Cultural methods can effectively control this weed. Dig deeply to remove roots and hand weed or hoe off any seedlings as they appear. Note that the tap root can develop in even quite young plants.
This is a deep-rooted perennial weed and it should be possible to eliminate the problem with repeat applications of a glyphosate-based weedkiller (e.g. Roundup).
Glyphosate is a non-selective, systemic weedkiller applied to the foliage. It is inactivated on contact with the soil, so there is no risk of damage to the roots of nearby ornamentals.
As glyphosate is not selective in its action, it is essential to avoid spray or spray drift coming into contact with garden plants. If treating weeds in the immediate vicinity of garden plants, apply carefully using a ready-to-use spray formulation in cool, calm weather. Branches or shoots can be held back, using canes, or by covering or screening while spraying, but make sure that the weed foliage has dried before releasing branches or removing the covering
Glyphosate is most effective when weed growth is vigorous. This usually occurs at flowering stage, but before die-back begins.
This particular weed may take more than one application to eliminate; if more than one treatment is necessary wait until the re-growing weeds have sufficient leaf area to take up the chemical.
Inclusion of a weedkiller product does not indicate a recommendation or endorsement by the RHS. It is a list of products currently available to the home gardener.
Weedkillers for gardeners (Adobe Acrobat pdf document outlining weedkillers available to gardeners; see section 4)
Chemicals: using spot and broad-scale weedkillers
Chemicals: using a sprayer
Chemicals: using safely and effectively
Weeds: non-chemical control