|I would like to know whether all cultivars of holly can be trained with pruning and when is the best time of year to do that.|
|There are many different types of holly (Ilex), and many different types of pruning techniques. Here are some of the more commonly seen.
The deciduous types (Ilex verticillata) can eventually be trained as an upright sort of multistemmed small tree form, although they are more naturally large rounded shrubs. These would be thinned in the spring, if needed.
Among the evergreens, the Japanese hollies (Ilex crenata) can be clipped if needed, but their natural shape tends to be quite symmetrical without much pruning at all. Some varieties have a specialized form such as especially tall and narrow or short and wide. They do take clipping nicely, however.
For more ornate evergreen topiary shapes, the Ilex cornuta types seem to grow quickly enough to give good results, and the larger American Holly our native holly tree, Ilex opaca, can also be trained although it will require very frequent clipping to maintain a smaller size.
Ilex glabra or inkberry holly tends to have a rounded outline and may sucker or spread at the roots, this is probably the most informally shaped of the evergreen hollies although some varieties are more compact and symmetrical than others. In my experience it would not make a good formal looking hedge or topiary.
The evergreens are best trimmed in the early spring and again as needed periodically during the summer. They should not be trimmed in late summer or fall as this might stimulate growth that would not harden sufficiently before the winter cold, thus risking winter die back.
I hope this answers your question.