It will take 12-24 months before your plants have grown a new root system that is deep enough to survive on its own. Until then, you are responsible for keeping the soil around them slightly moist and weed free.
Dealing with weeds is straightforward: hoe them down before they can set seed and mulch over them.
For best results, use glyphosate weedkiller well in advance of planting to clear the area and use a mulch fabric from the start to make your life really easy.
Watering is the single most important factor in the success of a new tree or hedge.
You must get to know your own soil, in particular: how fast it drains and how fast it dries out in warm weather.
You can water your plants too much: most plants dislike wet soil.
The aim of frequently watering is to stop the soil drying out in the small area where your plants roots are, not to make a permanent swamp feature!
Please remember that light rainfall onto warm soil won't do a good job of watering your plants. Only a good few hours of heavy rain counts as a free watering!
How to Test if Your Soil Needs Watering
Scratch and feel: dig down about 3-4 inches into the soil near your plants. If it is dry all the way down, or if it is only a bit damp right at the bottom, it's definitely time to water. Give your plants a good soaking.
The easiest way to water hedges and rows of trees is with an irrigation system like a leaky pipe, which can be buried out of sight.