Hey, because rooting cuttings of kumquat is that difficult, i have grafted it on a 2years old lemon and a 2years old sinensis (both grown from seed). I can only try:)
Are these suitable as understock?

The graftings are taped in and are standing in a fish tank with a little bit water on the bottom(humidity) and light on top of it. It's about 72°F inside of it.

Should i water more or less and with or without fertiliser? The roots aren't moved.

Thanx:) Laurens

Hi Laurens, I'm not an expert, but I asked a similar question regarding lemon as a rootstock. Millet said that leomon rootstocks always reduce the quality of the fruit, but increase the quantity-- I think lemon rootstocks are used commercially on juice oranges where they can increase yield and then adjust the taste by blending with other juices.

As for what you have done with your grafts, that sounds good, but no need to increase water. I remember reading T-buds do not do well at temps above 90 F. I do not know about fertilizer, but I would not add any until I see that the graft has taken or you get some new growth.

You hear people say that Kumquats do not do well on their own roots, however, I have never found this to be the case. Kumquats grown from seed seem to do just fine. Perhaps there is a understock that might provide better results if the tree was planted in certain types of soil, such as sand, wet land, high salt ground and so forth. Fertilizing newly transplanted trees should be no problem at all. Many commercial nurseries add fertilizer directly into their germinating medium. I add 25 grams STEM, 84 grams dolomite, and 250 grams 18-6-12 Osmocote per each cubic foot of germinating or rooting medium, and stick the cutting or the seed directly in the mix. This provide the energy required by the new plant, and they perform at a much higher level. Many commercial nurseries do the same. It is NOT the fertilizer that hurts new roots, it is addition of an over dose of fertilizer that causes the damage. An over dose would damage roots whether they were young or old roots.

Millet
Lewis Albert Hakes April 16, 1874

I have grafted Marumi, Fukushu, Mandarinquat, Limequat, Nagamin, Meiwa, Centennial Variegated, Variegated Calamondin, Nordman Seedless, Willowleaf unto Calamondin with excellent results.