Cold Hardy Citrus Project

Goals
1. Create fully hardy citrus to zone 6b that makes delicious sweet juice.
2. Create fully hardy citrus for making marmalade
3. Create a large enough population of diverse zygotic citrus, from which seed can be harvested for a cold-hardy complex hybrid citrus landrace.   

 

There are many species of citrus, and they readily hybridize. Most commercial citrus is a hybrid of two or more species. This allows for genetic complexity and unique fruit qualities. However, they are not hardy in my temperate zone. Luckily, one species is cold-hardy to zone 6, named Poncirus trifoliata or Trifoliate Orange, also known as Bitter Orange. The downside is that it has undesirable fruit qualities, most notably flavor. By crossing Trifoliate with other citrus, there is hope in retaining the cold hardiness from Poncirus trifoliata along with the good flavor of the other citrus species. There is a history of creating cold-hardy citrus by making crosses with Trifoliate, but with varying success. Many of the cultivars still have undesirable flavors or are not fully cold-hardy to zone 6b/7a. 

 

My process is to collect all the cultivars that are hardy to 6b and 7a, plant them outdoors, and evaluate fruit quality.  I will also collect the most cold hardy citrus that has good quality fruit, but still cannot survive outdoors in my zone.  These more sensitive plants are growing in 5-gallon buckets that can be brought indoors during winter.  I will then do controlled crosses of the best tasting fully outdoor hardy citrus with the almost-cold-hardy-enough indoor citrus.  Then plant as many seedlings outdoors as possible. 

 

Each citrus cultivar makes varying amounts of nucellar or zygotic seed.  Nucellar seeds are a genetic clone of the mother plant, whereas zygotic seeds are a unique offspring of the two parents.  For breeding, we want zygotic seed, so it's important to determine which mothers make a high percentage of zygotic seed and filter out the clonal nucellar seedlings. 

 

A resource cataloging the percentage of nucellar seeds of different citrus cultivars:

tropicalfruitforum.com/index.php?topic=51232.0

 

Acquired fully cold hardy citrus

Poncirus+  -This is an improved cultivar of Poncirus trifoliate with less bitterness, and less wax, brix 11, 4mm thick rind.

Poncirus trifoliata 'spring' -I harvested seed from a wild tree growing near a spring in Missouri.  The fruit was already old when I arrived, so I was not able to taste it, but based on reports it may have better fruit than a typical trifoliate orange.  

Flying Dragon citrus (Poncirus trifoliata var. monstrosa) -A contorted variety of trifoliate orange. They have a dwarfing habit. 

 

I'll make most of my initial crosses with hybrids of Poncirus trifoliata.  This jumps me ahead a generation.  I also want to make some with poncirus+. This allows me to start with a foundation of improved poncirus genetics, in case there is a bottleneck in trifoliate hybrid flavor because of the poor fruit quality of the original Poncirus line used in the initial crosses.

 

Acquired fully cold hardy hybrid citrus

Conestoga 060

Conestoga 010

Conestoga 011

The conestoga series are F2 seedlings of citrange cultivar 'C-35' and are cold hardy to zone 6b.  C-35 is a hyrbid of Rusk orange x Poncirus trifoliate, which is used as rootstock in the citrus industry.  The breeding of the conestoga series reveals how quickly the hardiness of hybrid citrus can be increased, and was a big inspiration for me to breed citrus. 

 

For details of the project:  

tropicalfruitforum.com/index.php?topic=30891.0

 

Acquired not-quite-hardy-enough but quality citrus cultivars

Brown's Select Satsuma Mandarin

Miho Satsuma Mandarin

Nagami Kumquat

Yuzu

10 Degree Tangerine

Nippon Orangequat

US-942

 

Citrus I started from seed in spring of 2024

Procimequat

Ichang Lemon

Thomasville Cirangequat

US-852 Citrandarin

Dunstan Citrumelo

US-639 citrandarin

 

Citrus I started from seed in spring of 2025

Tricimequat

10 Degree Tangerine

Nippon Orangequat

US-852

US-942