Cold Hardy Citrus Project

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

 

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. Although they are not hardy to 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 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 some off flavor 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 with the best tasting fully outdoor hardy citrus with the almost-cold-hardy-enough indoor citrus.  Then plant as many seedlings outdoors as possible.  

 

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

 

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

tropicalfruitforum.com/index.php?topic=51232.0

Aquired 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, incase 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 typically 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

 

Plants I started from seed in spring of 2024
Procimequat
Ichang Lemon

Thomasville Cirangequat
US-852 Citrandarin
Dunstan Citrumelo
US-639 citrandarin