Cold Hardy Citrus Project

This may be a shorter one, but I thought it’d be nice to cover the details of one of my favorite plant breeding projects I’ve been working on.

I’ll go over the research and context I needed to start, what my plan going forward is as well as some predictions on what I expect in the coming years!

My goal is to get some of you to start interesting citizen science type projects like I talked about a little while ago.

Research & Background

For a plant breeding project of this type, I really needed to do a lot of prep work and research in order to push the bounds of citrus tolerance.

I first had to decide what traits I was most interested in breeding. In this case the primary trait has to be cold climate tolerance, and the secondary was flavor since that can be bred into it later.

Next I had to figure out how citrus grows in general so I could strategize the best path forward. In this case, citrus species are generally late fall/winter/early spring producing fruits. This could pose a problem in an area where we get cold temperatures as early as October.

What does “cold hardy” mean for a citrus? Well, it can mean a few things:

Firstly, it can mean that the tree survives cold temperatures (which is actually not too hard to do).

Second, it could mean that it produces in a short season, so you get a harvest before the frost sets in for winter. In my case, I need both a tree that survives cold temperatures, and fruit that ripens before it gets too cold out!

TLDR, this is a challenge.

My next step was to research what genetic material I need to start with to even give myself a statistical chance to find what I want.

My go-to was a documentary on the history of the citrus family which explains that there are several ancestral citrus types that were all crossed (as seen above) to create modern varieties of citrus.

*Note: The documentary lists 10 or so ancestral species including a handful from Australia, but the chart above only has the main 5 from Asia

To my surprise, a lot of citrus actually originated in Asia and many of them actually grow in fairly cold climates (in particular the mandarin, kumquat, and yuzu).

What do we know about how plant genetics work? Well like I wrote in this article, growing from seed will yield new genes over time. So, this meant that I needed to get some citrus seeds from a source that had cold hardy genetics already.

One particular plant that stood out was the trifoliate citrus (Poncirus trifoliata) which is a close cousin of the citrus genus. The plant has gnarly thorns and is quite bitter and unpalatable but it can easily grow in pretty extreme cold. Traditionally this has been used as a rootstock for other citrus varieties, and as an ornamental citrus.

In my research, I also found that there had been attempts to breed cold hardy citrus in the US in the past! I found a guy who sells the seeds of already semi-hardy citrus and talks about them on his YouTube channel. There, I found tons of options to choose from and ended up starting with seeds of a cross that the USDA made early last century called a “Citrandarin” which is a hybrid between a trifoliate citrus and a mandarin. Supposedly this particular cross (I believe it was the US852 cross) tasted pretty good for a trifoliate cross.

Shout out to Tough Citrus for sending me the seeds plus the extra free seeds from the other trifoliate cross! I recommend buying from his store if this is something you’re interested in.

Current Plan

Ok so now I had my breeding stock. Now what?

I decided to go with the neglect method and basically leave them to their own devices. This means no watering, no fertilization, no protection. If only one survives, that’s going to be the one I want. If I have to baby the trees constantly then I don’t want it!

I planted the seeds in a few pots and for now I’m letting them grow out and seeing how they take the winter this year.

The parent tree can survive some pretty cold dips in temperature (0°F, or -17°C) and that seems to be the case for a handful of trifoliate crosses, so I’m less concerned about their survival rate. The main thing I’m interested in seeing is when they set fruit.

The type of mandarin my seedlings are crossed with is a “Changsha” mandarin which typically ripen in October or November. Those are pretty good starting odds to find what I want!

By my estimate I should find anywhere from 1-2 seedlings out of my 35 that will have all of the traits I want. Which is all I need to have!

Future Predictions

Mandarins also typically take anywhere from 4-7 years to bear fruit, and I'm growing from seed, so this is going to take awhile. Trifoliates generally take as little as 3 years so I'm expecting to get my first fruit around the 5–6-year mark. One year down, 4-5 more to go!

If multiple seedlings survive and thrive, then my next step will be to try the fruit from each and determine which one to grow more seeds from.

As you can tell from the pictures, the seedlings are already developing thorns which tells me that at least the tree itself is taking after it’s trifoliate heritage. It will be interesting to see if any fruits end up being mostly the same as a trifoliate citrus or if they’ll lean more towards a mandarin.

There is obviously a chance that none of them bear fruit at the right time in which case I’ll have to protect a tree enough for it to bear fruit so I can grow out seeds from it again. This should in theory give me bigger odds of success each time I have to do that even though it will take longer.

It’s a good thing I’m still young so that I have a few decades ahead of me to mess around with it!

End Goal

So to sum up, the goal is to create a citrus that:

• Survives cold temperatures

• Produces fruit at the right time

• Is at least palatable to eat

• Does not require protection in my climate

While it’s a challenge, we need this kind of experimentation to provide genetic material for future research and development!

Follow me on social media for behind-the-scenes videos and seasonal photos!

Leave a comment to show the algorithm how legit we are!

Thanks for reading The Naturalist. Please share on social media to support the work!