Potatoes will get along faster if they are "chitted" - sprouted in a cool room indoors - before being planted out. When the shoots are an inch or two high, plant in a bed which has been enriched with compost and well-rotted manure (if you prepared the bed last autumn, all the better). Never plant potatoes where you grew them the previous year as eel worm and other pests and diseases can build up in the soil. If possible they should form part of a four-year rotation. Year one: potatoes and root crops, year two: brassicas (the cabbage family), year three: peas and beans, year four: alliums (the onion family).
Rhubarb can be forced now by covering the crowns with upturned buckets or terracotta pots, surrounded by fresh manure. The heat from the decomposing manure will encourage the stalks to grow. As the plant elongates, you may need to replace the covering with a larger container.