Found this info (below) off the GYO website, which I hope will help? Sounds like it'll be ok in a raised bed, so long as it's deep enough to accomodate the roots. You could line it with a root-stop bag to prevent the perennial weeds from growing into the bed, then you'd only have to worry about pulling out the annual weeds.

Planting it
Though it’s possible to grow plants from seed, the commonest (and easiest) way to grow asparagus is from ready-raised crowns. They can be bought during the winter and planted out in early spring. Asparagus prefers a warm site and, despite the recent spate of mild winters, few areas of the country resemble its native Mediterranean climate and you’ll need to give it a boost.

One traditional method is to prepare a large ridge in the soil which creates a free draining layer around the crown and helps the sun heat up the soil. Make sure that they are sited in full sun and make the most of it by lining the ridges north to south – this enables the rising sun to heat one side and the setting sun to warm the other. Simply open up the top of the ridge, place the crown so its roots spread out over the top and cover the entire plant with soil. A layer of fleece will help keep the soil and crowns a touch warmer in their first season but make sure you remove it as the spears start to emerge in late spring.

If you’re lucky enough to have a free draining soil and live in one of the warmer parts of England, trench planting is another option. Dig out a trench 20cm (8in) deep with 8cm (3in) ridge running down the middle. Set the asparagus out across the centre of the ridge, about 45cm (18in) apart and plant in the same way as above. Earth up the spears as they grow.

Caring for it
Keeping asparagus beds weed-free throughout the year is vital for healthy crops. This can be a chore if you have not prepared the ground properly, as it has to be done by hand. Using a hoe runs the risk of causing serious damage to the roots and emerging spears. A light mulch applied early in the year will suppress most weed seedlings. It’s important to avoid cropping for the first couple of years, as the plant needs time to establish. Let the feathery shoots grow and support them with canes so that they do not blow over or snap in high winds. The leafy growth is vital for the plant to feed and fatten up the root system throughout the life of the crown but it is especially important in the first few years.

Once the foliage has turned yellow in the autumn, cut back to 5cm (2in) above the surface and draw up a small ridge of soil over the plants. Add a compost mulch to keep the soil fertile and suppress winter germinating annual weeds.

Asparagus is highly drought tolerant (thanks to its Mediterranean origins) and after it has become properly established in the first year, is unlikely to ever need watering.
Mick Lavelle


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__________________
Sarah

http://www.hypermobility.org


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