Bill Haller, Lyn Gettys, Margaret Glen
University of Florida/IFAS Center for Aquatic and Invasive Plants – Gainesville, FL
Syngenta Professional Products – Greensboro, NC
reprinted from Aquatics Magazine, Winter 2007 / Vol. 20, No. 4
Back in the 70s (that’s 1970s for the younger reader), when Elvis and disco were king and before iPods, PCs, HDTV, cell phones and whole-lake treatments with slow-acting enzyme-inhibiting herbicides like fluridone, most aquatic herbicide applicators were using weighted trailing hoses to treat the bottom acre-foot to control hydrilla and other submersed plants.
Say what?!? Yes, bottom acre-foot and weighted hoses! Read on…
Let’s say you had early-season hydrilla that was 3 to 4 feet tall and growing in water 7 to 10 feet deep. Back in the day, you could inject contact herbicides such as diquat, copper and endothall through hoses so that the herbicide was delivered into the plant bed at the bottom of the water column, resulting in less herbicide applied and less money spent (Figure 1).
The term “bottom acre-foot” may not be entirely correct, but that was what the method was commonly called. Bill McClintock (Director of Winter Park’s Aquatic Weed Control program at the time) had a pontoon boat loaded with a 500-gallon sprayer equipped with four 20-foot trailing weighted hoses spaced about 10 feet apart. Bill would tank-mix 500 gallons of water with the appropriate herbicide and treat 5 acres along the shorelines with an output of 100 gallons per acre (GPA) in the 30-foot wide swath, being careful to miss docks and diving boards. This type of system was not unique to Winter Park – everyone used some variant of this setup to increase efficiency and reduce costs (after all, diquat was $26/gallon, endothall was $20/gallon and copper sulfate was 15¢/pound).
The main goal of the bottom acre-foot treatment was to place the herbicide where the weeds were growing – in other words, why treat the upper half of the water column when the weeds were in the lower half? This philosophy explains why granular versions of several products have been developed – to facilitate the placement of herbicide directly in target weedbeds. The use of weighted hoses also allows contact herbicides to be placed below the thermocline, the area that separates the warm upper and cool lower “layers” of the water (ever stood in a pond and noticed that the water around your feet was much cooler than the water near the surface?). Temperatures above the thermocline can get downright toasty, especially on hot, still, summer days, and herbicides applied at or just below the surface of the water don’t mix with the cooler water below the thermocline. Surface-applied herbicides only come into contact with the upper 1 to 2 feet of topped-out hydrilla and don’t reach the lower portions of weeds, so regrowth occurs quickly. The use of weighted hoses allows applicators to ensure that contact herbicides are delivered to the lower portions of weeds below the thermocline; this system may be slightly more time-consuming than regular surface application, but provides much better, longer-term control of weeds.
If weeds are distributed throughout the water column (say, 8 foot tall weeds in 10 feet of water), you should apply most contact herbicides (such as diquat at 370 ppb or endothall at 3 ppm) uniformly throughout the water column. We frequently use weighted hoses for uniform application of herbicides, dyes and even alum treatments to the entire water column of experimental plots (see Figure 2). We typically treat ponds that are 8 to 10 feet deep; our setup uses three hoses (one each of 4, 8 and 12 feet in length) attached to a single boom that is the same width as the boat (8 feet) so we can avoid damage to diving boards and docks.
We use a standard Hypro pump with a 100-gallon tank set to pump about 5 gallons per minute (GPM) . The inlet to the 1-inch galvanized manifold (or boom) is shown in Figure 3. We use 1-inch galvanized because it is much stronger than smaller diameter pipes; after all, when a trailing hose snags on a stump or you turn too tightly and twist the hose around 2 tons of hydrilla, you want the manifold to remain intact (our motto: “Sink the boat, save the boom!”). The 1-inch boom is reduced to ½ inch at each hose (Figure 4).
The hydraulic hose connector is a key component and must be used – note there is no hose clamp or metal connection where the hose attaches to the brass connector (the yellow band is simply a washer). The hose and brass connector (sold as Parker Push Lok 250 psi, 5/8”) is designed so that the hose locks into the connector without any clamps; once locked, the hose will not come off. All hydraulic shops should have this or a similar product in stock. The shop where we purchased our materials also inserted the connectors for us.
The spring (hose protector coil) in Figure 4 prevents the hose from rubbing on the bow. The loose lower end of the hose is connected to a lead-filled weighted ¾” pipe as shown in Figure 5. Make sure there are no ridges or hose clamps in this area to avoid snagging the hose on weeds. We also ground the edges from the brass connector, the female connector and the enlarging connector to prevent hydrilla and other weeds from getting caught on the weighted hose. Check the edges on the left side of the connectors in Figure 5; that’s the direction the hose pulls the weighted pipe through the weeds. You may still collect some weeds, but a quick yank on the trailing hose usually frees them from the discharge end.
The weighted end of the hose is shown in Figure 6. The ¾” pipe is 2 feet long and filled with molten lead. Molten lead is dangerous stuff to handle and we are not very brave, so we had our pipes filled with molten lead at a plumbing shop; it cost $50 to fill three pipes with lead, which seems like a good investment. We have found that a 2-foot-long, ¾” pipe is about right for our treatments based on boat speed and the weeds we treat; ½” pipe was not heavy enough and 3-foot-long pieces of pipe got unwieldy. A lower cap may not be necessary if the lead remains in the pipe; if you do need a cap, be sure that the edges are ground off to prevent snagging weeds. The nozzle ain’t fancy, but it works just fine. The ground edges of the pipe fittings and the nozzle (hole) where the herbicide is discharged into the water are visible in Figure 7. Herbicide would be discharged into sediment or mud if the hole was on the bottom of the lead-filled pipe, so remember that the goal of this setup is to place the herbicide in the weeds or water column instead of on the bottom of the pond. If the pipe were vertical, the herbicide would discharge 2 feet from the bottom; however, when the pipe is dragging, it is not vertical and the herbicide usually discharges around 12 inches from the bottom (Perfect! The bottom acre-foot?!?).
It’s likely that the engineers among the aquatic applicators will find ways to improve on this design. A few cautions to keep in mind… first, you can’t drag hoses when the boat is on a plane! Second, you must go slowly to keep the hoses on the bottom of the pond (this is probably why Winter Park had a 30’ boom – to increase the acres treated per hour). Finally, avoid tight corners or the long hoses will collect lots of weeds. A good way to reduce the chances of this happening is to pull on the hoses every couple of minutes to shake loose any clinging weeds. Experience will be your best guide when working with weighted hoses – once you get the hang of it, you’ll appreciate how effective this system is!
So now you know what a bottom acre-foot treatment is and how to build a high-tech boom and weighted hose system. In fact, it’s so high-tech that if you apply copper, the money you save using this method will leave you with enough cash to build a new system each year (probably a good thing, since younger readers may not know that copper sulfate eats galvanized pipe!). We guarantee you’ll get more effective weed control and use less chemical if you switch to this system – don’t forget, this was proven way back in the 70s! For a bit of history, read the article “Development of the Bottom Placement Technique for Hydrilla and Eelgrass Control”. This publication is available through the Journal of Aquatic Plant Management Online, on the Aquatic Plant Management Society website. Now that’s old, but interesting, and yes, diquat was $25.85/gallon. Ah yes, back in the day… when we only had contact herbicides for submersed weed control.
*Note: If your pump output is 10 to 15 GPM, you will need a much larger hole than the one shown in Figure 7.
Line illustrations by Lyn Gettys, colored by Josh Huey. Photos by Greg Reynolds.
Figure 1. Bottom acre-foot treatment, with trailing weighted hoses. This system delivers the herbicide directly into the weedbed for maximum efficiency and lower cost.
Figure 2. Whole water column treatment, with trailing weighted hoses of various lengths. While this figure shows herbicide discharging from the ends of the hoses, it is actually discharged approximately 18 inches above the hose ends (see text).
Figure 3. Inlet setup from the spray tank to the boom.
Figure 4. One-inch galvanized boom reduced to ½” for attachment of the 5/8” trailing hoses. Trailing hoses can be any length and we use two sets for our 3-hose setup. We use three 12 foot hoses to cover the bottom acre-foot and one each of 4, 8 and 12 foot hoses to treat the total water column. If you typically treat water that is 12 to 16 feet deep, you should use hoses that are 20 to 24 feet in length.
Figure 5. Trailing hose attached to the ¾” drilled nozzle and followed by 2 feet of ¾” lead-filled pipe.
Figure 6. Lead-filled pipe at the end of the trailing hose. The depth of the lead-filled pipe will depend on the water depth and hose length and is greatly influenced by speed and weed density. Bill McClintock used a pontoon boat with a 30 foot swath (spray) width because he could treat almost 4 acres in an hour if he traveled at 1 mph (8 feet wide x 1 mile = 1 acre so 30 feet wide x 1 mile = 3.75 acres).
Figure 7. The nozzle for herbicide discharge can be drilled to any appropriate size. Note that the leading edges of the fitting are ground down to minimize snagging on submersed weeds. See note at end of article.*