Wild brassicas (rape, wild cabbage, wild turnip)

Scientific name: Brassica napus, Brassica oleracea, Brassica rapa subsp. sylvestris

Rape flowers

Rape plant buds

Rape seed pod

Rape seedling

Wild cabbage

Wild turnip leaf

Wild turnip young plant

Wild turnip infestation

Wild turnip plants

Wild turnip plant
  • Key characteristics
  • Biology
  • Impacts
  • Control
  • Further information

  • All three species usually have large, showy yellow flowers (sometimes white in wild cabbage) and the tall flowering stems rise in late spring from lax winter rosettes. The species can be distinguished from each other by the relative positions of buds and open flowers in the flower head. The stems of all three species are hairless and can become quite woody towards the base
  • Brassica napus (rape, oil-seed rape and swede) has blue-green leaves and its flowers are slightly below the level of the unopened buds. The petals are bright to pale yellow, 10-12 mm long
  • Brassica oleracea (wild cabbage) has large, pale yellow or white flowers with petals 15-25 mm long, and the open flowers are well below the level of the buds
  • Brassica rapa sylvestris (wild turnip) has its open flowers higher than the unopened buds. It is always an annual, flowering and dying within one year. Its petals are bright yellow and are 6-10 mm long
  • These three species are the only species in the genus Brassica with amplexicaul leaves (upper stem leaves clasping the stem at their base).

Similar weeds

  • Other species of Brassica also have yellow flowers but the individual flowers are mostly smaller and the plants much less common. These include Brassica fruticulosa (found in Palmerston North, Raumati and Wellington), Brassica juncea (Indian mustard, found in Northland), Brassica nigra (black mustard, a rare casual weed), Brassica tournefortii (Mediterranean mustard, an occasional weed of coastal areas and rail yards)
  • Wild radish (Raphanus raphanistrum raphanistrum) has large petals which can be yellow, lilac or white in colour. It can be distinguished by its bristly leaves
  • Two winter cresses, Barbarea intermedia and Barbarea verna, are generally similar with bright yellow flowers but the plants are smaller with shiny green leaves: individual flowers are only about 6 mm in diameter and the flower stems are only about 60 cm tall
  • Creeping yellow cress (Rorippa sylvestris) is again similar in appearance but the plants are much smaller, less vigorous and are spreading perennials with a rhizomatous root system.

Origin

  • All three species are native to Europe and were probably sown by Captains Cook and Furneaux during their visits in 1793. Wild cabbage in particular thrived and spread to all New Zealand coastal areas, probably with help from the Maori
  • The three species have all been cultivated as crops in New Zealand, and are still grown: Brassica napus as rape, oil-seed rape and swede, Brassica oleracea as cabbage, cauliflower, broccoli, Brussels sprouts, and kale, Brassica rapa as turnip
  • There are several recognised subspecies of Brassica napus and Brassica oleracea.

Life cycle and habitat

  • All these species of Brassica have similar flowers and leaves: Turnip (Brassica rapa) is only ever an annual; Brassica oleracea (wild cabbage) is a biennial or perennial; Brassica napus (rape) is an annual or biennial. These are the only species of Brassica with upper stem leaves that clasp the stems: the other species all have upper stem leaves that have stalks or attach directly to the stem
  • These three species produce lax-growing rosettes over winter. Their flower stems grow quickly in spring and become obvious by September or October (sometimes earlier). Flowering can continue until December or sometimes even later.

Benefits

  • When grown as crops all three species are very valuable as fodder crops for animals or as vegetable crops for people (see ‘Biology’). Brassica napus (rape) is commonly used as a fodder crop for livestock; Brassica oleracea is grown for animal (and human) food as kale, and for human consumption as cabbage, cauliflower, broccoli and Brussels sprouts. The cultivated turnip is classified as a different subspecies of Brassica rapa, subspecies rapa.

Occurrence

  • Brassica napus is locally naturalised along coasts and as an escape from cultivation throughout New Zealand
  • Brassica oleracea is also found throughout New Zealand, often on coastal areas and as an escape from cultivation
  • Brassica rapa is more common than the other species and is a persistent weed of pastures, arable crops, roadsides, waste places and gardens, again throughout the country.

Impact on pasture

  • These species are regularly grazed and some are in fact commonly sown as fodder crops, but all can sometimes be toxic to stock, which is why they can be a problem in pasture. They can also appear unsightly because of the tall-growing yellow flowers.

Impact on crops

  • All three species sometimes occur in crops, where they may reduce crop yields and possibly contaminate the product. Wild turnip is probably the most problematic of these species because it can go to seed early and then becomes much less productive, and it can also contaminate the seeds produced by the crop
  • All three species can be seriously problematic to the seed industry, in particular on farms growing brassica seed as the seeds from the weedy species cannot be dressed out.

Impact on livestock

  • Rape (Brassica napus) can cause rape scald, where lambs fed on the crop become photosensitised, developing oedema on the ears and head and sometimes along the midline of the back
  • Rape can also accumulate large quantities of nitrate and then becomes poisonous to lambs and ewes
  • Swedes (again Brassica napus) can cause redwater in cattle, as well as photosensitisation. Swedes too can accumulate nitrates and thus become toxic.

Grazing management

  • Young plants rarely survive a hard grazing with break-feeding being a good management option in autumn and early winter
  • Another option is to shut up an infested paddock for silage or hay as the mowing associated with these usually kills Brassica weeds.

Chemical control

  • In pastures, wild brassicas are easily controlled when young by using common herbicides such as MCPA or 2,4-D. The main key to successful control is to spray the plants while they are still young
  • Controlling wild turnip selectively in rape crops is difficult or even impossible, and one solution is to avoid using paddocks with a high wild turnip seed load for rape crops; here crop rotation becomes an important management tool.

References

Connor, H. 1992. The poisonous plants in New Zealand. DSIR Bulletin 99, GP Publications Ltd., Wellington, New Zealand. 247 p.

Harrington KC, 2015. Wild turnip, Brassica rapa ssp. sylvestris. Weeds database, Massey University (accessed 30 March 2016).

Popay A, Champion P, James T 2010. An illustrated guide to common weeds of New Zealand. New Zealand Plant Protection Society, Christchurch, New Zealand. 416 p.

Webb CJ, Sykes WR, Garmock-Jones PT 1988. Flora of New Zealand, Vol. IV: Naturalised pteridophytes, gymnosperms, dicotyledons. Botany Division, DSIR, Christchurch, New Zealand. 1365 p.