Spraying Transplant Vegetable Row Middles – What are Your Options?

by Kristen Callow, OMAFRA Weed Management Program Lead – Horticulture

There has been a lot of interest in spraying the row middles of vegetable transplant fields with herbicides using hooded sprayers.  Is this a good idea?  Well . . . it depends on the herbicide.  Some growers have questioned whether or not glyphosate can be sprayed.  This is not a labelled use and is definitely not a good idea.  Why?  Just take a look at Figures 1 through 3 which show the impact of glyphosate drift on tomatoes and cucurbits and Figure 4 which shows glyphosate drift on strawberry using a hooded sprayer.  Other reasons include:

  1. Drift could occur.  No matter how careful you are, wind can get under the hooded sprayer, you could drive over a rock, you could forget to turn off the sprayer when turning around on headlands, all resulting in drift.
  2. Drift of glyphosate typically results in dead plants.  Glyphosate is absorbed into green leaves or green stems. Once there, glyphosate moves or “translocates” throughout the plant, eventually causing wilting, yellowing – followed by complete browning, deterioration of plant tissue and ultimate decomposition of the underground roots and rhizomes (dead plants).
  3. Knowledge of how long glyphosate takes to break down on plastic mulch is not known.  Some formulations of glyphosate have been manufactured with surfactants included and the potential for residues in plastic mulch are also not known.  So, can the glyphosate stay active on the plastic mulch resulting in exposure when growing plant foliage reaches it?  This is not known.  We do know that glyphosate is broken down quickly by microbes in the soil.

The only herbicide currently labelled for weed control in row middles with a hooded sprayer is carfentrazone-ethyl (Aim EC).  Carfentrazone-ethyl is taken up through the foliage, not readily translocated and has no residual in soil.  If drift occurs with carfentrazone-ethyl the leaf tissue will simply brown and burn on contact.

White flash in growing point of tomato, diagnostic of glyphosate injury
Figure 1. Glyphosate drift on tomato
Dying tomato plant due to glyphosate injury
Figure 2. Glyphosate injury on tomato
Yellowing leaves on cucurbit plant due to glyphosate injury
Figure 3. Glyphosate injury on cucurbit
Discolouration of strawberry leaves due to glyphosate injury
Figure 4. Glyphosate drift on strawberry