Contact II is perfect for this application. It can be used both in ponds and on land and when mixed at 16 ounces per gallon of water (for land use), it will give a non-selective kill of all vegetation it contacts. Because it is inactivated upon contact with soil, it will not spread out and kill more vegetation than you wanted to – it will only kill where you put it. Products like Contact II are sometimes called “liquid trimmers” due to their ability to provide precision results. NOTE: areas treated with Contact II should not be watered (or rained on) for 24-36 hours after application, so be careful about the weather when planning to apply Contact II.
Double 6 has a drift reducer ingredient incorporated into the formulation. When sprayed, the drift reducer helps the spray to develop into larger droplet size than formulations without a drift reducer. Because of the larger droplet size, the spray tends to go where it is aimed rather than riding on the wind and drifting where you don’t want it to go (just like when you use a garden hose with a coarse spray vs. a fine spray). The particular drift reducer used in Double 6 (called Paratac) also makes the drops have a slight increase in “tack” which helps them stay where they are delivered better than a formulation without a drift reducer.
Essentially, the difference is the solvent carrier. In Double-6 the carrier for the active ingredients is off-road fuel oil, along with small amounts of other solvents needed to keep the mixture stable. It is illegal to use fuel oil for a carrier (due to VOC regulations) in several states, so an alternative mix of solvents that were exempt from the VOC regulations was developed to carry (essentially) the same active ingredients in N.S.H. 2025 as are found in Double-6. Practically, N.S.H. 2025 may give you a slightly faster kill on a warm sunny day but that is balanced out by Double-6 having a bit more of residual kill due to having a bit more bromacil in the formula. In general, there is not that much functional difference between the two products.
Select Weed II has stump killing instructions on its label. They read:
TO KILL STUMPS: Mix 1 pint of this product in one quart of kerosene or diesel oil. Spray all of the stump area thoroughly.
Whack is a concentrate that is diluted with water. This opens up the possibility for things to go off the rails in two areas: dilution and application. Whack has easy basic dilution directions: dilute one gallon of Whack with 10 gallons of water. So far, so good. Then the user should apply ALL of the 11 gallons of diluted product solution onto an area of 800-3100 square feet (for annual weeds that are easier to kill) or onto 400-800 square feet (for perennial weeds that are more difficult to kill). The key thing is to use Whack on the low end of the application range given on the label. For annual weeds (foxtail, ryegrass, crabgrass, ragweed) apply the entire 11 gallons to 800-1000 square feet. For perennial weeds, apply the entire 11 gallons to 400-600 square feet. Note that this is a lot more liquid than you would use with a product like Double 6 or N.S.H. 2025. The user may think that using that much product is a waste or is over-treating the area, but it’s not. The product needs that much water in the solution to get it down into the root zone where it will be most effective. Most of the time when using a water based dilutable product like Whack, problems occur in the application step rather than the dilution step.
In areas where you might have concern about overspray causing damage to desirable vegetation, such as on a narrow patch of weeds with grass around it or on a hillside next to a pond.
Summary: Being a granular product that is applied with a spreader gives you the ability to apply it with greater precision than Double 6 which is applied via spray.
The difference is that a non-selective weed killer will eliminate ALL the vegetation you apply it to, while a selective weed killer will be selective about what it kills. Selective weed killers are good for killing broadleaf weeds (like dandelions) without killing the grass areas that the weeds are growing in. To know what a broadleaf is compared with other vegetation, look at a leaf. You will see a branched out vein structure in the leaves, but in grasses the vein structure runs parallel.