To promote root growth, you'll need to use a product containing Potassium (or potash). Here in the States, I use an organic product called Roots which can be found in garden centers and is often used by professionals in my area when they transplant trees and have to "guarantee" them.

The key to using Potassium is the pH of your soil. If you don't know the pH then test the soil and find out. Ideally, you want the pH to be 6.5 -- lower than that affects the plant's ability to absorb the Potassium.