When it comes to reproduction, citrus species have no limits.

All of them are interfertile, meaning they can cross-pollinate freely and produce offspring that are a combination of the two parent species or hybrid species involved. A grapefruit can pollinate a lime. An orange can pollinate a lemon. A pomello can pollinate a tangerine.

This is highly irregular in nature since a species, by definition, is an organism that can only reproduce with members of its own distinct kind. Yet the parentage of every kind of citrus may be traced to three distinct species: pomello (Citrus maxima), mandarin (Citrus reticulata), and/or citron (Citrus medica). There are hundreds of varieties of apples – red, yellow, and green – yet all of them are of the same species (Malus pumila). There are numerous varieties of wine, each fermented from a different variety of grape, but all of them, red or white, come from the same grape species (Vitis vinifera). Oranges, grapefruits, lemons, and limes, on the other hand, are all separate hybrid species with one or more parent in common.

Here, it is worth noting that hybridization of grape species in nature is rare but hybridization with human assistance is quite simple. Traditional grape varieties are hand-pollinated with pollen from varieties that have demonstrated superior cold tolerance or disease resistance, resulting in grape varieties that can be grown in cold climates and will not be decimated by disease. While traditional grape varieties may need to be sprayed with chemicals a dozen or more times during the year to prevent fungus disease from ruining a crop, hybrids may require only two or three annual sprayings or none at all. And then there are plumcots or pluots (hybrids of plums and apricots) and broccoflowers (broccoli-cauliflower hybrids) that have also been created by human agency.

As if citrus trees were not already rich enough in their capacity to hybridize freely, they possess another characteristic that is quite rare and valuable: the seeds they produce contain unfertilized embryos that develop into seedlings and trees that are exactly like the mother plant. In the case of nearly every plant species, seedlings come from embryos that are the product of pollination, typically facilitated by bees, other insects, or birds, followed by sexual reproduction or fertilization. Those seedlings, like children, share some characteristics of their parents but have their own distinct identities. After a pollen grain is transferred from a male stamen to a female pistil (consisting of a sticky stigma where the pollen lands, a stigma stalk or style, and a bulbous ovary), the pollen grain produces a tube that elongates down the style into the ovary, where male sex cells are released, intermingling with and fertilizing their female counterparts, resulting in a single embryo that develops within each seed. Yet, in the case of citrus, through a phenomenon known as apomixis (apo = away from, mixis = mixing), citrus embryos are frequently created without any mixing of male and female cells. Instead, a single embryo produced by fertilization is accompanied in the same seed by embryos that spring up from unfertilized female sex sells. These apomictic embryos grow into seedlings and later trees with predictable, clonelike characteristics of the mother tree, including the taste and texture of its fruit.

If you plant a seed from an orange or a lemon, you are likely to observe two or three seedlings germinating from it. One of those, being a mixture of genes from two differing trees, will be unpredictable and most likely bear fruit of inferior quality, while the other seedling(s) will produce fruit that is a carbon copy of the fruit from which their seed was taken. This is highly irregular as embryos in seeds from Pink Lady apples, Bonanza peaches, Santa Rosa plums, Blenheim apricots, and Hass avocadoes, for example, will develop into trees whose fruit quality will be inferior to that of the named fruits from which they came. However, you might have to wait more than ten years to see fruit from a citrus tree planted from a seed.

Incidentally, the multiple seedlings you get when planting beet or Swiss chard actually come from separate seeds. The reason beet and Swiss chard seeds are roughly hewn is because each seed is, in fact, a cluster of many seeds, each the product of a separate fertilization event, fused together. Thin out all but the most robust seedling from each sprouting group.

The history of California, and especially Southern California, is inextricably linked with that of the orange groves that were planted here. The first California citrus trees came from seeds planted by monks outside a mission near San Diego in 1796. Later on, oranges and lemons were grown at the San Gabriel Mission, and seeds from the fruits on those trees were planted in Los Angeles, site of the area’s first commercial citrus farm in the 1840s. During the gold rush, oranges and lemons were sent to northern California because of their reputation for preventing scurvy, a debilitating, even life-threatening condition due to lack of vitamin C; just ten weeks without vitamin C can bring on scurvy. In the 1490s, when the Portuguese explorer Vasco de Gama made a long sea voyage to India by way of South Africa, his men became sick with scurvy and were only saved when they anchored in Kenya and were given citrus fruit by the local king. Unfortunately, on their way back to Portugal, they were 12 weeks at sea and, lacking in the sustaining citrus, 55 men died of scurvy.

During that same period, sea voyagers to Asia brought sweet oranges back to Spain and Portugal, giving Europe its first opportunity to grow them. Around 1530, oranges were first planted by Portuguese Jesuits in Brazil in several areas including Bahia, where the first navel oranges were discovered in the 1820s. In 1870, naval orange budwood cuttings arrived in Washington, D.C. and were grafted by an official of the USDA onto citrus seedlings growing in a greenhouse. The official had been a neighbor of Eliza Tibbets before she set off for Southern California and he now sent her two grafted navel orange trees since he surmised that Riverside, California, where she had settled, would be hospitable to the trees’ growth.

Eliza Tibbets was a spiritualist who believed in contact with the dead and conducted seances. She was an abolitionist and a suffragette. The arrival of her navel orange trees was an adventure in itself. They had been shipped by rail to San Francisco and  from there by carriage to Gilroy. Tibbets took her own wagon up to Gilroy to secure the trees. She planted the two navel orange trees sent from the nation’s capital and irrigated them with dishwater since potable water was in short supply. The fruit from these trees was displayed and tasted at a nearby fair. It was so astonishingly sweet, with the bonus of being seedless, that the trees’ budwood became a source for the propagation of huge groves of navel orange trees.

The California citrus industry rapidly expanded. There had been a citrus industry in California before the advent of the navel, but the fruit was not of consistent quality. The uniformity of clonally propagated navels was an innovation that made growing citrus trees highly lucrative. Between 1870 and 1900, navel oranges became California’s main crop, replacing wheat with that honor, and Riverside boasted the highest per capita income in the United States. Cities created around navel orange groves included Rialto, Fontana, Redlands, Corona, Etiwanda, and Ontario. The Valencia orange was created (hybridized) by a Southern California orange grower. He sold the patent for Valencias to the Irvine ranch where 100 square miles (64,000 acres) of Valencia orange trees were planted beginning in the 19th century.

One of the original trees of Tibbets is still alive and producing at the corner of Arlington and Magnolia Avenues in Riverside. It almost died in 1918 and again in 1951 but was saved on both occasions by a procedure known as inarching, where roots of various citrus species were grafted onto the trunk and dug into surrounding soil. In 2019, a screen house was built around the tree to protect it from contracting citrus greening disease, a deadly bacterial infection vectored (transmitted) by the Asian psyllid, an aphid-sized insect. Now more than 150 years old, it is apparently still producing sweet and juicy fruit.

If you have a citrus tree of which you are proud that is currently bearing fruit, please send along a photo to the email address at the end of this column. Include your location, the variety of the tree, details of the tree’s history, and a description of the care that it receives.

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Tip of the week: Finger or caviar limes (Microcitrus x asutralasica) are truly a marvel to behold. The fruits are 2-3 inches in length, capsular in form, deep purple when ripe and, when split open, full of clear vesicles that will remind you, by their size, color, and shape, of caviar. The vesicles are easily squeezed out of the surrounding skin and, when applied to your tongue, yield a tart limey flavor. Finger limes are popularly used for livening up fish dishes, especially sushi, and certain cocktails. I once saw them growing near Mulholland Drive but, like all lime species, they are sensitive to cold and should probably not be planted north of there. You can procure finger lime trees from a few Internet vendors and Moon Valley Nurseries do list the tree as a species that they grow. If anyone knows of any other sources for this tree, please advise.

Please send questions, comments, and photos to [email protected].