Knowing when to harvest any citrus fruit involves the question of its degree of maturity. Technically speaking, there is no ripening process in citrus fruits and no such thing as "tree ripe" fruit. Citrus fruits pass from immature to mature and finally to an overmature condition while remaining on the tree, but the changes are slow and spread over several months. The only way to determine maturity is to taste the fruit.
Mandarin. T. Williams, Roose lab, UC Riverside.
Fruit color is a poor indication of ripeness, because many fruits have fully colored rinds a long time before they can be eaten.
Don't expect citrus fruits to increase in sweetness or ripen more fully once you've picked them, as do peaches and some other fruits. When picked at any stage of maturity, citrus fruit does not change after picking, except that it may decay or slowly dry out.
Unless damaged by frost, citrus fruit keeps longer on the tree than if picked and stored. Once you begin to harvest, pick fruit from the lower branches first, leaving the high fruit until later in the season. There are two reasons for this; one is that frost is often more severe near the ground, so low hanging fruit is more likely to be damaged when the weather is cold; secondly, a fruit-rotting fungus disease called brown rot may splash from the soil, where it lives, onto fruit hanging low in the tree. Brown rot can penetrate unblemished citrus fruit rind, unlike other decay organisms which require a break in the rind to cause injury.
Brown rot on lemons, Jack Kelly Clark, UC IPM.
When you're picking citrus fruit that you plan to store for awhile, be careful not to bruise or break the skin. Fruits that are cut or scratched during harvesting will rot fairly quickly in storage. Citrus fruits with perfectly sound skin are fairly decay proof, and will last in cool, moist storage for several weeks (38 to 48 degrees F, 85 to 95 percent relative humidity). Under dry conditions at room temperature, fruits develop off flavors and shrivel within a week to 10 days.
Ed Perry is the emeritus Environmental Horticultural Advisor for University of California Cooperative Extension (UCCE) in Stanislaus County where he worked for over 30 years.