Chilling Hours Are Necessary for Fruit, Nuts and Bulbs

Deciduous fruit and nut plants are genetically programmed to need a certain number of chilling hours to fulfill and break dormancy. Certain bulbs need a predetermined amount of chilling time before they sprout and bloom. The process is called vernalization and it is an essential factor in selecting and growing many fruits, nuts and bulbs.

Choosing the right fruit and nut varieties for our yards requires more than matching trees and bushes to our Zone 8 climate. We must also select varieties that require the right number of chilling hours for our location.

Hardiness zone and chilling hours are not directly related. Cold tolerance is different from the accumulation of chilling hours.

In order to bloom deciduous fruits and nuts need a dormancy period with a particular number of chilling hours during which the temperature is between 32 and 45 degrees Fahrenheit. Freezing temperatures do not count toward chilling time. Hours are cumulative; they do not need to be consecutive.

Insufficient chilling can delay spring growth. It may result in lack of synchronization between male and female flowers. That leads to delayed pollination, as well as late foliation, and reduced fruit yield and quality.

The chilling requirement, expressed in chilling hours or units, of a fruit is the minimum period of cold weather after which a fruit-bearing plant will blossom.

The weather is different every year. Chilling hours are variable and counting them can be imprecise. Accrual of hours varies over short distances. Micro climates affect accumulation.

There are a number of scientific models for counting chilling hours, but tallies vary. Consequently, chilling units are best stated in ranges.

Most of the fruit varieties recommended for our broadly identified chilling region require 600 – 800 hours; however, Clemson suggests selecting varieties with a low chilling requirement for the coastal area.

Gary Forrester regularly recommends finding “the variety with the lowest chilling index possible.” He explains, “We generally average between 600 and 800 hours but have seen as low as 400 and high above 800 to 900. Last couple of years we were in the 500 range but with ‘climate change’ who knows what the next 10 years will bring. There can also be a fairly big difference between the coast and further inland as well as rural compared to urban sites.”

Some fruits already produce well in our region. Figs, pomegranates, quince and persimmon all require 300 or fewer chilling hours. Muscatine grapes require 200 - 600 hours.

Low chill varieties of peaches, nectarines and plums need 100 – 525 hours of temperatures below 45 degrees. The Ein Shemer apple requires only about 300 chilling hours and grows as far south as Zone 9. Anna apple, another low chill variety, needs only 250 chilling hours. Southern Highbush blueberry varieties were bred for low chill areas with high summer heat.

Generally, with the exception of amaryllis and daffodils, winter in the coastal region does not provide enough chilling time for spring blooming bulbs (unless bought prechilled) to properly develop and flower. Unlike trees and bushes, we can provide bulbs with proper chilling time in our refrigerators. The best temperatures are around 40 degrees Fahrenheit. This allows them to slowly develop their root system.

Chill bulbs like large-flowered tulips, hyacinths, and crocus 8 – 10 weeks before planting them outside. In the coastal region plant bulbs after the ground has cooled. That can be as late as December, and even early January is not disastrous.

Caution: Never freeze bulbs. Never chill them in the refrigerator with fruits and vegetables where the ethylene gas from produce can cause the bulbs to abort.

Bulbs for forcing require a bit longer in the refrigerator. Chill crocus 14-15 weeks, hyacinths 10-14 weeks, tulips 13-17+ weeks depending on variety. Don’t cut short chilling. Extra time is fine. Remember, this is just the chilling time. They still need to sprout and bloom.

For bulbs, it’s time to buy them for holiday, midwinter and spring blooms. The lead time is for chilling.

Look for low chill varieties of fruit and nut trees for fall planting. Be careful at big box stores; regionally offered varieties are not necessarily low chill.

This story was originally published September 3, 2016 at 5:00 AM with the headline "Chilling Hours Are Necessary for Fruit, Nuts and Bulbs."