Dogfennel is a native perennial plant found across much of the southeastern United States. It is an aggressive weed species that can invade under-improved, unimproved, or overgrazed pasture. Infestation with dogfennel can lead to a decline in overall forage yield and quality. Cattle tend to bypass dogfennel when grazing, but have been known to consume the plant if more suitable forage is not available. This can be problematic as dogfennel contains low levels of tremitol, which can cause dehydration in cattle.
Most dogfennel growth occurs from rosettes that have overwintered low to the ground during the cool season. However, once the soil temperature reached a constant 65°F, seeds will sprout as well. Dogfennel tend to grow in a single shoot without any large branching up to eight feet in height. Dormancy typically occurs from January to March. Bolting of the plant occurs from April to June, with flowering in September and October. Seed dispersal occurs during November and December. Dogfennel plants are also able to spread via underground rootstocks that branch from the main taproot of the plant. This results in plant colony formation in the pasture.
Dogfennel Control with Herbicides
Unlike many other plants, there is no month or time of year that is specifically recommended for the use of herbicides on dogfennel. The recommendations for herbicide treatment are based on the height of the plant. In general, smaller plants are easier to control than larger plants. This holds true with dogfennel as well. Plants that are under 20 inches in height are controlled with 2,4-D amine itself or with herbicides that contain 2,4-D amine, such as WeedMaster). As the plant height increases over 20 inches, the amount of control offered by 2,4-D amine decreases. Plants that are between 20 and 36 inches in height will typically see a control rate of 80 to 85%. However, once the plants are above 36 inches in height, control with this herbicide decreases dramatically.
Large dogfennel plants (those over 36 inches in height), require a different approach for control as compared to smaller plants. When a pasture is infested with large dogfennel plants, it is also important to consider both the type of grass (forage) in the pasture along with other weeds that are present. If the large dogfennel plants are the primary weed, the recommendation is to use Pasturegard HL at 1.5 pints per acre.
If there are additional weed species present that need to be controlled, such as tropical soda apple, it is recommended to use a combination of GrazonNext HL (at 1.5 pints per acre) along with one of the following: 1) Pasturegard HL at 0.5 pints per acre, 2) 2,4-D amine at 3 pints per acre, or 3) 2,4-D amine + dicamba at 2 pints per acre. The combination of these chemicals will provide control with low injury to your forage crop. There is an exception to this rule: if you are growing limpograss for forage, do not apply any product containing 2,4-D between May 1 and November 1, as this will result in severe injury to the limpograss.
While the height of the dogfennel plant is very important to consider when choosing an herbicide option, there is another consideration that must be taken into account. That consideration is drought condition. Early spring tends to be one of the driest times of year in North Florida, but it is also the time of year when weeds are small and tend to be controlled easiest. If it is dry enough that plants are wilting during the day, then application of herbicide is probably not going to be effective against dogfennel.
For additional information on the biology and control of dogfennel, please visit the UF/IFAS Extension EDIS database at http://edis.ifas.ufl.edu/ag233. The landing page for the EDIS site can be found at http://edis.ifas.ufl.edu/.
Another weed that is commonly found in pastures containing dogfennel, is Flat-top Goldenrod. An article describing the biology and control of this weed can be found at http://edis.ifas.ufl.edu/ag361.