Roundup Systemic Weedkiller acts through the leaves of weeds and other undesirable plants. It is trans-located within the plant and carried down to the roots, which it kills. When used as a spray it makes up to 27 litres of mixture and can treat up to 540 sq metres of weeds.
Weeds are a major obstacle to successful hedge plant establishment and they should be eradicated either by removal, permanent mulching or weed-killing before planting. Roundup does the killing bit and it can be applied around hedge plants without damage provided it does not touch any green tissues as it does not work through the bark. Another of the joys of Roundup is that it is perfectly safe to plant the area as soon as the spray has dried (so you can spray in the morning and plant in the afternoon...)
Just make sure you spray when there will not be any rain for at least 8 hours and ideally 24, when the weather is relatively warm and when the plants to be treated are in some growth (March - late October is best).
Please always follow the instructions on the packet.