A: Duckweed is a tiny menace that definitely needs to be managed. Brought to your pond or lake by humans and their equipment or on the feet and feathers of visiting waterfowl, dense colonies of these plants can proliferate and eventually cover the water surface. It's not something you want in growing your pond.
Duckweed or Watermeal?
Duckweed a very small, light green, free-floating plant with a single hair-like root and three 1/16- to 1/8-inch long leaves, or fronds. It tends to grow in dense colonies in quiet water that's undisturbed by waves. You can fit six to eight of these plants of the tip of your finger.
Watermeal – another invasive plant that can be mistaken for duckweed – is also light green and free-floating, but it has no roots and is more of a grainy, seed-type plant. It's also much smaller than duckweed; at less than 1 millimeter in size, you can fit 10 to 20 of them on the tip of your finger.
Duckweed and watermeal colonies can provide a habitat for microscopic critters and forage for hungry ducks, but the plants can reduce oxygen in the water if they grow to cover a lake's or pond's surface. That could compromise your fishes' wellness and cut off sunlight to underwater plants.
To control duckweed, think short-term and long-term.
Short Term: Ultra PondWeed Defense or KnockDown Defense used with Treatment Booster Plus are your go-to herbicide products for short-term control of duckweed and other invasive aquatic weeds. They provide broad-spectrum pond weed control in slow-moving water and kill what's actively growing in your pond. If duckweed hasn't completely taken over your water surface, you may notice algae growth mixed in with the weeds – in which case you'll need to treat the algae first. (Pro tip: Propeller will control both algae and duckweed.)
Long Term: For long-term control, you'll need an herbicide like fluridone, which is found in Airmax WipeOut or Sonar A.S. When applied in early spring (or when you begin to notice weed growth), you'll see the product controlling established plants in 30 to 60 days, and in 90 days, you'll have full pond protection. Because exposure to sunlight can reduce Sonar's effectiveness, use in combination with Pond Dye. If you use your pond water to irrigate, you will need to wait 30 days following treatment.
Improve Overall Pond Quality
In addition to managing your menace with herbicides, you should also reduce muck and aerate the water to keep your overall pond balanced. The products in the ClearPAC Plus Pond Care Package – including PondClear, MuckAway and EcoBoost – will help reduce the submerged and suspended organic debris. Combine that with some Airmax Aeration, and your water will stay crystal clear all season long.