Citrus Hybrids and Original Citrus Fruits

Did you know that oranges, lemons, and limes didn't occur naturally? By that I mean they are all hybrids.

Each of these was a cross made by pairing two or more original (landrace) citrus varieties to create something completely new.

All of these citrus varieties are essentially human inventions (with nature's help).

What's Up With Citrus Lineages?

When I first got into vegetable gardening I was shocked to find out how many plants were related to one another. You may have had this experience too.

Before veggie gardening I assumed that broccoli, cauliflower, romanesco, broccolini, turnips, radishes, rutabaga, bok choi, tatsoi, kale, collards, wasabi, horseradish, arugula, mustard, brussel sprouts, etc. were each totally unique things.

Turns out they are all essentially cousins.

If you've got the same notion for citrus, that lemons and limes and grapefruits and oranges are all totally different things, I've got some news for you, lol.

The bright side is that discovering the connections between various citrus fruit species through their lineages is pretty sweet.

Learning that a lemon is a man-made hybrid of a bitter orange crossed with a citron is a fun recontextualization!

We'll talk more about how hybrid citrus varieties came about later on, but first let's jump into where the heck citrus came from...

Origin of Citrus Fruits

All of the 'pure' citrus varieties or wild citrus types have been traced back to a common ancestor, Citrus linczangensis.

Citrus linczangensis lived in the Himalayan foothills between 5 and 10 million years ago, and would later expand its range throughout Asia and into Australia (roughly 4 million years ago).

Though Australia had long-since broken off from Gondwana, these wild citrus were able to spread across the Malay Archipelago

Parent Citrus Species

After citrus linczangensis spread from its Himalayan homeland and its range expanded across Australasia, several distinct 'parent' species of citrus evolved.

Environmental pressure, climate, rainfall, humidity, pest pressure, and all manner of endemic flora and fauna all drive evolutionary divergence. For citrus this was no different.

Distinct landrace varieties of citrus have been identified through genetic testing, and though hundreds of unique citrus species evolved, only around 10 native citrus species are the progenitors to almost all modern citrus hybrids.

Let's take a look at those main 'parent' citrus species:

Native to Asia

• Pomelo (Citrus maxima)
• Mandarin (Citrus reticulata)
  • Not to be confused with modern hybrid mandarins which are also referred to as 'mandarin'
• Citron (Citrus medica)
• Kaffir Lime (Citrus hystrix)
• Micrantha (Citrus hystrix var. micrantha)
• Ichang Papeda (Citrus cavaleriei)
• Mangshanyegan (Citrus mangshanensis)
• Kumquat (Citrus japonica)
• Oval Kumquat (Citrus japonica var. margarita)

Native to Australia

• Desert Lime (Citrus glauca)
• Round Lime (Citrus australis)
• Finger Lime (Citrus australasica)

Citrus Hybrids

Of the species above, the 3 most prominent ancestral citrus species used to create modern hybrids are:

• Pomelo (Citrus maxima)
• Citron (Citrus medica)
• Mandarin (Citrus reticulata)

These 3 species have been crossed to create most of the citrus fruits you're familiar with today.

Let's take a look at the "recipes" or crosses which comprise some popular modern citrus hybrids:

• Sweet Orange (pomelo x mandarin)
  • Most of the orange hybrids you're familiar with are varietals of the sweet orange cross.
• Bitter Orange (pomelo x mandarin)
• Lemon (bitter orange x citron)
• Meyer Lemon ((pomelo x mandarin) x citron)
  • Though this cross appears the same as a lemon, the hybrid pomelo x mandarin used for Meyer lemon is distinct from both the sweet and bitter oranges.
• Key Lime (micrantha x citron)
• Persian Lime (key lime x lemon)
• Grapefruit (sweet orange x pomelo)
• Bergamot (bitter orange x lemon)

Citrus Cold Tolerances

In order from most-cold-tolerant to least-cold-tolerant:

• Mandarin (Tangerine) - 20F
• Kumquat - 20F
• Meyer Lemon - 22F
• Sweet Orange - 24F
• Grapefruit - 26F
• Lemon - 27F
• Lime - 28F

These temperatures are rough estimates, and you'll want to do more research about the tolerances of the specific citrus species that you choose.

Keep in mind here that that the amount of time spent below the given temperatures matters tremendously. These trees will likely not do well for extended stretches of time below the given temps.

Additionally, citrus cultivar matters. Within each variety there are numerous cultivars which may fare better or worse with cold temps. You may be able to grow a variety which wouldn't survive your zone by utilizing a cultivar which will.

Mandarin and kumquat do better in the cold than any other citrus varieties. Lemons and limes are the least-tolerant of cold and will struggle more with frost.

It's worth noting that ,ature citrus trees will always tolerate a wider range of conditions than immature saplings.

Additionally, citrus trees with fruit on them cannot tolerate the temperatures listed above - the critical cold temperatures rise a few degrees for citrus trees with fruit, as frost easily damages citrus fruit.

Think about micro-climates on your land if you're going to plant citrus in a location where winter temps approach or exceed the lows listed above. Or get a greenhouse or tunnel.

Sometimes a specific corner of you property will be warmer or the protection of other trees will yield a warmer winter micro-climate than full exposure.

Stone, brick, or stucco walls and buildings can absorb heat during the day and radiate it back out at night. Perhaps your citrus tree will find a nice home aside a warm protecting wall

Also consider protecting citrus saplings from frost or bringing young trees indoors for the winter. If you're cold, they're cold; bring them inside.

Diversity in Citrus Orchards

Embracing a diverse mix of species and varieties in citrus cultivation can enhance sustainability, resilience, and overall ecosystem health.

Whether you're a commercial orchard or have a little backyard food forest thing going on, diversity in the orchard has more benefits than I can possibly mention. Here are a few:

Disease and Pests

Diverse ecosystems are less susceptible to disease and pests than monocultures. This means reduced possibility for catastrophic loss, and no need for pesticides.

Weed Suppression

Citrus trees won't suppress weeds but a lot of ground cover crops certainly will - and they can improve your soil while doing it.

Sustainability in Citrus Cultivation

I talk a lot about regenerative agriculture on this site, and it certainly applies to a sustainable citrus cultivation strategy as well.

Regenerative agriculture is all about looking forward and ensuring that what works today will continue to work long-term. Ideally, things will improve with each year, and certainly they will not degrade.

Nutrient Requirements for Citrus Cultivation

Compost and well-rotted manure make for great citrus nutrition, as do wood chips, leaf moulds, and biochar.

Compost teas and fermented plant juice are incredible organic liquid fertilizers as always, along with fish hydrolysate.

If you want to buy a product to kick off your citrus nutrients quick, I dig this citrus stuff as an occasional supplement.