Clearing grass etc round newly planted trees

[Delhiwood]

Hullo, This is my first post. I have 12 acres of newly planted trees - 2012 planting - which are doing very well but the problem is that the grass, thistles etc are doing well as well.
I've read about spraying both organic and non organic and burning off. The trees are about 2 metres apart and have tree shields.
Does anyone have advice on the best approach? The worst area is about 2 to 3 acres.
Many thanks Chris

[oldclaypaws]

Once the trees have grown tall enough and bushed out, which should be another 2 or 3 years, they should shade out the weeds which will then gradually disappear, so I'd have thought there's no need to do anything other than wait, unless they start to smother and choke the saplings, in which case a brushcutter should keep them down.