Chemical herbicides are immediately applied to the sown seedbeds. These "pre-emergence" herbicides selectively kill the germinating weeds but do not harm the tree seedlings. If weeds become a serious problem later in the growing season, then a "post-emergence" herbicide is sometimes applied over the top of the seedlings. Unfortunately, not all weeds can be controlled with herbicides so nursery workers must remove them by hand-weeding.
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