Weeds and grass around young trees are the biggest threat to the short term future of your planting, as they take rainfall from your newly planted trees. An area of one metre square around each tree should be maintained until the tree is about your height, usually 3-4 years.  There are a 3 forms of control,  mats, sprays, and mulches are available and I can talk you through the pros and cons, or come on a course to find out the whole story.