Oilseed rape in flowering broccoli

The wild cabbage Brassica oleracea is the origin of most of our brassica crops such as broccoli, cauliflower, cabbage, swedes and sprouts. The wild form is perennial, living and flowering year after year on sea cliffs and coastal scree [1].

Many of the cultivated forms, growing in the rich soil of fields, also have this tendency to last more than one season. If they get the chance they will flower and if neglected some of them will recover after the winter and grow again.

This desire for  life among such short-season plants, grown for their leaves or compact floral heads, can sometimes be seen in late summer and early autumn, after a brassica crop has been harvested and before it is destroyed and the soil ploughed. The cabbages are not the only ones to flower illicitly – weeds of a similar type will also take the chance.

lf_5_plnt_crpwd_brocclsrmll17_gs_1100The photographs above show a field of broccoli in east Perthshire, flowering in mid-September after most heads had been harvested. The cultivated plants have the same pale yellow flower colour as their wild relative. In this field, some rows took to flowering more than others, perhaps indicating the planting of different varieties.

Among the pale yellow appear plants of a taller, rangier stature and with flowers of a much stronger yellow (examples circled in the top image). These are ‘volunteer’ oilseed rape [2], surviving as seeds buried in the soil from a previous crop and emerging and growing  when conditions allow.  Oilseed rape Brassica napus is half-cabbage, originated from a combination of the genomes of Brassica oleracea and Brassica rapa, the turnip.

These volunteers are hard to control in crops of cabbage and turnip. Any seedlings emerging in a corn crop are likely to die, but in cabbage or turnip and sometimes in potato, they can flower and re-seed. Volunteers of oilseed rape and also the main cereals, are now all common in the buried seedbank.


The photographs above show, top, buds and open flowers of broccoli. The flowering branches usually arise as a side head after the main, central broccoli head has been harvested. Each floral sub-branch of the side head extends quickly resulting in the floral ‘bunches’ shown lower left. The large grey-green leaves are of the broccoli; the leaves of flowering oilseed rape are a similar shade but tend to be much smaller.

Such a mass of flowers within the cultivated parts of fields has become rare at this time of year. Flowering broccoli offers a few weeks of food for insects and other small life forms that make up the cropland food web. Bumble bees were foraging (lower right), as they have been doing among the flowering cabbages in the Living Field garden.

The remains of this vegetable crop will soon be dispensed with and the soil ploughed. Seeds on some of the oilseed rape might mature and drop to the soil and join the buried, living population of volunteer weeds. They will germinate, emerge and re-seed at the next opportunity, maybe seven or more years hence, depending on what is grown in between.

Waste and plenty ….. for the past few weeks, this field has held a mass of edible food in the form of broccoli side shoots, compact enough to cut easily before they elongated to flower. Though edible and nutritious, such small broccoli heads are probably not saleable to the buyers that want standard uniformity to a schedule. Yet in this instance, the wasted food quickly became useful to the late summer food web.

Sources, links

[1] The 5000 Years page on Crop-weeds gives background to wild cabbage and its relatives that form the Brassica complex in studies of weediness, geneflow and persistence.

[2] Seedbank studies at the Institute stretch back to the 1980s, started by Harry Lawson and Gladys Wright. At that time, few oilseed rape were found as buried weed seeds in cropped fields. In the last 15 years, they have risen to become among the most common four or five broadleaf (i.e. not grass) seedbank species.

Other pages and posts on Brassica crops and weeds on this site:

Growing cabbages and their relatives in the Living Field Garden: Vegetables and The Garden’s vegetable bounty.

Brassica root sections: The Beauty of Roots exhibition, Sectioned.

Swede and turnip in Burns’ time: SoScotchBonnet.

Contact for this page: geoff.squire@hutton.ac.uk