Q. I transplanted a dozen Rosemary from the planter in front of my house into pots to see if I could salvage them. I know Rosemary likes full sun, but I’m curious if they could use some shade because of the shock of transplanting?
A. Yes, you should shade plants in containers after removing them from the planter, but you should also severely cut them back. Let me explain.
Prune Transplants Hard
General rule of thumb, when you transplant any plant from the ground to a new location, including containers, cut back the top about 1/3. In other words, reduce the amount of top growth the plant has because it loses so much of its roots during transplanting.
I've read estimates that as much as 80% of the roots remain in the ground when moving plants to a new location. The older the plant, the more roots remain in the ground.
If the plant is watered with drip irrigation, the roots grow closer to the plant and transplanting is more successful. For these reasons, and my own personal experiences, when plants are more than 3 or 4 years in the ground, the chances of successfully moving it to a new location are fewer. Root pruning helps.
Plan a Year in Advance
Transplanting plants from the ground to a new location or container is more successful if you can plan a year in advance. One year ahead of its move, cut the roots with a sharp shovel as deep as possible where you’ll be digging the following year. “Root pruning” causes roots to grow closer to the mother plant and improves the chance of transplanting.
If moving a plant into a container, put them in the shade the first growing season before moving them into intense sunlight. Remove one third of the top by eliminating entire branches rather than cutting these branches into a “butch haircut”.