I've read that Trifoliate makes a decent rootstock for kumquat trees since they tend to have dwarf characteristics anyway, but I was wondering if putting Kumquats on a Sour Orange Rootstock would make the tree any larger or have any better fruit production.

Phillip

I have heard that sour orange is not very compatable w/ kumquat and may decline after 4 or 5 years.

In europe many Kumquats are grafted to Sour Orange. And most endure longer than 5 years.
These incompatibility problems often claimed, IMHO come from virus infected budwood.
That's now past, so most virus free kumquats hold much better on sour orange and do not show any weakness.

Some is told about kumquat on trifoliata, which will give unhealthy looking combinations....

So for me the rootstock has to ensure larger fruits with a thicker peel, so the usage of Citrus macropyhlla seems very good and I've seen trees older than 10 years growing without any sign of incombatibility. Thus, such rootstocks seem recommendable.

Philip, Sour Orange is a great root stock. Sour Orange is rated as good for salinity, high pH soils, clay soils, wet soils, and freezes (although in the case of freezes not as good as trifoliate). Sour Orange is fair with drought. It's one MAJOR drawback is that it is highly susceptible to Tristeza virus. If your area has not been infected with Tristeza, it should be OK. Myself, I would prefer using Flying Dragon, but each to his own. - Millet

Sure, that's the major backdraw of sour orange.... it's CTV suspectility...

But about freeze hardiness:
Poncirus trifoliata imparts a very strong cold tolerance to each scion, but only if fully domant and adapted to cold weather. But this adaption process to retain full dormancy requieres a couple of days, before fully frost tolernance is gained.
If this adaption process is disturbed, or if a fully dormant Poncirus trifoliata rootstock is set out of dormancy by only one day of warm weather, followed by only a clear freezy night, frost damage can be much more severe as on Citrus aurantium.
So Poncirus trifoliata is a good stock, were it's winter dormancy and adaption process is not disturbed, to play off is full frost protection potential, but were temperature switches to often, usually betwen day and night, well, the cold tolerance isn't superior to any other cold tolerant rootstock like Citrus aurantium, and is simple just compareable.
So no real advantage here...

Steve, your above posting certainly offers some good points. I would offer, that I certainly agree with Steve, that Poncirus Trifoliata obtains it cold hardiness, much the same as other stocks, and that is my a gradual adaptation to cold temperatures. However, I would differ in that "only one day of warm weather" or for that matter, several days or longer of warm weather, will bring Poncirus out of dormancy. Of all the stocks for citrus trees, trifoliate is the hardiest and it imparts to the variety worked upon it some of it hardiness and its resistance to both HEAT AND COLD.. Its resistance to heat is quite as important as its resistance to cold, for if the tree responds to heat by starting to grow, it becomes very tender; the measure of a variety's hardiness is frequently its resistance to high temperatures by remaining dormant. Besides imparting to the cion some of its ability to withstand cold, it also imparts its ability to retard growth in the spring. Trifoliata has been known to withstand a temperature below zero. It is also distinctly periodic in its growth, and it is not responsive to sudden changes of temperature during its dormant period. As a rootstock it imparts both of these tendencies to the cions grafted upon it. Due to its resistance to heat, it is well known that Poncirus Trifoliata blooms much later in the spring than other citrus varieties, even kumquats. - Millet

Philip, Sour Orange is a great root stock. Tristeza is not a big deal in California. The variety collection at Riverside removed several Tristeza infected trees not long ago, and if I remember correctly that was why the bud wood distribution was cancelled for that particular distribution. My guess is that your tree would not have much of a threat. Your tree would certainly grow quicker and larger. By the way, I really like the picture you have selected as your Avatar, what a great picture. - Millet

Well, depending on what Dr. Castle from the University of Florida has told me, and what Dr. Wutscher frm the Horticultural Research Lab. told me, is Poncirus trifoliata usually mich quicker in it's domancy disturbed, as most guess about.
According to Dr. Wutscher, it only to a day wiith tmperatures around 80° F to get it out of full dormancy, and making the fine twings very prone to frost damage.
Mainly, and that's my xperience, Poncirus trifoliata seems to act more to soil tmeperature, rather than air temperature.
Thus if the soil remains cold, the dormancy won't be broken, but if the periode of cold leads Poncirus trifoliata to dormancy, but the soil won't be hold on the cold, the dormancy get's fast enough lost, to loose all the good abilities to widstand cold.

And depending on bloom: Well, Poncirus trifoliata flowers later in Spring, because of other mechanics in cold protection compared to normal citrus. That's something biological, nothing about dormancy or heat tolerance....
Did you ever spot a Poncirus trifoliata flower in Summer? No? I did!
Thus the biological factors influencing bud break and bloom are quite different in Poncirus trifoliata as in Citrus sp.
Because we talk about a evergreen tree and a decidous species, which both have fully different mechanics in biological system.
It's like comparing a Zulu with a Inuit, both are Humas, but are very different in behavior and adaption to environment in climatics, and have different working biological system, even if it's 100% anatomical the same.....

So, nothing againt consideration that Poncirus trifoliata is the cold hardiest rootstock known, that we can surely lay down as fact.
But this superior cold hardiness isn't only imparted to the scion, the cold hardiness depends also on the scion variety.
Thus if I gro a very tender Key Lime onto Poncirus trifoliata, it evenly will react quite properly to cold temperatures, often much mure than the rootstock itself. This leads, IMHO to the fact of great imbalance betwen Stock and Scion, which can, and you will agree, even influence cold hardiness...
And none as able to see, if his rootstock is fully dormant and fully adapted to cold.... So for us, these differences, often only in the margin of 2 up to 3 degrees Centigrade, are less important. For a comercial growers, this maybe important, if he grows citrus in a cold lclimatic, and frost appears, which harms his trees, and his Poncirus trifoliata stocks could widstand the frost some hours longer without damage as other comericial rootstocks, but for us, this little effect, which you can read in most commercial rootstock chapters in various horticultural books about citrus, won't realy matter.
So we can consider most other cold tolerant rootstocks for non-cold-tolerant scions, as realy compareable. The Plus of Poncirus trifoliata will usually in most container cultures less important and even in many in ground areas were normaly no citrus is grown with a typical citrus crop very minimal.
But: For cold hardy stocks, like Citrangequat, Citrange and other Poncirus trifoliata-Habrids, the chioce of this stock is more a must, as a can do.... to archive best cold tolerance for prolongued cold periodes...

This posting has developed into an interesting discussion, with somewhat differing views. The duration of cold hardiness, nor the depth of cold hardiness, is not really much of a concern to Philip as he lives in one of God's great citrus growing areas San Diego, California (named after Saint Juan Diego (1474-1548). Tree size, and faster growth are Philip's main concerns.. Philip, good luck to you and to your new tree. - Millet

Tristeza is going to be a big concern in California now. It started about a couple of years ago when several foundation block parent trees were tested to be infected with Tristeza. The culprit are nearby citrus fields beyond the controls of the university research facilities.

Now there are no more Foundation Block orders as more and more foundation trees are tested positive for Tristeza.

Thank you Joe for the Tristeza information, I thought I remembered something about the diseased UCR trees. I would make one last comment concerning the cold hardiness of Poncirus. As I live in Colorado at an elevation of 5,540 feet (1,813 meters) I must grown my citrus in a warm greenhouse, as the Colorado winters are quite severe at times. Many members on this forum know that I maintain the root zones of all of my containerized trees between 64 - 70F (18 - 21C) throughout the winter and spring months. Even with these elevated soil temperatures, the Poncirus Trifoliata, Flying Dragon, Poncirus Hybrids, and Kumquats are the last citrus varieties to come out of dormancy and begin blooming. Some a month later then the rest of my collection. Lastly, next to the trifoliate orange, the Sour Orange is the hardiest of all the varieties or species of citrus COMMONLY used as root stocks. As compared with Poncirus trifoliata, it is not nearly so hardy, but it is considerably hardier than the sweet orange. Of course, an exceedingly low temperature causes all differences in hardiness between sweet and sour orange trees to disappear, and after such low periods of cold no difference can be detected, but the fact remains that a degree of cold which frequently injures the sweet orange has no effect on the sour orange. Sufficient cold to destroy the young growth and defoliate sweet orange trees does little or no injury to the sour orange. It is impossible to state the relative hardiness in so many degrees, as so much depends on the condition of the trees. - Millet

Philip, thank you for the link about Father Serra. I have been very fortunate to have been abel to visit almost all of the missions founded by Father Serra. I also have seen and prayed in the very room in which Father Serra died at Mission Carmel, and have been to Mission San Juan Capistrano on the Feast of Saint Joseph, which is the day the swallows return to the Mission (and yes they in fact came back on that day). In fact my most recent visit was to Mission San Diego with The American Ivy Society, followed by a personal tour through the Canon of Palms in the park. By the way I am also a native Californian, as I was born in San Francisco. Thanks for your membership, you add a lot to this forum. - Millet