My own rather sad experience of this lovely plant bears out what John says. I was given a good-sized specimen in a two-litre pot, which I planted out in the autumn in our hot Greek garden. It did not survive the following (unwatered) summer, and I now believe that that was probably because it was already too old and established, in its pot, to be able to plunge its roots down in that first winter in the way that a younger plant might have been. So I'm trying now to raise some Ebenus cretica from seed, in the hope of planting them out very young in the late autumn.
Generally, with other plants, we try to give them a monthly deep watering in their first summer, then leave them. As our Mani summers are long and hot, usually with virtually no rain from May to October and sometimes even longer, we have to accept that some plants simply don't survive. You have to remember that with the native plants which do survive in the wild, for every seedling that is so luckily placed and so luckily treated by circumstances in its first winter that it has managed to get its roots down well and survives to become eventually a grand old plant, probably many hundreds less fortunately placed have perished in their first very few months or years. If we get a 50% or even 75% survival rate with our own plantings, we are almost certainly doing very much better than Nature!