Roundup is a popular, effective and widely available herbicide, but it kills EVERYTHING, so use with caution. To make an "after planting" spray barrier, you can cut the bottom off a 2 liter [or cut a length of PVC pipe or carpet tube], slide that over each tree until it is flush with the ground, and then carefully spray around the 2 liter...but be careful that the Roundup doesn't drip off the 2 liter and onto the tree as you remove it. It is also important to avoid creating a fine mist which can blow onto the trees by the wind. Set the sprayer to produce droplets which fall to the ground.
Treflan, Biathlon is a pre-emergent weedkiller, meaning it does not kill germinated/growing plants. Apply Treflan to bare soil around each tree and then cover treflan with mulch or sawdust to create an effective barrier against weeds and a good insulator against moisture loss.
There are commercial post emergent herbicides which can be directly applied to evergreens without hurting them, Fusilade II, Grass killer Vantage, Goal 2 XL. Goal is quite popular in the wholesale industry.