How do I keep a pond from turning green?

The lasting fix for a green pond is to cut the nutrients going in, not to kill the algae that result. Fertilizer, manure, septic leakage and bare, eroding banks feed algae every summer. Treatments only clear the water for a while, and a pond designed with the wrong depth or slopes will keep turning green.

Why ponds turn green

Algae grow when nutrients and warm weather coincide. The Northern Virginia Soil and Water Conservation District says "in the summer months, inputs of nitrogen and phosphorus stimulate plant growth",^[1] and adds that "excess pond vegetation is an indicator of an unbalanced ecosystem where too many nutrients are entering the water".^[1]

Green water is sometimes even deliberate. Virginia's Department of Wildlife Resources says "fertilization makes the water turn green",^[2] because fertilizer feeds the microscopic algae that fish food chains depend on. But the same agency warns that "adding too much fertilizer can cause unsightly algal blooms with unpleasant odors".^[2] It also notes that "most Virginia ponds do not need fertilization".^[2] If you or a previous owner ever fertilized the pond, that is the first place to look.

Why a bloom is more than ugly

When blooms collapse the water loses oxygen. DWR says "algae die-offs during summer months can cause low oxygen levels, which may result in fish kills".^[2] The district describes the same loop for all pond plants: growth, death and decay use up dissolved oxygen, and in severe cases cause die-offs in the fish community.^[1] A pond that smells and has dead fish after a hot spell has probably been through one.

Stop the nutrients first

The district's advice is plain: "the best way to manage overly prolific aquatic vegetation in your pond is to control nutrient runoff at its source".^[1] Its list is a good checklist for any owner:

• "limit the use of fertilizers" on the lawn and land that drains to the pond^[1]
• "manage runoff from horse pastures and manure piles properly"^[1]
• "ensure proper maintenance of septic systems on lands surrounding and draining to the pond"^[1]
• keep a "2-3 foot wide vegetated buffer of native trees, shrubs and grasses" around the water to filter runoff^[1]

Every item on that list is something you can see on your own property. Walk the area above the pond after a rain and note where water and soil reach it.

Why chemical fixes rarely last

Algaecides and herbicides can clear a pond quickly, but they leave a problem behind. The district says that after treatment "dead plant material settles in the pond where it decays, releasing additional nutrients and jump starting the next cycle of plant growth", and that the usual result is "a never-ending cycle of increasingly frequent, and increasingly expensive, applications".^[1] Treat chemicals as a short-term tool, used alongside nutrient control and not instead of it.

They also carry rules. DWR says "certain herbicides can be purchased and used only by Certified Pesticide Applicators",^[2] so the work may have to be hired out, and the label has to be followed exactly. The district recommends hiring a certified pesticide applicator to choose the product and rate.^[1] Stocking plant-eating fish is regulated too: "a permit (PDF) is required from the Department of Wildlife Resources to stock triploid grass carp into any Virginia waterbody".^[2] Grass carp eat soft tissue pond plants such as hydrilla and coontail,^[1] so identify the problem before you buy anything. DWR says your Cooperative Extension office or local fisheries biologist can help identify the plants.^[2]

Design that works against algae

Shape matters. DWR's guidance says "shoreline depths of 2 to 3 feet with bank slope ratios of 3:1 are ideal" for aquatic plant control.^[2] If your pond keeps returning to green no matter what you do, an excavation contractor can look at regrading the shoreline or deepening shallow areas. See how a backyard pond is built for what that kind of work involves, and how much it costs to build a pond in Virginia for the cost drivers. A pond that is also dropping in level has a different problem: see why a pond loses water.

Permits

Reshaping banks or dredging is land disturbance, and in a stream or wetland it can need state and federal permits. Read whether you need a permit to build a pond in Virginia before anyone starts digging.

When to call a licensed pro

For regrading, dredging or shoreline reshaping. Virginia's Board for Contractors says that "if the work you are considering is valued at $1,000 or more, a valid Virginia Contractor’s License is required for the license category in which the contractor is to work".^[3] Under the Code of Virginia a Class C licence covers a single contract over $1,000 but less than $30,000.^[4] Check the licence before you sign, and get any chemical treatment in writing from a certified applicator.

Loclis lists licensed local businesses. Browse excavation services, read more excavation answers, or describe your pond project once and licensed excavation businesses will quote it.