Land preparation before planting a pecan orchard often leads to widespread germination of a broad spectrum of weeds, generally requiring mechanical, biological or chemical methods for adequate control after tree establishment. Disking was commonly used in many older orchards but is now generally discouraged due to possible root injury, spread of pathogens such as the crown gall bacterium and detrimental effects on micronutrient uptake. Many growers graze livestock in pecan orchards, but this can only be done after the pecan trees have grown large enough to minimize physical damage caused by large herbivores. One of the best management strategies for pecan orchards is the use of a vegetation-free zone down the pecan row with a grass alley between rows. The vegetation-free zone reduces water and nutrient competition for the trees, resulting in optimum vegetative growth and nut production. The sod strip improves soil aeration and permeability while reducing soil erosion in high-rainfall areas. As fuel costs continue to rise, it is beneficial for the grower to minimize the area in the orchard which requires mechanical mowing to reduce production costs. A wide range of herbicides is labeled for use in pecan orchards and can be divided into several classes.

Herbicides are divided into several classes based on target activity site:

1) Soil application

A) Fumigant – non-selective herbicide killing all organisms exposed to it (biocide). Generally used before the orchard is planted but can be used to treat trouble spots after planting.

B) Pre-emergence – herbicide applied to the soil which kills germinating broadleaf and grass seeds due to absorption and translocation of the herbicide as the plants are growing through the treated soil layer. Requires moisture from rainfall or irrigation to activate the herbicide.

C) Postemergence + residual soil activity – herbicide which will “burn down” young, emerging weeds and still have a prolonged soil activity on new germinating seeds.  Requires moisture from rainfall or irrigation for soil activity.

2) Foliar application

A) Systemic – herbicides which must be absorbed and translocated in the target plant, usually requiring 1 to 3 weeks to kill the weed. Can range from broad-spectrum to selective activity.

B) Dessicant or contact – herbicide which kills only the part of the plant it contacts; requires excellent spray coverage for good results.

Fumigant Herbicides

Methyl Bromide: Brom-O-Gas, Meth-O-Gas 100. Preplant; do not apply around existing trees. Temporary soil sterilant which kills living plant, stem and root tissue, most seeds, nearly all insects, and most soil microorganisms. Applicators should wear self-contained breathing apparatus during application, and the air should be tested before re-entry. A respirator must be worn if air concentration exceeds 5 ppm methyl bromide.

Sodium Methyl: Vapam, Vapam HL. REI = 48 hours. Preplant; do not apply around existing trees. Controls or suppresses weeds, nematodes and soil-borne pathogens. Application is a minimum of 14 to 21 days before planting.

Pre-emergence Herbicides

Isoxaben: Gallery 75 DF. REI = 12 hours. Nonbearing trees only. Preemergence control of some broadleaf weeds; requires rainfall within 21 days of application for activation.

Napropamide: Devrinol 50 DF. REI = 12 hours. PHI = 35 days. Nonbearing and bearing trees. Preemergence control of annual grasses and some broadleaf weeds; requires rainfall within 2 to 3 days of application for activation.

Norflurazon: Solicam DF. REI = 12 hours. PHI = 60 days. Nonbearing and bearing trees. Preemergence control of annual grasses and some broadleaf weeds; requires rainfall within 28 days of application for activation.

Oryzalin: Surflan AS, FarmSaver Oryzalin. REI = 24 hours. PHI = ? days. Nonbearing and bearing trees. Preemergence control of annual grasses and some broadleaf weeds; requires rainfall within 21 days after application for herbicide activation.

Pendimethalin: Prowl 3.3 EC, Prowl H20. REI = 24 hours. Nonbearing trees only. Requires rainfall within 21 days of application to activate the herbicide. Apply after ground has settled around transplanted trees; no visible cracks.

Simazine: Princep 4L, Princep Caliber 90. REI = 12 hours. PHI = Do not apply when nuts are on the ground. Nonbearing and bearing trees. Preemergence control of annual broadleaf weeds and annual grasses. Trees must be established for a minimum of 2 years.

Trifluralin: Bayonet. REI = 12 hours. PHI = 60 days. Nonbearing and bearing trees. Preemergence control of annual grasses and some broadleaf weeds; requires incorporation after application.

Post-Emergence + Soil Activity Herbicides

Oxyfluorfen: Galigan, Goal 2XL, GoalTender. REI = 24 hours. Nonbearing and bearing trees. Preemergence control of broadleaf and some grass weeds. Also provides burndown of young, emerged weeds. Dormant season application; apply prior to budswell or after trees have initiated dormancy in the fall.

Diuron: Karmex DF, Direx 4L, Diuron 4L. REI = 12 hours. Nonbearing and bearing. Preemergence control of broadleaf and some annual grass weeds. Will provide some burndown of young, emerged weeds. Trees must be established for 3 years and soils contain 0.5% organic matter.

Systemic Herbicides

Clethodim: Prism, Select 2 EC. REI = 12 hours. Nonbearing trees only. Post-emergent control of grasses; will not control broadleaf weeds.

Glyphosate, isopropylamine salt: Credit, Durango, Glyphomax, Glyphomax Plus, Glyphosate, Rattler, Roundup Original, Roundup Original II, Roundup Ultramax. REI = 12 hours. PHI = 3 days. Nonbearing and bearing trees. Provides broad-spectrum control of many annual and perennial weeds, woody brush and trees.

Glyphosate, potassium salt: Roundup Original Max, Roundup WeatherMax. REI = 4 hours. PHI = 3 days. Nonbearing and bearing trees. Provides broad-spectrum control of many annual and perennial weeds, woody brush and trees.

Glyphosate, monoammonium salt: Roundup Ultra Dry. REI = 12 hours. PHI = 3 days. Nonbearing and bearing trees. Provides broad-spectrum control of many annual and perennial weeds, woody brush and trees.

Glyphosate, diammonium salt: Touchdown. REI = 12 hours. PHI = 3 days. Nonbearing and bearing trees. Provides broad-spectrum control of many annual and perennial weeds, woody brush and trees.

Fluazifop-p-butyl: Fusilade DX. REI = 12 hours. PHI = 30 days. Nonbearing and bearing trees. For control of annual and perennial grasses.

Halosulfuron-methyl: Sempra CA. REI = 12 hours. PHI = 1 days. Nonbearing and bearing trees. Selective herbicide for the control of nutsedge and some broadleaf weeds.

Sethoxydim: Poast. REI = 12 hours. PHI = 15 days. Nonbearing and bearing trees. Selective, broad-spectrum herbicide for the control of annual and perennial weeds. Does not control sedges or broadleaf weeds.

Sulfosulfuron: Outrider. REI = 12 hours. Nonbearing trees only. Selective herbicide for the control of some annual and perennial grasses, nutsedge and broadleaf weeds.

2,4-Dichlorophenoxyacetic Acid: Weedar 64, Orchard Master, Unison. REI = 48 hours. PHI = 60 days. Nonbearing and bearing trees. For control of certain broadleaf weeds; trees must be at least 1 year old and in vigorous condition.

Contact Herbicides

Diquat Dibromide: Reglone. REI = 24 hours. Nonbearing trees only, can be used up to 1 year before expected harvest. A dessicant herbicide for control of annual grass and broadleaf weeds. Essential to obtain complete coverage of the target weed to achieve effective results.

Paraquat Dichloride: Gramoxone Max, Boa, Cyclone Max. REI = 24 hours. PHI = Do not apply with nuts on the ground. A dessicant herbicide for control of annual grass and broadleaf weeds. Essential to obtain complete coverage of the target weed to achieve effective results.

Glufosinate-ammonium: Rely. REI = 12 hours. PHI = 14 days. Nonbearing and bearing trees. A broad spectrum contact herbicide used to control a wide range of broadleaf and annual grass weeds.

Carfentrazone-ethyl: Aim EC, Shark. REI = 12 hours. PHI = 3 days. Selective post-emergent control of broadleaf weeds.