Our recent tree planting survey results from county production meetings across the state indicate over 120,000 pecan trees planted and over 3500 new pecan acres added to the state in 2014. These trees need adequate water and protection from weed competition. As long as the leaves and bark are protected we have a variety of choices for burn-down herbicides including glyphosate, paraquat, and glufosinate. Pre-emergence herbicides added to the tank will help manage weed resistance and extend the interval between needed applications. We have several good pre-emergence options available. Prowl H20, Surflan and Chateau (flumioxazin) can be used in the year of planting after the soil has settled.
Alion can be used after year 3. There has been talk of the label restriction changing to available use in the year of planting but I have not been able to find this on a current label so for now we will recommend it for use only after year 3
Simazine can be used after year 2
Diuron can be used after year 3 (avoid on sandy soils)
There are a few others but these are the most commonly used materials in commercial orchards.