Hydrothol 191 Granular 40 lb & 20 lbs Herbicide
Description
Granular Weed & Algae Control for Lakes, Ponds & Aquatic Sites
HYDROTHOL 191 Granular is a fast-acting contact aquatic algaecide and herbicide for controlling labeled algae and submerged aquatic plants in lakes, ponds, drainage canals, and other labeled aquatic sites.
Its granular formulation sinks through the water column and delivers the active ingredient into the treatment zone. Depending on the target species and site, HYDROTHOL Granular can be used against planktonic, filamentous and branched algae as well as labeled submerged weeds including hydrilla, watermilfoil, coontail, naiad, elodea, and multiple pondweed species.
The product contains 11.2% mono(N,N-dimethylalkylamine) salt of endothall, equivalent to 5% endothall acid.
Know the Target. Measure the Water. Follow the Label.
Need help determining acreage, average depth, or the correct labeled rate? Contact Weeders Digest Product Support at 877-224-4899.
Weeds & Algae
What Does HYDROTHOL 191 Granular Control?
HYDROTHOL 191 Granular is labeled for control of a broad range of algae and submerged aquatic plants. The exact concentration varies by species and site.
Planktonic Algae
Labeled for planktonic algae. Light to moderate infestations are commonly treated within the lower algae-rate range on the label.
Filamentous Algae
Labeled for filamentous algae including certain labeled species and site-specific restrictions.
Branched Algae
The label includes branched algae such as Chara and Nitella under applicable use directions.
Watermilfoil
Labeled Myriophyllum species include watermilfoil at the concentration range specified on the label.
Hydrilla
Hydrilla is listed on the label with its own labeled concentration range.
Pondweeds
The label includes multiple Potamogeton species, including curlyleaf, floating-leaf, largeleaf, Illinois, narrowleaf, threadleaf and others.
Coontail
Coontail is a labeled submerged aquatic plant with a species-specific concentration range.
Naiad & Other Weeds
The label also includes naiad, horned pondweed, elodea, Brazilian elodea, water stargrass and additional labeled species.
Application Rates
Label Rate Conversion Table
The label provides the following general conversion from desired endothall acid concentration to HYDROTHOL Granular quantity.
| Desired Rate | lb per Acre-Foot | oz per 1,000 ft² per Foot of Depth |
|---|---|---|
| 0.05 ppm | 3 lb | 1.0 oz |
| 0.10 ppm | 5 lb | 1.9 oz |
| 0.30 ppm | 16 lb | 5.8 oz |
| 0.50 ppm | 27 lb | 10 oz |
| 1.0 ppm | 54 lb | 20 oz |
| 2.0 ppm | 108 lb | 40 oz |
| 3.0 ppm | 162 lb | 60 oz |
| 5.0 ppm | 270 lb | 100 oz |
General Label Ranges
Light to Moderate Planktonic / Filamentous Algae
The label gives 0.05–0.3 ppm, equivalent to approximately 3–16 lb per acre-foot.
Higher Algae Rates
Certain difficult algae situations may use higher labeled rates up to 3.0 ppm, subject to the label's fish-protection precautions.
Submerged Aquatic Plants
The general plant-control section lists 0.5–5.0 ppm, or approximately 27–270 lb per acre-foot, with species-specific rates elsewhere on the label.
Species Rates
Selected Labeled Submerged Plant Rates
The table below summarizes selected species-specific ranges from the label. State restrictions and other label footnotes still apply.
| Target | Labeled Rate | Approx. lb per Acre-Foot |
|---|---|---|
| Hydrilla | 1.0–5.0 ppm | 54–270 lb |
| Coontail | 1.5–2.5 ppm | 81–135 lb |
| Watermilfoil | 1.5–2.5 ppm | 81–135 lb |
| Horned Pondweed | 1.5–2.5 ppm | 81–135 lb |
| Naiad | 1.5–2.5 ppm | 81–135 lb |
| Sago Pondweed | 1.0–2.5 ppm | 54–135 lb |
| Curlyleaf Pondweed | 0.75–1.5 ppm | 41–81 lb |
| Floating-Leaf Pondweed | 1.0–2.5 ppm | 54–135 lb |
| Largeleaf Pondweed | 1.5–2.5 ppm | 81–135 lb |
| Illinois Pondweed | 1.25–2.5 ppm | 68–135 lb |
| Narrowleaf Pondweed | 1.0–2.5 ppm | 54–135 lb |
| Threadleaf Pondweed | 1.5–2.5 ppm | 81–135 lb |
Ponds & Small Lakes
Special Directions for Ponds & Small Lakes With Little to No Outflow
The label includes a specific whole-waterbody-style use pattern for ponds and small lakes with minimal outflow. These rates differ from many marginal or sectional treatment rates.
| Target | Rate per Acre-Foot | Concentration |
|---|---|---|
| Planktonic & Filamentous Algae | 15 lb | 0.25 ppm |
| Branched Algae: Chara / Nitella | 30 lb | 0.5 ppm |
| Labeled Aquatic Plants | 30 lb | 0.5 ppm |
How to Apply
HYDROTHOL Granular Application Basics
HYDROTHOL 191 Granular is a contact aquatic herbicide and algaecide. The label directs users to apply it as evenly as possible throughout the treatment area while the target algae or plants are present and actively growing.
- Identify the target species.
Determine whether you are treating algae, Chara, milfoil, hydrilla, pondweed, naiad, coontail, or another labeled target. - Measure the treatment area.
Determine square footage or acreage. - Measure average water depth.
Treatment amount is based on water volume, so depth is essential. - Select the labeled rate.
Use the label to determine the correct concentration for the target species and application type. - Calculate the required quantity.
Use the pounds-per-acre-foot or ounces-per-1,000-ft²-per-foot-of-depth conversion. - Review fish-protection restrictions.
Higher rates may require sectional or marginal treatment and commercial applicator involvement. - Apply evenly.
Distribute the granules as evenly as possible throughout the labeled treatment area. - Monitor the treated area.
Dead algae and vegetation consume oxygen as they decompose, so dense infestations require additional caution.
Application Tools
Ways to Apply Granular Aquatic Treatments
Hand Broadcasting
For small labeled treatment zones, granules may be distributed evenly using an appropriate scoop or handheld granular spreader while wearing all required PPE.
Boat Application
Larger treatment zones may require application from a suitable boat or professional application equipment to achieve the even distribution required by the label.
Extreme Blower Granular Spreader
The optional Extreme Blower attachment converts a compatible round-tube leaf blower into a portable granular spreader. Application equipment does not change the pesticide label rate or PPE requirements.
Water Use
Swimming, Fishing, Irrigation & Drinking-Water Guidance
| Water Use | Current Label Guidance |
|---|---|
| Swimming | No general federal swimming restriction is stated for labeled use, but New York requires no swimming until the day after application, 24 hours. Follow all state and local restrictions. |
| Fishing | No general fishing-use restriction is stated by the current product information when used according to label directions. Fish-protection precautions still apply during treatment. |
| Irrigation | The label allows treated water to be used for irrigation subject to its directions and restrictions. Do not use treated water for chemigation. |
| Food-Crop Irrigation in Flowing Water | Do not apply more than 30 ppm per growing season, more than 5 ppm per application, or more than a total of 5 ppm within a 7-day interval. |
| Potable Water | Specific drinking-water intake restrictions apply. See below. |
Potable-Water Intake Setback
For lakes, ponds, and other quiescent water bodies, the label requires HYDROTHOL Granular applications to be at least 600 feet from functioning potable-water intakes in the treated water body.
For flowing water bodies, the applicator is responsible for preventing treated water above the drinking-water maximum contaminant level from entering a functioning potable-water intake. Follow the specific closure or downstream-treatment directions on the label.
Fish & Environment
Important Fish & Environmental Precautions
0.3 ppm Threshold
The label states that fish may be killed by dosages above 0.3 ppm.
Commercial Application
Because of potential fish toxicity, the label suggests applications above 0.3 ppm be made by commercial applicators as marginal or sectional treatments.
Dense Vegetation
Decomposing algae and plants consume oxygen. Heavy infestations should be treated in sections to reduce fish-suffocation risk.
Above 1 ppm
For slow-moving or quiescent water, do not treat more than 10% of the area at one time with doses above 1 ppm.
Koi & Hybrid Goldfish
Do not use HYDROTHOL Granular in water containing Koi or hybrid goldfish.
Freshwater Only
Do not use HYDROTHOL Granular in brackish or salt water.
Not for Small Garden Ponds
The label states that HYDROTHOL Granular is not intended for small-volume garden pond systems.
Safety & PPE
Required Personal Protective Equipment
Mixers, loaders, applicators and other handlers must wear the PPE required by the current label:
- Long-sleeved shirt and long pants
- Socks and shoes
- Waterproof gloves
- Protective eyewear
- A NIOSH-approved particulate respirator with the filter / approval configuration specified on the current label
After Handling
- Wash hands thoroughly before eating, drinking, chewing gum, using tobacco, or using the toilet.
- Remove contaminated clothing or PPE immediately if pesticide gets inside.
- Wash the outside of gloves before removing them.
- Wash thoroughly and change into clean clothing after handling.
- Keep contaminated PPE separate from other laundry and follow label cleaning instructions.
Package Sizes
Current Weeders Digest Size Options
20 lb
A practical option for smaller labeled treatment areas or customers who need less product per treatment.
40 lb
A larger quantity for broader treatment areas or applications requiring higher total labeled quantities.
Specifications
HYDROTHOL Granular Specifications
| Official Product Name | HYDROTHOL® Granular Aquatic Algaecide and Herbicide |
|---|---|
| Legacy Search Name | Hydrothol 191 Granular |
| EPA Registration Number | 70506-174 |
| Formulation | Granular aquatic algaecide and herbicide |
| Active Ingredient | 11.2% mono(N,N-dimethylalkylamine) salt of endothall |
| Endothall Acid Equivalent | 5% |
| Mode / Type | Contact aquatic herbicide and algaecide |
| Labeled Sites | Lakes, ponds, drainage canals and other labeled quiescent, slow-moving or flowing aquatic sites |
| Algae | Planktonic, filamentous and branched algae subject to label directions and state restrictions |
| Submerged Plants | Multiple labeled submerged species including hydrilla, milfoil, coontail, naiad, elodea and pondweeds |
| General Algae Rate | 0.05–0.3 ppm for light to moderate planktonic and filamentous algae; additional labeled rates may apply |
| General Submerged Plant Rate | 0.5–5.0 ppm, with species-specific rates and fish-protection requirements |
| Koi / Hybrid Goldfish | Do not use in waters containing Koi or hybrid goldfish |
| Brackish / Salt Water | Do not use |
| Small Garden Ponds | Not intended for small-volume garden pond systems |
| Potable-Water Setback | At least 600 ft from functioning potable-water intakes in treated lakes, ponds and other quiescent water bodies |
| Current WD Sizes | 20 lb and 40 lb |
| Shipping | Continental U.S. only on current Weeders Digest listing; state restrictions may apply |
Label & SDS
Read the Current Label Before Every Application
Pesticide labels can change. Application rates, PPE, state restrictions, potable-water requirements and environmental precautions should always be checked against the current registered label before use.
Label
EPA Reg. No. 70506-174, current EPA-listed label accepted January 10, 2024.
Product Label & SDS Resources
Current product resources include access to the full product label and Safety Data Sheet.
FAQs
HYDROTHOL 191 Granular FAQs
Is Hydrothol 191 Granular the official product name?
The current EPA-registered granular product is named HYDROTHOL Granular Aquatic Algaecide and Herbicide. "Hydrothol 191 Granular" is commonly used as a legacy product/search name. Hydrothol 191 itself is also the name of a separate liquid formulation.
Is this the granular or liquid Hydrothol?
This page is for the granular formulation. The separate Hydrothol 191 liquid has a different concentration and is not the same formulation.
What is the active ingredient?
HYDROTHOL Granular contains 11.2% mono(N,N-dimethylalkylamine) salt of endothall, equivalent to 5% endothall acid.
What is the EPA registration number?
EPA Reg. No. 70506-174.
Does HYDROTHOL Granular kill weeds and algae?
Yes. It is labeled as both an aquatic algaecide and contact herbicide for multiple algae types and submerged aquatic plants.
What algae does it control?
The label includes planktonic, filamentous and branched algae. Species susceptibility and state restrictions vary, so use the current label to select the proper rate.
Does it control Eurasian watermilfoil?
The label includes Myriophyllum species, or milfoil, at a labeled concentration range. State restrictions and fish-protection precautions apply.
Does it control hydrilla?
Yes. Hydrilla is listed on the current EPA label.
Does it control curlyleaf pondweed?
Yes. Curlyleaf pondweed is listed at 0.75–1.5 ppm on the current federal label, subject to all applicable use restrictions.
How much HYDROTHOL Granular should I use?
There is no single universal rate. The amount depends on the target species, treatment type, water volume and labeled ppm concentration. Use the label and calculate using acreage and average depth.
Why does average water depth matter?
Aquatic herbicide rates are based on water volume. Two areas with the same surface acreage can require very different quantities if one is substantially deeper than the other.
What is an acre-foot?
One acre-foot is the volume of water covering one surface acre to an average depth of one foot. For example, one acre averaging four feet deep contains approximately four acre-feet of water.
What is the rate for light to moderate algae?
For light to moderate planktonic and filamentous algae, the label gives 0.05–0.3 ppm, equivalent to approximately 3–16 lb per acre-foot. The exact rate still depends on the target and label instructions.
What rate is used for submerged weeds?
The general submerged-plant section lists 0.5–5.0 ppm, but individual species have more specific ranges. Due to fish toxicity, plant-control applications are suggested for commercial applicators as marginal or sectional treatments.
Can rates above 0.3 ppm harm fish?
Yes. The label states that fish may be killed by dosages above 0.3 ppm. Applications above 0.3 ppm are suggested for commercial applicators as marginal or sectional treatments.
Can I treat the entire lake at once?
Not automatically. Dense vegetation and algae can create oxygen depletion when they decompose. In slow-moving or quiescent water, the label specifically states that doses above 1 ppm must not treat more than 10% of the area at one time.
Why can killing too many weeds at once hurt fish?
Dead algae and aquatic plants consume dissolved oxygen while decomposing. A rapid large-scale kill can reduce oxygen enough to suffocate fish.
Can I use HYDROTHOL Granular in a Koi pond?
No. The label specifically states not to use HYDROTHOL Granular in waters containing Koi or hybrid goldfish.
Can I use it in a small decorative garden pond?
No. The label states that HYDROTHOL Granular is not intended for small-volume garden pond systems.
Can I use it in salt water?
No. Do not use HYDROTHOL Granular in brackish or salt water.
Can I swim after treatment?
The federal product information does not state a general swimming restriction for labeled use. However, New York requires no swimming until the day after application, or 24 hours. Other state or local requirements may also apply.
Can I fish after treatment?
No general fishing-use restriction is stated in the current product information when the product is used according to the label, but fish-protection application restrictions still apply.
Can treated water be used for irrigation?
The label allows irrigation use subject to its directions and restrictions, but HYDROTHOL Granular-treated water must not be used for chemigation.
Can I apply near a drinking-water intake?
For lakes, ponds and other quiescent waters, the label requires applications to be at least 600 feet from functioning potable-water intakes. Flowing-water sites have separate intake-protection directions.
Do I need a permit?
Possibly. Aquatic pesticide requirements vary by state and water body. The label specifically directs users to consult appropriate state or local water authorities before treating public waters because permits may be required.
What PPE is required?
The current label requires long sleeves and long pants, socks and shoes, waterproof gloves, protective eyewear and the specified NIOSH-approved respirator for mixers, loaders, applicators and other handlers unless a specific label exception applies.
Can I broadcast the granules by hand?
Granules must be applied evenly and according to the label while the handler wears the required PPE. Small treatment zones may be handled with an appropriate scoop or granular spreader, while larger areas may require more suitable application equipment.
Can I use the Extreme Blower spreader with HYDROTHOL Granular?
The Extreme Blower attachment is designed for compatible granular aquatic treatments. Using a spreader does not change the HYDROTHOL Granular label rate, PPE requirements, or application restrictions.
What package sizes does Weeders Digest sell?
The current product page offers 20 lb and 40 lb size options.
Can this product ship to Alaska or Canada?
No. The current Weeders Digest listing restricts shipment to the continental United States. State registration and shipping limitations may also apply.
Where can I get help calculating the proper treatment amount?
Contact Weeders Digest Product Support at 877-224-4899 with the treatment-area dimensions, average water depth, target plant or algae, and state where the waterbody is located.
Videos
HYDROTHOL Granular Videos
Hydrothol Granular Herbicide for Lakes & Ponds
HYDROTHOL Label & Safety Information
9 Reviews
-
Hydrothol
This product is amazing. We have a big event coming up and I had a huge weed spike. Hydroyhol crushed it! The pond looks amazing.
-
Kills!!!
This stuff is great it kills the weed right to the bottom of the lake. You just have to be patient it take a little time and you need a lot of it, it does get costly but if you want the weeds gone this is what it takes.
-
Great product
This product killed off all of my underwater weeds in about 10 days!
-
Hydrothol grandular
Product works as advertised
-
Weed control
Works well for for my needs. Best results of the different products I've tried
Related Products
Renovate OTF Granular Aquatic Herbicide - Triclopyr 40 Lbs.
Aquathol Super K Granular Herbicide
Green Clean Pro Granular Algaecide, 50 lbs
