The first year, the plant will emerge as a seedling and grow to form a rosette, producing only leaves and a large whitish tap root. In the second year reserves from the tap root will produce a stem that will emerge, flower and set seed and in the fall, die and disperse the seed. Wild carrot disseminates only by seed so anytime you can prevent the plants from going to seed you will reduce future infestations. Seeds generally germinate within 2 years of dispersal, however, they can survive for up to 7 years in the soil.
To germinate, the seeds require large amounts of water and therefore, usually germinate in early spring but can germinate later in the late summer to early fall if moisture conditions are favorable. Plants can flower as early as the end of June and continue to flower until the first frost meaning seed maturity and dispersal can occur over a long period of time. Primary control methods are aimed at preventing the plant from going to seed.
There are three stages of growth where wild carrot can be controlled:
- early season pre-emergence or post-emergence,
- established plants with fall herbicide applications and
- prevention of seedling emergence. The overwintered and established plants are usually the most difficult to control.
Several pre-emergent herbicides can prevent the initial germination of seed - simazine, diuron, terbacil and norflurazon will prevent seed germination if applied early enough in the growing season.
If the rosettes are already present then early season burndown with glyphosate with ammonium sulfate plus a non-ionic surfactant (1.0qt/A +17 lb. AMS/100 gal. + ½% NIS). The best control with this combination will occur if the application is made during the first warm period in the spring following an initial greening up. This same treatment applied in late September to early October should also be made to catch any newly germinated rosettes. Light frosts that do not cause any visible injury will not reduce effectiveness and should be applied when daytime high temperature is at least 60 degrees F.