Hello Slowfood

Just found this for you on the net and hope it helps !
Good luck !

Zelkova bonsai need deep, fertile, well-drained soil. Repotting should be done in early spring. Young plants should be repotted yearly, and older plants should be repotted less periodically - when plants have outgrown their pot and roots become matted.
Be careful when wiring and handling Zelkovas. Although it is a hardy tree, its branches are thin and fragile. Wiring should be done in the summer.
Shoots should be pruned back to one to two leaves during Zelkova is suited to almost all styles - the exception being literati. It is most suited to the 'broom' style, naturally developing a thick crown of branches - as shown below. Zelkovas are also good in group plantings - voted as one of the best for the forest style in the August '99 survey.
General Care : Although reasonably frost hardy, they do prefer shelter.