

Dormancy is broken when sufficient cold temperature breaks down the growth inhibitors within the tree. This is called vernalization, “chilling,” or “winter chill.” A specific number of cumulative hours of chilling (temperatures between 32°F–45°F), are required to break dormancy, which varies from variety to variety. Once the appropriate number of hours of chilling have been achieved by a variety, active growth resumes in the spring, but only after trees are exposed to warm enough temperatures for natural growth processes to begin. Most of Northern California receives between 800 and 1,500 hours of vernalization each winter. Southern California may only receive 100–400 hours.

Temperate trees and shrubs grow best in climates in which the winters are warm enough that plant tissue is not killed from extreme low temperatures, but not so warm that buds receive inadequate chilling to break dormancy. Flower and shoot buds of deciduous fruit trees and olives (evergreen) will grow normally in the spring only after exposure to sufficient winter cold. After winters with inadequate chilling, the plants leaf out late in the season (delayed foliation), blossoming is prolonged, buds may deteriorate, buds may drop, and few, if any, flowers are produced. Without flowers, there is no fruit to harvest.

The number of hours below 45°F is a fair index of the adequacy of winter chilling. Both the absolute number and distribution of the hours below 45°F need to be considered. The chilling requirements of selected temperate tree fruits and nuts expressed as the number of hours < 45°F needed to break dormancy are shown below. December and January are usually the most critical months. If each of these two months has approximately 400 hours of temperatures below 45°F distributed fairly evenly, then troubles related to mild winters are less likely. However, if seasonal totals based on accumulated chilling are concentrated in a few weeks interspersed with periods that are sunny and too mild, then the seasonal total chilling hours are less effective. Regular accumulation of chilling weather is essential for many varieties to perform optimally. Periods of a few days to a week or more of mild weather may offset or reduce the effectiveness of accompanying periods of good chilling weather. Greater seasonal totals are usually necessary in those years or districts with interrupted periods of adequate low temperatures and warm sunny days.

* See [PDF] Variety Table for low chill varieties of these fruits, which have been reported to require <300 hr of temperatures <45°F to break dormancy.

Over the years, plant scientists and breeders around the world have been selecting and developing varieties that require less chilling to break dormancy and resume growth, usually <300 hours of cold temperatures <45°F. The development of these varieties has extended the range of climates and latitudes in which temperate tree fruits and nuts can be produced in California and elsewhere. Varieties that require fewer hours of winter cold temperatures to break dormancy in the spring are known as “low chill” varieties. Specific varieties of apple, pear, apricot, nectarine, peach, Japanese and hybrid plums, and kiwifruit that are reported to have low chilling requirements, which would render them particularly adapted to the low latitudes of Southern California, are noted in Varieties, below.

In general terms, the relatively low chilling requirements of quince, figs, persimmons, almonds, olives, chestnuts, pecans, and some walnuts has enabled many varieties of these fruits and nuts to thrive in low latitudes without specific breeding programs to develop “low chill” varieties. On the other hand, sweet and sour cherries and filberts (hazelnuts) are not adapted or recommended for the low latitudes of Southern California because of the lack of availability of low-chill varieties and the high chilling requirements of current varieties.

Frost

[PDF] Frost Protection for Citrus and Other Subtropicals

Frost and its management are important in the home orchard. Clear skies and cold, still air at night can be reliable signs before frost strikes. Certain terms help in understanding this subject:

Frost damage is low temperature injury during some stage of the growing season. Parts affected are flower buds, flowers, and young (spring) fruit or near mature fruit or other tissues (fall).

Winter injury (or winter kill) is low temperature injury during rest (dormancy) and affects buds, stems, main limbs, and trunk.

Winter burn is associated with desiccation. Certain atmospheric conditions (wind) along with frozen ground and/or reduced soil moisture may not meet transpirational demands (especially with evergreens).

Cold hardiness is the ability of plants to withstand cold injury (autumn-winter). Complex physiological mechanisms occur within the plant.

Hardening (acclimation, “hardening off”) is the process of achieving cold hardiness. In a broad sense it's the process that increases plants ability to survive the impact of unfavorable environmental stress. Water, nitrogen, and other practices can affect this process.

Plant parts and their susceptibility vary:

• Opening flower buds are more susceptible than unopened buds
• Young fruit are more susceptible than flowers at full bloom

Managing frost requires full use of site factors, understanding species requirements, and cultural practices. It begins with species choices to fit the site and microclimate. Understanding still, clear, night time inversions and the movement of cold air over terrain helps us plan and prepare for frost. Past observations and low temperature records on location are useful. Know the relative bloom sequence among fruits and nuts. Place the most susceptible to damage (early bloomers) at the warmest location if the site permits and situate later blooming species in the colder areas. A maximum-minimum thermometer is a useful investment for home orchards. In addition to recording highs and lows each day, it is portable and comparisons can be made between different sites on the property. Management techniques include:

• Optimizing the orchard floor (the environment over vegetation free or clean cultivated ground is 2° to 4° F warmer than when cover crops or sod are present)
• Delayed pruning (grapes) or pruning after bud break
• Use of water on or under trees. Water releases a large amount of heat as it cools and upon forming ice. Commercially, 50 to 70 gallons per minute per acre are required to keep a mixture of ice and water on trees or vines.
• Structures – tarps, blankets, cages, etc.
• Lights, heaters – in or under trees
• Planting frost sensitive trees next to dark, south-facing walls that re-radiate heat at night
• Planting under awnings or overhangs allows plants to have some of the trapped, re-radiated heat
• Frost and cold sensitive species such as citrus can be grown in containers and moved indoors or to protected areas

Varieties

[PDF] Selecting Fruit, Nut, and Berry Crops for Home Gardens in San Mateo and San Francisco Counties, K. S. Jones, Laurence R. Costello
[PDF] Fruit, Nut, and Grape Varieties for Home Orchards (El Dorado, Amador, Calaveras Cos.), Ken R. Churches, Lynn Wunderlich
[PDF] Suggested Fruit and Nut Varieties for San Luis Obispo and Santa Barbara Counties' Backyard Orchardists, John H. Foot, Frank Laemmlen, Mary Bianchi

The region, or [PDF] climate zone, where you live in California will determine which varieties of temperate tree fruit and nut crops will perform best in your home garden, when fruits and nuts are harvested, and which pest and disease problems are more common.

[PDF] Temperate Tree Fruit and Nut Varieties for Planting in the Home and Garden Landscape describes selected varieties of the major and minor temperate fruit and nut crops that are suitable for home gardeners in California. The table could easily be doubled or tripled in size if all heirloom varieties and newer varieties available at nurseries or through mail order were included.

Certain varieties are superb eaten fresh; whereas, other varieties tend to be used more often for cooking, canning, and freezing. Experts do not always agree about which varieties are best suited for various uses because individual tastes differ; thus, the comments in the table regarding these issues are offered as points of interest only, not as official advice endorsed by the UC. Low chill varieties of apple, pear, peach, nectarine, Japanese and hybrid plums, and kiwifruit are also listed.