My Swingle Citrumelo blooms on new growth so far as I see. It blooms for the second year and still not very much.
Flower buds on old wood seem to be quite common for trifoliate hybrids. Swingle 5 Star blooms mainly on old wood but also on new growth. Another Citrumelo of mine (Sacaton / Yuma Citrange ?) also blooms mainly on old wood and on new growth. My Morton blooms primarily on old wood but also on new growth. My C35 blooms for the second year and still has not many flowers. It also has winter buds, so blooms on old wood but also on new shoots. African Shadock x Poncirus primarily blooms on short flowering sproots, so not quite on old wood but almost so. It resembles my Chandler in that regard.
Sanford Curafora blooms on new growth as does Staraji. Both are F2 hybrids in which the citrus elder has gained dominance.
Yuzu, as far as I remember, blooms on short flowering sproots, so not on old wood as C. ichangensis but also not on fully developed shoots like mandarins.
Kumquat hybrids bloom year around, so not only in summer as kumquats but also in spring like citrus.
To sume up, it seems to me that the flowering habit of hybrids is in the middle of the respective parents. That is to say that Poncirus hybrids have winter buds like Poncirus but they are not so fully developed and big. They appear a little bit later and seem to require a bit more growth or development than pure Poncirus flowers. Anatomically, Poncirus flowers are also short twigs but the flowers of hybrids tend to appear on more fully developed twigs, i. e. on very short flowering shoots with more than one flower. So they are extented Poncirus inflorescences or shortened citrus (mandarin) inflorences depending on the perspective. The first flowering wave tends more to the Poncirus type the second more to the citrus (mandarine) type while the first wave is the main wave.
Pure Citrus is not uniform in its flowering habit. Compare for instance C. ichangensis, pumelo and mandarins. Poncirus can also flower on new growth. But that is extremly rare.) So Poncirus hybrids are also not all alike.
I prefer plants like Poncirus in my cool climate. They bloom earlier and so fruits have more time to ripen. A plant like Sanford Curafora blooms one month later than Poncirus, just because it blooms on new growth. This month is the one month that is lacking in autumn to allow the fruits to ripen before frost.
