Selective

• Growers use selective herbicides to treat several types of annual grasses and broad-leaved weeds that are a problem for fresh market tomato crops. These herbicides are "selective" in that they are only effective on grasses and broad-leaved weeds. They operate systemically by working their way through the soil and down to be absorbed into the root system of the weeds and grasses. This stops growth and development and kills the weed or grass. Napropamide is the active ingredient used in herbicides for fresh market tomatoes. Trade names for herbicides with napropamide include Devrinol and R-7465.
  
  

Nonselective

• Nonselective herbicides are ideal for eradicating vegetation. In other words, they kill everything. As a result, nonselective herbicides are typically used in trenches that border the rows of the actual tomato plants to form a defensive barrier so the weeds do not grow on the soil for the tomato plants. One of the most widely used nonselective herbicides for fresh market tomatoes and more than 100 other types of crops is the chemical weed killer named Paraquat. Paraquat works on both annual and perennial weeds and kills on contact. Paraquat trade names include Crisquat, Dextrone, Dexuron and Ortho Weed and Spot Killer. Another popular nonselective trade brand for residential garden use is Round Up, whose active chemical ingredient is glyphosate.
  
  

Pre-Emergent

• Farmers use pre-emergent herbicides as a preventive measure to control weeds. These herbicides are applied before the tomato plants go into the ground. One widely used pre-emergent Is Trifluralin. It is a member of the dinitroaniline chemical family. Trade names for Trifluralin include Treflan, Trefanocide, Treficon and Ipifluor. Pebulate is another popular pre-emergent used for fresh market tomatoes. Pebulate controls both grasses and broad-leaved weeds. It is applied before planting and worked into the soil. Pebulate can only be used by agricultural and commercial growers.
  
  

Pre- and Postemergent

• Pre- and postemergent herbicides work to prevent weeds from growing and are applied to control them once they appear. Metribuzin is a chemical that is used in many pre- and postemergent herbicides. Metribuzin works by inhibiting photosynthesis -- the biological process that allows vegetation to develop. This chemical is effective on annual grasses and many types of broadleaf weeds. Trade names for metribuzin include Sencoral, Sencorex, Lexone and Bay 94337.