The lower desert areas experience freezing temperatures several times during the winter and it is not uncommon winter vegetables injured. There are two types of injury that low temperatures can cause on vegetables. Chilling injury is what happens to some vegetable crops of tropical origin held at the wrong storage or transit temperature, but a temperature above 32°F (0°C). Generally these are temperatures around 41-50°F (12.5°C). Chilling injury occurs at temperatures well above freezing point. The tissue becomes weakened leading to cellular dysfunctions. Symptoms include surface lesions/pitting, internal discoloration, water soaking of the tissue, failure to ripen normally and increased susceptibility to decay organisms such as Alternaria. Maturity at harvest also affects the susceptibility to chilling injury in products such as tomatoes, honeydew melons and peppers.

The second type of injury is frost/freezing injury and it can occur in a field when temperatures drop to or below 32°F (0°C). It can also occur during cold storage if temperatures were below the freezing point of the product. This exposure to freezing temperatures may have a drastic effect upon the entire plant or affect only a small part of the plant tissue, resulting in reduced yields or poor product quality. Ice formation on the plant and in the plant tissue is what causes the damage, especially when the tissue thaws. Postharvest researchers have worked on identifying the freezing temperatures for most crops and have published tables with the recommended optimum storage temperatures. No vegetable or fruit cold storage facility should be without these tables and they are available for download at the UC Davis Postharvest web site (http://postharvestucdavis.edu ). Researchers have categorized fruits and vegetables into three groups based on their sensitivity to freezing (Table 1): most susceptible are those that are likely to be injured by one light freeze, moderately susceptible are those that will recover from one or two light freezes and least susceptible are those that can be lightly frozen several times without serious injury. Table 2 summarizes freezing symptoms for a range of common desert grown vegetables.

Table 1. Susceptibility of fresh fruits and vegetables to freezing injury.

Most susceptible

  • Asparagus
  • Beans, snap
  • Cucumbers
  • Eggplant
  • Lemons
  • Tomatoes
  • Lettuce
  • Limes
  • Okra
  • Peppers, sweet
  • Potato

Moderately susceptible

  • Broccoli
  • Carrots
  • Cauliflower
  • Celery
  • Grapefruit
  • Grapes
  • Onion (dry)
  • Oranges
  • Parsley
  • Radishes
  • Spinach
  • Squash

Least susceptible

  • Brussels sprouts
  • Cabbage, mature and savory
  • Dates
  • Kale
  • Kohlrabi
  • Parsnips
  • Turnips
  • Beets