How to Get Rid of Weeds in a Farm Dam: Species Guide + Methods

Dam weed removal is the single most common request we get from farmers between spring and late autumn. Water lettuce chokes the pump inlet. Cumbungi reeds march out from the bank until half the dam is a swamp. A green film of azolla makes the whole surface look like a golf-course lawn. And every year, from the Northern Rivers down to the Southern Highlands, someone asks us the same question: what is the fastest way to get this stuff out?

The honest answer is that speed matters less than picking the right method for the right weed. Spray glyphosate on a floating water hyacinth mat and you will drop tonnes of decaying plant matter to the bottom, spike the oxygen demand, and kill every fish in the dam within a week. Dig out cumbungi without treating the rhizome and it will be back inside 18 months. Getting weed out of a farm dam properly means identifying the species, understanding why it is there, choosing a method that suits the biomass, and closing the door so it does not come back.

Start by identifying what you are actually dealing with

Australian farm dams carry roughly a dozen problem species, and the treatment depends entirely on which one you have. The NSW Department of Primary Industries splits them into four functional groups (emergent, floating attached, free-floating and submerged) and each group needs a different tool. Cumbungi is Typha, an emergent reed anchored in silt on shallow margins. Water hyacinth is Eichhornia crassipes, a free-floating plant with glossy round leaves and purple flowers that can double its mass every 7 to 10 days under favourable conditions (NSW DPI Primefact 30). Salvinia is Salvinia molesta, a free-floating fern that also doubles every 7 to 10 days. Azolla is a small native free-floating fern that turns red-brown when stressed. Duckweeds are the tiny bright-green discs (Lemna, Wolffia, Spirodela) that carpet still water.

Water hyacinth and salvinia molesta are declared Weeds of National Significance. If you have either, you have a legal obligation under state biosecurity law to control them, and in some jurisdictions you must not move fragments off your property.

Common farm dam weeds and what actually kills them

Species	Type	Best first response	Do NOT do this
Cumbungi (Typha)	Emergent reed	Cut below waterline in autumn, then spot-spray regrowth with aquatic-registered glyphosate	Slash and walk away. Rhizomes regrow within one season
Water hyacinth	Free-floating	Physical removal with booms, nets or a dam harvester. Report to Local Land Services	Broad-scale herbicide on a full mat. The decay will kill the dam
Salvinia molesta	Free-floating fern	Physical removal plus release of the Cyrtobagous salviniae weevil for long-term control	Move fragments off-site on machinery, boots or nets
Azolla	Free-floating native	Skim off with a scoop net and dry on the bank, then fix the nutrient source	Treat with herbicide. It is usually a symptom, not the disease
Duckweed	Free-floating native	Skim, aerate, reduce nutrients	Ignore it. A full cover blocks oxygen and kills fish
Water lettuce	Free-floating declared	Physical removal plus report to biosecurity	Compost near a waterway

Cumbungi is the one most farmers try to fix themselves. It grows from a woody rhizome in the top 300 to 600 mm of silt on the dam margin, and if you leave a single node behind the whole clump comes back. The Queensland biosecurity guidance is clear: repeated slashing gives some control, but the only lasting method is spraying aquatic-registered glyphosate on actively growing plants before seed set, then burning the dead stems 6 weeks later.

Why is my dam full of weed in the first place?

Aquatic weeds are a symptom of three conditions: excess nutrients, still water and shallow margins that stay warm and lit. Fix any one and the problem gets smaller. Fix all three and it usually goes away. Nutrients come from stock defecating in the water, fertiliser runoff from upstream paddocks, and organic matter already in the dam. Azolla is a reliable indicator of high nutrient loading. Agriculture Victoria notes that azolla grows fastest at pH 4.5 to 7 and 18 to 26 °C water, and vigorous growth almost always means the dam is nutrient-rich.

Shallow water is the second factor. Cumbungi, phragmites and waterlilies only establish where they can reach the bottom. A dam with steep 3:1 (H:V) batters and 2.5 to 3 m at the margins does not give emergent weeds anywhere to root. A dam with flat 5:1 batters and a 300 mm shelf around the whole perimeter is a nursery.

Still water is the third factor. Floating weeds double every 7 to 10 days on undisturbed surfaces. A small aerator or shallow-set fountain running a few hours a day slows that down by disrupting the surface and adding oxygen. If you also have blue-green algae in the same dam, the fix is the same: cut nutrients and get the water moving.

How to physically remove weed from a farm dam

Physical removal is the first-choice method for every free-floating weed and small emergent infestations. It is slow, but it is the only method that does not dump organic matter back into the dam to rot.

1. Confirm the species and, for water hyacinth, salvinia or water lettuce, notify your state biosecurity authority or Local Land Services before you start.
2. For small floating infestations, use a garden rake or a purpose-built weed rake from a boat or the bank. Work outward from the shore.
3. For larger floating mats, string a boom (a rope with floats) across the water and drag the mat to one corner where it can be lifted out.
4. For emergent stands like cumbungi over 20 m², bring in a 5 to 13 tonne excavator with a long reach. Scoop the rhizome mat cleanly and place it well back from the water so no fragments can wash back in.
5. Let removed weed dry on the bank for at least 72 hours before moving or composting it. For declared weeds, dispose of the dried material at a licensed green-waste facility, not in a paddock compost heap.
6. Never move machinery from a salvinia-infested or hyacinth-infested dam to a clean one without a full pressure wash. A single leaf fragment can start a new infestation.

Dam harvesters (floating platforms with a conveyor and cutting bar) suit dams over 1 ML with heavy free-floating cover. For smaller farm dams, a long-reach excavator clears the same volume in a day at a fraction of the cost.

When to use herbicide, and when to stay away from it

Herbicide has a place, but it is not a substitute for physical removal on a large infestation. When a heavy mat of hyacinth or salvinia is killed with glyphosate, the plants sink and decompose over 2 to 6 weeks. That decomposition consumes dissolved oxygen and releases nutrients back into the water column. A 2024 industry briefing notes that spraying a full water-hyacinth mat without prior physical removal typically causes a fish kill and a turbidity spike lasting months (Berries Australia dam water quality workshop, 2024).

For dams used for household, garden or recreation, only glyphosate formulations registered for aquatic use may be applied, and only on emergent parts above the waterline. In dams not used for drinking, gardening or swimming, actives such as 2,2-DPA and diurex WG are registered for cumbungi and some emergent weeds. Always read the label, check for a stock withholding period, and match the product to the water use.

The best chemical strategy on any large infestation is staged: physically remove 70 to 80% of the biomass first, wait 1 to 2 weeks for the water to clear, then spot-spray the remnant and any regrowth. On a Roma property in 2023 we cleared a 4 ML dam of dense cumbungi, ran the excavator for two days, gave it three weeks to settle, then knocked back the regrowth with a low-volume glyphosate spray the following spring. That dam has stayed clear for two full seasons.

Biological control for salvinia is a genuine success story

Salvinia molesta has one biological control agent that actually works: the salvinia weevil, Cyrtobagous salviniae. Released in warm-climate Australian dams it can knock a full salvinia infestation down to a few scattered plants within 12 to 18 months. In temperate regions the weevil struggles in winter, so control is slower, but it remains the only long-term biological answer (NSW DPI biological control field guide). No comparable weevil works for water hyacinth in southern Australia. Two Neochetina species are established in northern catchments but perform poorly south of the Tropic of Capricorn. For water hyacinth on a QLD or northern NSW property, ask Local Land Services about weevil availability; south of that, plan for physical removal.

What dam weed removal actually costs

Costs vary with dam size, weed density and access, but three benchmarks are useful. A 5-tonne excavator on wet hire runs $140 to $170 per hour across most of Queensland and NSW as at 2026; a 13-tonne machine with an operator runs $180 to $220 per hour (iSeekPlant excavator rate guide). A moderate cumbungi clear on a 2 to 5 ML dam typically takes 1 to 2 days of machine time. A heavily infested 1 ML dam of water hyacinth or salvinia can take 3 to 5 days plus disposal costs. Herbicide costs are much lower (aquatic glyphosate at label rate covers most farm dams for under $200 in product), but the labour and fish-kill risk means it is a follow-up cost, not a primary one. Salvinia weevil release through a state department programme is generally free, with full effect at 12 to 18 months.

How to stop dam weeds coming back

Every removal method fails if you do not fix the underlying conditions. Prevention comes down to four measures, and any two together will keep most dams substantially weed-free.

• Keep stock out of the water. Off-dam troughs supplied by a solar pump or gravity feed cut nutrient loading by 60 to 80% on most properties. Agriculture Victoria specifically recommends troughs as a first-line intervention for nutrient-driven weed problems.
• Build steeper batters. A dam with 3:1 (H:V) downstream batters and no shelf shallower than 500 mm gives emergent weeds nowhere to establish. If you are planning a farm dam rebuild, this is the single most powerful change you can make.
• Vegetate the catchment upstream. A 5-metre grassed or lightly treed buffer between fertilised paddocks and the dam intercepts a large share of the nitrogen and phosphorus that would otherwise wash in.
• Aerate deeper dams. A small solar aerator or a bottom-set diffuser running during warm months prevents thermal stratification, keeps oxygen levels up, and slows the still-water conditions that free-floating weeds prefer.

Lindsey Hughson has built and rebuilt farm dams across Queensland and northern NSW for over two decades. The dams that stay weed-free long-term are the ones designed correctly at the start with the right depth, the right batter and the right catchment management. Reactive weed control is always more expensive than good design.

Frequently Asked Questions

Can I use Roundup in a farm dam?

Not standard Roundup. You need an aquatic-registered glyphosate formulation such as Roundup Biactive or an equivalent product with an aquatic label. Standard agricultural glyphosates contain surfactants that are toxic to aquatic life. Even the aquatic formulations should only be sprayed on plant material standing above the waterline, and a full-mat spray on a large floating infestation will kill fish through oxygen depletion as the plants decompose.

How fast does water hyacinth grow?

Under favourable conditions water hyacinth can double its mass every 7 to 10 days. A single plant can cover a small dam in a season. That is why water hyacinth is a Weed of National Significance in Australia and why early physical removal is critical. Waiting even one summer to act typically means dealing with 10 to 100 times the biomass the following autumn.

Do I need a permit to remove weeds from my farm dam?

For most private farm dams on your own property, no. But if the dam is connected to a watercourse or classified as a wetland, or if you are using herbicide near a stream, controlled-activity approvals may apply under state water legislation. If the weed is a declared species like water hyacinth, salvinia, water lettuce or alligator weed, you have a legal obligation to control it, and your state biosecurity authority can advise on approved methods and disposal.

If your dam has been taken over by weed and you want it cleared properly, with the right machine and a plan to keep it clear, book a site inspection with Big Ditch and we will assess the species, the causes and the right removal method for your dam.