Beta vulgaris L.
Weed beet occurs as a weed in sugar beet and a range of other crops. It is also found on wasteland and on the verges of newly constructed roads. It can originate from wild beet, from hybrids between wild and cultivated beet, and from bolters in open pollinated cultivars, and their resulting volunteer seedlings. Sugar beet was first grown for sugar around 75 years ago and this biennial plant has always had a proportion of plants that flower prematurely in the first year. Bolting is partly inherent and partly under climate control. Early sowing can expose seedlings to cooler conditions that promote bolting and roots left in the ground after harvest can also flower and set seed in following crops. The resulting seed may produce seedlings with the potential to become annual rather than biennial plants. Weed beet can therefore evolve from seed of any beet left to flower in the field.
Weed beet was first seen as a problem in the 1970s.
Bolters become visible in late May and begin to flower in July. The average date of flowering is the 12th July. Individual flowers on the inflorescence open over a period of 3-4 weeks. The flowers produce large amounts of wind blown pollen and are more self-fertile than normal sugar beet. Beet is visited by pollen bearing insects that may also contribute to pollination. There is the potential for pollen from wild beet on seashores and from weed beet to be carried onto sugar beet being grown as a seed crop. Diploid wild beet tends to release its pollen earlier in the day than modern triploid cultivars so stray pollen can lead to the contamination of a seed line as has happened in the past. The annual character is dominant.
Seeds start to become viable in mid-August, 28 days after the start of flowering. The flowers on cut down stems or pulled plants can produce a small number of viable seeds. Intact bolters left to grow to maturity produce an average of 1,000 to 1,919 seeds per plant. Weed beet begins to shed seed in September.
Persistence and Spread
There is no evidence of dormancy in beet seed but weed beet seeds appear able to remain viable in soil for at least 7 years. Ploughing to depth may prolong the life of the seed. In burial experiments, sugar beet seed gave up to 35% germination after 6 years.
To limit weed beet infestations grow sugar beet on clean land, use bolt resistant cultivars, sow the crop after mid-March and destroy bolters in July-August to prevent fresh seed entering the soil seedbank. Leave shed seeds on the soil surface to germinate, die or suffer predation. Shallow cultivation will stimulate seed germination.
It is difficult to distinguish weed beet seedlings from sugar beet seedlings unless they are out of line with the drilled crop row. Annual bolters tend to have seedlings with red petioles but so does a proportion of cultivated beet. Tractor hoeing of the inter-rows will remove 70% of the weed beet.
The tall flower spikes of the bolters allow height selective control. Hand pulling is very effective but not practical. Good control is achieved by cutting down bolters 3 times at 2-week intervals, starting 14-28 days after flowering. Two cuts will give reasonable control but a single cut does not give a reliable result. Cutting 42 days after flowering is not satisfactory as viable seed is already formed. Rotary cutters and toppers have been developed that remove the flower heads of bolted beet and other tall weeds in the sugar beet crop.
Weed beet is less of a problem in competitive crops like cereals. Increasing plant density reduces flowering and seed production by weed beet. In crops such as peas that are harvested relatively early, weed beet has insufficient time to produce viable seed.
Updated November 2007.