Dandelion bloom is right around the corner. Mid-Sept through late October is the best time to control dandelions and other perennial broadleaf weeds for a number of reasons including:
Most effective control since the herbicide will translocate to the roots with the photosynthate as the weeds prepare for winter.
Few or no ornamentals or garden plants are actively growing and susceptible to off-target damage.
Holes left by the dying plants will fill-in yet in the fall or early next spring before crabgrass fills in the holes.
However, some areas must be treated in the spring. New lawn care accounts, dry weather last fall delaying or cancelling fall herbicides applications, and/or fall seedings that could not be treated last fall are a few examples of areas that will likely require treatment this spring.
Our 3-year study from the late 80’s shows clearly that spring applications of broadleaf herbicides can be inconsistent if applied too early in the spring. Amine formulations do not penetrate well and provide inconsistent control until air temperatures reach 70F or so or 150 GDD50 (Growing Degree Days base 50F). Ester formulations penetrate the leaf better and provide more consistent control at cooler air temperatures.
Therefore, choose the more expensive ester formulations early in the spring whereas amine formulations can be used later. One serious precaution is to reduce the potential off-site movement of these herbicides, either through spray drift on windy days or volatilization on warm days shortly after the application. Many trees are just leafing out at this time of the year and are extremely susceptible to herbicide drift. Newly planted vegetables are also extremely susceptible as are most ornamentals. The latest concern is off-site drift to grapes growing in vineyards, and these also can be potentially damaged from lawn applications hundreds of yards away depending on wind and temperature. To minimize drift, spray on relatively calm and cool days (few and far between in NE), choose low volatile formulations, and stay a safe distance away from susceptible plants.
Another option for spring dandelion control is a new product from Dow called Defendor (florasulam). This product is packaged with Dimension and this is applied at the same time as the preemergence herbicide for crabgrass. This timing should be at least two weeks prior to dandelion flowering and will not only control the blooms, it will also provide long term control The benefit of this product over typical spring-applied phenoxy herbicides is that it provides much more consistent control while minimizes chances for damage to non-target plants.
Dandelion control options in reseeded or overseeded lawns are limited and in most cases, it is better to wait until seedlings are fully established before attempting control. FMC’s QuickSilver (carfentrazone) has the most flexible label and can be applied any time before or after seeding, but it is a contact herbicide and may not provide long term control without multiple applications. FMC’s SquareOne (quinclorac+carfentrazone) can be applied 7 days after emergence DAE, whereas Tenacity can be applied to Kentucky bluegrass or tall fescue at 28. Most three-way
phenoxy herbicides (Trimec, etc.) cannot be applied until after the second mowing of the seedlings.-Zac Reicher, Professor, Turfgrass Science, University of Nebraska-Lincoln, zreicher2@unl.edu