Forcing Rhubarb (Rheum rhaponticum)
Rhubarb (Rheum rhaponticum)
Early forced rhubarb is a commercial crop of some significance. It is forced in dark heated sheds at 10-13°C (50-55°F), usually fan ventilated to avoid the risk of fungal and bacterial diseases. On an amateur or professional scale it is first of all necessary to build up a stock of two-to three-year-old rhubarb crowns.
Commercial practice is to divide established crowns, this being quicker and more dependable than. Each section must bear one or more terminal buds, these being planted in well-cultivated with a pH of 5.5-6.0, phosphate and potash index 2, on a 90cml.2m (3-4ft) square-planting system. Large areas are treated with a simazine based weedkiller at 2.2kg/ha (2lb active ingredient per acre). Vigorous growth is encouraged by dressing with nitrogen at 251 —376kg/ha (2 —3cwt per acre) 34-40g/m2 (1 – 1-1/2oz per sq yd).
Preparation for forcing
Forcing crowns are those which have been established for at least two years. They are ploughed, dug or forked out of the ground and allowed to become frosted. A points system is used to determine the optimum time for housing the various varieties.
To install the necessary soil thermometer make a vertical hole 10cm (4 in) deep with a cane the same diameter as the bulb. Insert the thermometer so that the bulb is in direct contact with the soil, with the graduated part of the stem lying horizontally along the surface. Fill the hole gently with sifted soil, disturbing the soil as little as possible. Leave the thermometer in position undisturbed until the end of the season and do not cover with a box for protection. The temperature should be read every day at 9am (10am before British Summer Time ends). Any readings above 9°C (49°F) are ignored. All readings below are subtracted from this base of 9°C (49°F) and the daily differences added up: these are the ‘cold units’.
The lifted crowns are placed in forcing sheds or in boxes under the greenhouse staging, packed tightly together with soil or peat to fill up the spaces and cover the crowns. If given warmth and moisture and complete darkness, blanched shoots will be formed according to the temperature level. After forcing the crowns are discarded. Planting in large whalehide pots and lifting into a warm, dark place for forcing is now practised.