According to the Iowa State University Weed Science program, competition with weeds is the primary cause of death for tree plantings. The first three years are the most crucial for the establishment of new tree plantings. Planning your weed control strategy before planting trees means you'll have time for preventative measures that will ease and complement later control methods while you're establishing the trees.
Weed Barrier Fabric Benfits
Weed barrier fabric is black polypropylene fabric that allows air and water through, but blocks out sunlight. It is laid around the base of the tree to form a skirt 3 to 6 feet in diameter. Existing weeds underneath the cloth die from lack of light, and new seeds fail to germinate properly. Weed barrier fabric tends to be inexpensive, and can be re-installed repeatedly during the establishment period of young trees.
Weed Barrier Fabric Drawbacks
Eventually, sunlight will break down weed barrier fabric. Once this happens, weeds can push through. It must also be anchored so it won't be carried off in high winds. Even if cuts are made in the fabric to give room for the trunk of the tree to expand, the fabric may not break down significantly to prevent the fabric from choking the base of the tree. Dirt and organic material that builds up on top of the fabric provides weed seeds an ideal area to germinate.
Mechanical Cultivation Benefits
If begun the fall before planting, mechanical cultivation can help greatly reduce annual weeds while nearly eradicating perennial weeds. Use mechanical cultivation before weeds begin to flower, each time they come into season. Cultivators that churn the weeds back into the ground help with on-site organic decomposition, while cultivators that move the weed matter to a specific location can be used to build compost piles for later soil amendment.
Mechanical Cultivation Drawbacks
The need for repeated mechanical cultivation can add to the labor and expense of weed prevention. Perennial weed seeds may be brought to the surface where they can germinate. Cultivation cannot be used to a depth of more than 4 inches, and no closer than 9 inches away from the tree once it is planted. Heavy cultivation machinery can cause compaction if used repeatedly in the root zone. This makes mechanical cultivation a poor choice for weed control after the tree has been planted.
Thick layers of organic mulch will suppress many weeds (though not all), as will annual green manure crops. The advantage of both of these methods is that the ground need not be disturbed while the tree is establishing, and the weed control method also becomes essential nutrition for the growing tree. Consider using a combination of methods: disking before tree planting, then adding plenty of organic mulch topped with weed barrier fabric, or alternating weed barrier fabric and organic mulch during the establishment period.