As soon as asparagus foliage has dried off, cut off stems to a few centimetres above soil level. The yellowish-brown colour of asparagus stems means that the plants have withdrawn nutrients and carbon compounds into their crowns to provide energy for new spring growth. Cutting back the stems while they are still green will gradually weaken the plants, and reduce the number of asparagus spears in coming seasons.
After cutting back the stems, remove any weeds from the bed, apply a generous drink of seaweed extract tea to the bed, and add a dusting of dolomite or agricultural lime. Asparagus are heavy feeders with a high nitrogen requirement. Give the bed a 3-5 cm layer of mature compost, or a 2 cm layer of worm castings, or a generous application of poultry-based organic complete fertiliser and a 3 cm layer of aged manure. Then cover the bed with a 5 cm layer of fluffed-up organic mulch. Fluffing the mulch allows rain and irrigation to trickle through to the soil. That done, apart from an occasional watering in during dry spells, you can leave nature to do its thing until spears start to poke their heads above ground in spring.