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Rob the Bank

Posted: August 8, 2012

Seed-bank, that is. The weed seed-bank. Right now, across Pennsylvania, huge deposits of weed seed are being accepted by soil. Stop it now and this will pay dividends later.

Enough joking. Weeds are serious business and late summer is a time when annual weeds are maturing…that means seed production. The numbers are mind boggling. Textbooks tell us that a single mature redroot pigweed plant produces more than 100,000 seeds. Purslane, 50,000. Giant foxtail, 10,000. Let's say you have one plant per square foot.  Do the math and you'll run out of words for the total potential, per acre seed production…and your calculator will say "ERROR!"

Not every field is this overrun with weeds and not every seed will come back to haunt you, but you get the idea. Time spent on a mower or disk will cut that seed production dramatically. Sure, you have plenty of jobs to accomplish every day that will yield highly satisfying, short-term rewards: pull sweet corn, spray sweet corn, get ready for market, plant the next crop. But if you can find the time to deal with weedy sites, you'll be rewarded. Here are sites that need attention:

Cropped-out plantings -- Early maturing plantings of beets, beans, carrots, onions and other non-competitive crops often become overwhelmed with annual weeds. The pre-emergence herbicides you may have used have degraded and are no longer providing weed control. Most of the crop is out and what's left is not competitive. Rather than let these sites "go to seed," put the mower or disk to them. Mowing and disking is best since many annual grassy weeds such as crabgrass and purslane will set seed at low mowing heights. If you can find another spare chunk of time, cover crop these sites with quick establishing annuals such as buckwheat or ryegrass (annual or perennial). While you may have plans to seed rye as an overwintering cover crop, there is no reason to leave that site in weeds for two months.

By mid-August, consider covers that overwinter such as mustard or rapeseed. Or consider the combination of tillage radish and oats which winter kill. In addition to improving soil quality in many ways, these cover crops compete with winter annual weeds that germinate in early fall. So, cultivating or mowing cropped-out fields reduces summer annual weed seed production. Add a cover crop and you limit winter annual weed establishment.

So far we've been targeting annual weeds…crabgrass, purslane, pigweed, galinsoga, ragweed. Perennial weeds need more than cultivation for maximum control. Got patches of bindweed, quackgrass or nutsedge in those fields? Consider two quarts of glyphosate per acre as broadcast or spot treatment. Canada thistle? Use glyphosate or Stinger. And, the better the weed is growing, the better the herbicide will work. Plenty of healthy weed foliage is critical. Remember that drought stressed or otherwise compromised weeds do not translocate herbicides well. If you can't treat these perennial weed-infested fields in August, budget time to nail them in September. Shoot for treatment about two weeks before a killing freeze to allow for translocation.
 
Row Middles Between Plastic -- First, evaluate the weed control program you designed this year. Grade it. Can you do better? There are excellent herbicide options for every crop outlined in Penn State's Commercial Vegetable Production Recommendations.  Make a note to investigate this for the 2013 season. For now, mowing may be your best option for weed problems between rows, but wicking applicators can apply glyphosate very safely. In some situations, weeds may be too mature to get decent control with selective, post-emergence herbicides; or days-to-harvest limitations may rule out application. However, be aware that many excellent post-emerge options exist. The post-emergence grass herbicides, Poast or Select Max, are labeled for almost every vegetable crop. They have excellent crop safety and control almost all emerged grasses. Herbicides for post-emerge, broadleaf weed control are also available.

Field Borders -- Easy one. Brush hog. But, some of those borders might be good pollinator forage or refuge. Goldenrod, milkweed, Joe-Pye weed are all super bee food that you probably recognize. Read on.

Leave Some for the Pollinators -- If you don't get to any of these weed control suggestions, you can always say it's pollinator refuge. But this will be incorrect because most of the weeds that infest our vegetable fields are not great insect forage. If you really want to beef up your population of pollinators and other beneficial insects, you could consider getting serious about pollinator refuge. Heck, the Natural Resources Conservation Service (NRCS) will pay you if you need the incentive.

Scott Guiser, Penn State Extension Educator, Bucks County, (sxg6@psu.edu)