Hedge Mustard (Sisymbrium officinale)
Hedge mustard is a sneaky weed. Initially it grows as a flattish rosette, and may, to the uninitiated, look like a dandelion. But the indentations (or teeth) in the leaves go right to the stem with a slight gap between each, and the stems themselves turn purple from the centre outwards.
Even at the rosette stage it will have produced a strong tap root which can be 10cm or more long.
The rosette is but a brief phase and the plant quickly produces a wiry stem which then grows to a cluster of branchlets. This stem can be producing tiny yellow flowers, and then small seed pods which grow parallel to the stems, when no more than 20cm high, although the branched stem can grow to 1 metre or more in height and the adult plant be half a metre or so wide.
Once the flowering stem grows the plant seems to dispense with all but vestigial leaves and becomes instead a cluster of purple/black wiry stems tipped with yellow flowers. In this form it is often called ‘wireweed’ although true wireweed is another plant entirely.
Stock avoid it, so an adult plant can cause quite large patches to not be grazed, resulting in clusters of juvenile plants germinating around each one, mainly in spring or autumn. It is often found growing in untended areas such as the sides of stock races.
Hedge mustard is a member of the brassica family, and is therefore difficult to eradicate when growing among other brassicas, such as turnips or cabbages, as it is resistant to the same chemicals as they are.
In newly sown pasture with hedge mustard at the seedling stage, herbicides such as Select or Preside will work without suppressing clover. In established pasture broadcast herbicides such as MCPA will work (best at seedling stage also), although clover will be temporarily suppressed.
For small blocks where an infestation is fairly localised, hand pulling can be very effective, although it is best to carry a rubbish bag and load plants as they are pulled, to prevent the seeds escaping when the plant dries. Plants may grow as annuals or biennials, but the seeds remain viable for long periods and infestations may recur for several years after all plants appear to have been removed, so a keen eye should be kept for new plants at the rosette stage appearing in the sward.