Problem Weeds in Strawberries

Now that strawberry harvest is finished and renovation well underway, we are getting questions about specific weeds and making changes to weed management systems for next season. Here are some comments about some of the common problem weeds in strawberries:

First, let's deal with 4 perennial weeds that are impossible to control in strawberries and need to be handled in rotational crops before strawberries are planted:

1. Toadflax: Try using Amitrol before cereals, corn or beans, or after cereals. Tillage is also useful to suppress this weed, but avoid dragging roots to clean fields. Toadflax is on our Roundup label for 15cm vegetative regrowth after summerfallow tillage for established plants. Dacthal will control germinating seedlings.
2. Oxalis (wood sorrel): Of the registered herbicides, Dacthal is your only hope and really doesn't give good control, even when applied several times per year. Growers who integrate Sinbar into their program several times through the season see much less oxalis in their fields. Use frequent tillage to knock down small seedlings, and try to prevent seed set by mowing where possible (eg. field edges). Goal applied premulch will reduce oxalis, but will not clean up a totally infested field.
3. Yellow nut sedge: Rotate to corn or a vegetable crop where you can use Dual II Magnum as a PPI treatment. Use Dual II Magnum PPI in new strawberry plantings. Cultivate before nutlets begin forming in July, and avoid dragging nutlets into uninfested fields. Sinbar can suppress nut sedge if high rates can be used. This is a weed that is never eradicated forever, but try to start with clean strawberry fields.
4. Horsetail: In strawberries, horsetail will be suppressed by 2,4-D or Sinbar. Try rotating to corn to use MCPA or Amitrol (before or after the crop). Also, after wheat, Buctril M, MCPA or Amitrol can be fairly effective on horsetail.

Some of the newer field crop herbicides are effective on these problem weeds, but watch for carryover residues.

Because these 4 weeds are not well controlled with glyphosate (eg. Roundup), innovative approaches are needed to clean them out of strawberry fields before planting. Pay close attention to their life cycles (especially their propagation method eg. seed, rhizomes, spores, etc.) to hit them when it hurts. Mowing of field edges and patches of these weeds and seeds can keep problems from getting worse.

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Next, let's look at 4 weeds that are difficult to control in strawberries, but can be eradicated in the preplant year with glyphosate. Here are the keys to helping glyphosate work:

Apply to actively growing weeds

• Use clean water and low water volumes
• Choose the correct rate for the weed:
  • high labelled rate for most broad-leaved perennials
  • medium rate for quackgrass
  • low rate for annual weeds
• Apply at the specific growth stages for each weed.

If you have worked your field this spring, and have had rain, your weeds may be approaching the proper stage for glyphosate:

1. Bindweed - apply at 10% to full flower
2. Milkweed - apply at early flower bud
3. Canada thistle - apply at early flower bud
4. Quackgrass - apply at 3-6 leaves
5. Coltsfoot - apply at full leaf

Consider buying a wick wiper (around $50) and spending a little time each month targeting patches of problem weeds. An ounce of prevention…..

So what do you do if these weeds creep into your strawberry fields? Here are some strategies to try in strawberries:

1. For bindweed, try 2,4-D at renovation, or use a spot spray of 2% glyphosate when bindweed is in bloom.
2. For quackgrass, apply Venture or Poast Ultra + Merge in the spring OR after quackgrass grows back after renovation, or use a spot spray of 1% glyphosate when quackgrass has 3-6 leaves.
3. For milkweed, a wiper applicator with a 33% solution of glyphosate at flower bud can be very effective.
4. For coltsfoot, try a spot spray of 2% glyphosate after harvest but before mowing. Good coverage is essential to try to control this weed.
5. For Canada thistle, use a spot spray of 2% glyphosate at early flower bud, or a wiper application of 33% solution of glyphosate. 2,4-D or Lontrel at renovation will help, especially if the thistles are in full leaf to flower stage. Regular use of Sinbar will also suppress thistles.

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Now let's look at some tricky weeds in strawberries:

• Field violet (pansy) has become a real problem in many areas. Dacthal will control germinating seedlings, but will only give 6-8 weeks of control. Because of the cost, target it on fields where both field violet and dwarf snapdragon, likely in early spring and possibly at renovation (especially if they have gone to seed). Sinbar used at Labour Day will also control weeds that would germinate through the fall, and Goal used at premulch will reduce the populations next spring.
• Dwarf snapdragon is also showing up in more areas. Again, Dacthal should be used early in the spring before they germinate.
• Groundsel is a problem because it flowers and produces large numbers of seeds all season. Devrinol is effective if applied before they germinate, but works best if applied at either Labour Day (with irrigation) or premulch. Once groundsel is established, Lontrel used at renovation will control established plants. Avoid allowing groundsel to go to seed, especially in the fall (these seeds have longer dormancies). Use of Gramoxone as chemical renovation can greatly reduce groundsel populations. Where oxalis is also a problem, the premulch Goal application has also reduced groundsel populations.
• For vetch, ox-eye daisy, thistles and sheep sorrel, applying Lontrel at renovation will control established weeds. Do not apply Lontrel after mid-August to avoid injury to developing fruit buds in the early fall.

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