by Sanne Kure-Jensen
Good weed control leads to stronger plants, higher crop yields and higher farm profits. “Effective weed control starts with proper identification,” said Dr. Richard Bonanno, UMass Extension and GAP Educator, at a recent pesticide-training workshop held in Warwick, RI. He recommended growers verify weed ID using the “New England Vegetable Management Guide” or a good a field guide.
Growers should note weeds, treatments and weather each season and use past years’ notes to determine likely locations and times for outbreaks. Scout potential trouble sites to verify the need for treatment. For peak effectiveness, apply just enough of the right treatment at the weed’s most vulnerable life stage.
Wood chips, hay or other organic mulches work well to control many annual weeds. Organic mulches will keep soils cool and should be avoided on fruiting vegetables until the soil has warmed sufficiently. Beware of mulches that contain weed seeds.
Stale bedding helps control weeds when rotating crops. Delay planting after tilling in last season’s crop residues or cover crops. Allow weed seeds to germinate. Repeat shallow cultivation weekly for several weeks to reduce the seed bank of weed seeds. Deep cultivation moves surface seeds deep into soil layers and brings other seeds up to germinating depth. Dr. Bonanno recommends using a flame weeder, or herbicide or shallow cultivation to kill young weed sprouts.
Dr. Bonanno applies a “Rule of seven times” to seed germination. Seeds can germinate at soil depths up to seven times their diameter. For many seeds, this means depths of only 1/4 to 1/2 inch.
To control unwanted grasses, wait until they are tall enough so the growing point is above the soil surface. Then cultivate or use a post emergent weed killer. Be sure to minimize further soil disturbance.
“When you find a new weed in a farm field, get off your tractor and pull it!” Dr. Bonanno said. Do not let the weed become established, go to seed and create a long-term problem. Many weeds produce 3,000 to 5,000 per plant and some produce as many as 100,000 in a season.
Increasing soil temperatures in the spring trigger seed germination. Summer annual weeds like common ragweed and purslane need warmer soils to germinate. Some weeds like dandelions spread by seed and regrow from cut root segments. Creeping perennial weeds like quackgrass, bindweed, bittersweet and poison ivy spread by rhizomes. Cut rhizome segments become new plants.
Pre-emergent soil applications do not need surfactants. These treatments need irrigation or rainfall to be effective but should not be tilled in unless the label recommends it or they might end up too deep to work.
Surfactants can help make herbicides more effective when sprayed on plant leaves. Check product and surfactant labels to verify they can be used together, at that temperature, season and with that crop. Shield crops when recommended. Be sure to pay attention to days to harvest requirements (also known as preharvest intervals) and which crops can follow applications this season and even next year.
During hot summer days, post-emergent treatments should be applied late in the day as temperatures are falling (never on sunny mornings when temperatures are rising). For peak post-emergent effectiveness and minimal crop injury risk, the sum of the temperature and relative humidity should be less than 150. Surfactant labels will list the effects of high temperatures and humidity. Dr. Bonanno reiterated, “The label has your best advice.”
Before using herbicides on beds with plastic mulch, verify that the treatments may be used on or in rows between plastic mulch. When cultivating between rows of plastic, be cautious of bringing up new seeds. Shallow cultivation is best.
In any planting, there is a race between the chosen crop and the weeds, until the crops can shade out the weed seeds. Dr. Bonanno explained, “I’m a fan of weed control in cucurbits during the first six weeks and then letting the plant canopy close and shade between rows to provide weed control for the rest of the season.”
Unexpected rain or temperature swings after applications may cause crop injury like leaf deformity, yellowing or stunting. Pumpkins and winter squash are more likely to grow out of these injuries than cucumbers or summer squash.
Pay attention to herbicide group numbers. Use herbicides from different groups from year to year to minimize risk of developing weed resistance. The group numbers do not correspond between herbicides, fungicides and insecticides.
Be sure to mix treatments accurately and according to label recommendations. Rates are given in acres of coverage, not per 100-gallon tank. Do not count the area under plastic-covered rows when calculating soil coverage. Mixing in a little extra will not make treatments more effective and this may cause injury. Dr. Bonanno recommended everyone calibrate sprayers at least once a season or whenever changing tires, gauges or nozzles.
The mixing order matters. Adding ingredients in the wrong order prevents solids dissolving, changes application rates and efficacy as well as clogging nozzles. “Tanks are not for making potpourri,” said Dr. Bonanno. Product labels will tell you if a “tank mix” is recommended. Unless you have experience, test first in a small batch. When tank mixing pesticides, mix in the proper order: Wettable Powders (WP), Water Dispersible Granules (WDG), Flowables (F) (DF) (SC), Water-dispersible liquids (AS), Emulsifiable Concentrates (EC), and finally Solutions (S). Non-ionic surfactants (NIS) are safest. Even these surfactants should be used according to labels. Use beyond recommended levels may be toxic to crops.
Train everyone on the farm to follow all worker protection standards. Wear the right protective gear, stay out of fields until the reentry interval and pay attention to preharvest intervals.
Weed control for vegetable crops
by Sanne Kure-Jensen