In Matt Garcia’s A World of Its Own: Race, Labor, and Citrus in the Making of Greater Los Angeles, 1900-1970  he writes:

Among oranges, Valencias and Washington navels occupied different places in the citrus belt. Since Valencias possessed seeds and were used primarily for juice, farmers cared more about their quantity than quality. Consequently, ranchers grew Valencias near coastal ranges and further down the inland valleys. In the interior, a combination of dry desert breezes, low atmospheric moisture, and the threat of high-elevation frost produced oranges of higher sugar content with a deep reddish-orange hue. Farmers took advantage of such conditions by planting the seedless navel in these districts. Regarded as the sweetest eating-orange and the “autocrat of the price list,” the navel thrived in a narrow belt that extended along the San Gabriel Valley’s inner-mountain foothills from Pasadena to San Bernardino and Redlands.

Which is probably common knowledge, but news to me.  It helps to explain why the citrus colonies (Pasadena, Arcadia, Monrovia, Pomona, Claremont, Cucamonga, Etiwanda, Alta Loma, Ontario, Fontana, Upland, San Bernardino, Redlands, Riverside to a lesser extent, and probably others) were located where they are: there’s water from the mountains, transport in the form of the new transcontinental railroad, and the right climate for the “autocrat of the price list,” the newly invented (in the mid-nineteenth century) Washington navel orange.  But to this list I think you have to add the lure of a healthy California lifestyle, an idea that is older than the place itself.

(By the way, it’s worth noting that the Inland Empire developed independently from Los Angeles; many of the same factors, for sure, were in play, in the growth of both regions, but they are actually fairly distinct.  The fact that people commute to LA now makes the IE a distant Los Angeles suburb, which is how I initially understood it.  This, however, is a new reality and not productive for understanding the development of the region, which, still, in many ways, is very different from Los Angeles.)

What might be called “the colony mechanism” helped this process along.  There were many colonies established in the citrus belt during the crucial 1880 – 1910 period.  For example, Pasadena was settled by a group of investors from Indiana, fleeing the cold and looking for a better life, and lifestyle, in the orange groves of southern California.  They delegated one of their members to search out the right spot and, like the mother mallard in Make Way for Ducklings, he searched and searched:

San Diego seemed an ideal spot, and the price was right, but a series of windmills would have to be set up to pump water. The Company rejected the idea. Of San Bernardino he said, too hot. Of Anaheim he didn’t care for the superabundance of fleas nor the number of “musketers” (gun toters). Of San Fernando he said, the price at $2 per acre was acceptable, but the area was only good for growing grain. There was too little access to water for citrus growing. The Indianans had their hearts set on orchards. Rancho Santa Anita was the collective lands of today’s Arcadia, Monrovia, Duarte, El Monte, and Baldwin Park. The property had absolutely everything required for citrus growing, but at $20 per acre the place was too expensive.