Large mature rhubarb crowns should be divided every five years according to the books, this will give you several new plants for your own allotment and probably some spare to share with other plot holders. Rather than put a time frame on it, I grab for my spade when I notice weak growth ie leaves becoming noticeably smaller and stems becoming thinner. It is best to divide a rhubarb crown when it is dormant, making this a perfect job to carry out during the winter months but avoid if the ground is frozen.
Use a sharp spade to slice down into the crown to create several smaller sections, each with a portion of root and a few strong buds. Discard the woody sections from the centre of the old crown as these will be less vigorous, take sections from the outer part instead.
Replant the new sections straight away, slightly proud of the soil to avoid rotting and water well. Mulch with well-rotted manure or fresh compost. Water frequently especially during dry weather and do not pull any stems for at least a year to allow the plant to concentrate on producing a strong root system.
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