Too Late To Treat Pond Weeds?

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Today I asked about duckweed taking over a pond around Meigs. On the way up, I passed another pond that was nearly 100% infested with duckweed. When we look at weeds like duckweed, we need to make sure another weed called watermeal is not also present. If that is the case, our treatment options change. To see more on these two weeds, click on my blog post titled Pond Weeds – Duckweed & Watermeal.

This pond is almost 100% full of both duckweed and watermeal. Some large bass have died as result of low oxygen. This pond needs to be treated, but we are very late in the season to do it.  I’m getting this question more this year since temperatures are so warm. There are few general rules we can follow, but every situation is different depending on weed present, water temperature, who treats, chemical availability, etc.

When is too late?

UGA Extension Aquatic Scientist Dr. Gary Burtle says our water is borderline for treatment right now. 70 degrees F is when we are generally safe to treat ponds. We still need to consider, however,  that some compounds work better at 80 degrees F though 70 is our cutoff. Right now, plants are not growing as fast, and chemical reaction is slower.

What does this mean? Some of our compounds are systemic and others contact. Diquat, cutrine and clipper are contact herbicides. Sonar, which is good on watermeal and duckweed, is systemic. Systemic chemicals needs 3 – 4 weeks of actively growing plants to be effective. We are certainly too late in the season for this kind of treatment. Clipper also works good on duckweed when watermeal is mixed. But the issue with contact herbicides is though they may provide a kill with borderline temperatures, timeliness is key. If we do not already have a plan, water temperature could drop to 60s in a short time.

We must to also consider pond converge. Dr. Burtle reminds us that if the pond is 80-100% covered, two treatments of a contact herbicide would be needed. Considering the cost, one application of a systemic herbicide (for a small pond < one acre) would be less expensive – which we are too late to apply.

Conditions now are essentially the same as conditions will be in late Febrary. At this time in the season oxygen in a pond is actually higher. This will keep us from losing too many fish and should also be considered. Cooler weather will also slow weed growth. Dr. Burtle says it is not totally bad to treat during these conditions; however, once all factors are considered, waiting until spring would be more beneficial.

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Duckweed & Watermeal




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