I have a mature Ceanothus ‘Victoria’ that I’d like to prune and transplant. When is a good time to do this? It seems as if it has a deep root system.
Ceanothus ‘Victoria’ can be a bit difficult to transplant because the root systems are extensive, as you noted, but it is worth a try. I have transplanted this cultivar both successfully and unsuccessfully.
I would recommend either that you do not prune them or that you wait until August. You do not want them to grow much before you transplant them, and pruning during the growing season will encourage growth. If you prune them in August, they will grow very little.
Extensive pruning before transplanting sets up competition between the root system and the upper plant (responding to the pruning), as far as the plant’s resources are concerned. After transplanting, you want energy directed toward the roots so that they might take hold and also so that growth above ground slows. If you choose to prune the shrubs, I recommend that you prune as little as possible. Prune from the inside, thinning and taking out dead branches, and removing a few lower limbs. You can also cut back some of the longer limbs, as this shrub can handle ‘heading back,’ as this type of pruning is called. Please note that this shrub is genetically programmed to get quite large, and pruning will not prevent this. Be sure the new spot can handle a shrub that wants to grow 8 to 10 feet up and out (possibly more!).
With this in mind, you can consider transplanting the shrub in the fall. I recommend October or later so that you can avoid a hot spell (which may promote upper growth and/or place the plant under stress). When you dig up the root system, retain as many of the roots and their native soil (surrounding them and holding them together) as you can. You will have to cut the deep taproot(s); that is unavoidable. The tiny, thread-like roots are more important to retain.
When you dig the hole, make it big enough to accommodate the soil around the roots as well as a bit of filler. You don’t need to add new soil; simply backfill with the soil you dug out. You may have to water a bit, even in the fall, until our rainy season begins. You don’t need to saturate the roots, but don’t allow them to dry out.