T.E.R:R.A.I.N - Taranaki Educational Resource: Research, Analysis and Information Network

Brassica rapa ssp sylvestris (Wild Turnip)

Kingdom: Plantae
(Unranked): Angiosperms
(Unranked): Eudicots
(Unranked): Rosids 
Order: Brassicales 
Family: Brassicaceae 
Genus: Brassica 
Species: B rapa ssp sylvestris
Scientific name: Brassica rapa ssp sylvestris
Common name: Wild turnip

Brassica rapa ssp sylvestris is a tall, competitive annual weed. It is is a persistent weed found all over New Zealand, but it is particularly troublesome in Canterbury, Otago and Southland. It is capable of causing large reductions in crop yields due to competition nutrients, light and water. Seeds can germinate all the year round. Since it is a member of the Brassicaceae family - which gives us many of our cultivated vegetables and condiments such as broccoli, cabbage, kale, bok choi, radishes, turnips, cress, horseradish, wasabi, mustard it is very difficult to control selectively in such crops as it is resistant to the same selective herbicides as the crop.

This family of plants are recognised by their four-petaled flowers with six stamens. They all have flower heads that look like little broccoli heads when budding. 
The vegetative stage of Brassica rapa ssp sylvestris is a rosette at ground level and leaves radiate out from this growing point. Leaves on the rosette are covered with bristly hairs. The leaves present near the top of this flower stem are quite smooth, lacking the bristly hairs of rosette leaves.

When it flowers, a tall upright flower stem forms. The open, bright yellow flowers are 6-10 mm long and are at the ends of the branched stems and are taller than the plant. The flowers later form pods. The formation of pods is typical of plants in the Brassicaceae family.
The root system of Brassica rapa ssp sylvestris swells with age, forming a storage organ similar to the crop species turnip, but much smaller.

A juvenile plant.

A leaf hight on the stem

The lower leaf

Thanks to Wikipedia for text and information: https://creativecommons.org/licenses/by-sa/3.0/