7 March 2014
Most jobs go better if you get the preparation right, and growing asparagus is no exception, but given that asparagus plants can stay productive for up to 20 years it will be time well spent.
An asparagus plant is composed of ferns, crown and a root system. The crown is a collection of rhizomes and lateral roots that initiate new ferns, while the spears, the harvested part of the plant, are actually immature ferns. If the spear is not harvested, it grows into a large fern, which manufactures and stores energy in the crown for next year’s crop.
Asparagus is a perennial vegetable, and benefits from growing in a bed where it won't be disturbed, as it takes a long time to grow to maturity. It is best grown in a large garden or allotment as it likes plenty of space, ideally in a sunny position in well-drained soil and protected from the wind. A soil pH of 6.5 - 7.5 is ideal. It isn't suitable for container-growing or gardens with heavy clay soil but will grow happily in a raised bed.
Ideally prepare your bed in autumn by digging over thoroughly, removing all perennial weeds, then mixing in plenty of well-rotted farmyard manure. Fork in some general fertiliser granules over the area at a rate of roughly 90g/sq m about a week before planting, then rake the ground level.
Asparagus can be grown from seed, but it is easier to plant one-year-old dormant plants, known as crowns, in March. Make sure you buy fresh crowns, as they often dry out while on display. To plant your crowns make a straight trench, 30cm wide by 20cm deep, then pour soil down the length of the trench to make a 10cm high mound. You can first work in some well-rotted manure to the bottom of the trench if you have some available.
Carefully place your asparagus crowns on top of the mound, 30cm apart, spreading the roots out either side, and then cover with about 5cm of sieved soil.
As the stems grow, cover the plants with more sieved soil, so that the trench is completely filled by autumn. Further rows should be 45cm apart, with the crowns staggered between the rows.
Thoroughly water newly planted crowns, keeping them damp during dry weather. Spears may appear soon after planting, but harvesting them will weaken the crowns so leave them alone.
For the first two years of their growth, plants should be left to form lots of ferny foliage. If you leave the foliage to yellow in autumn before cutting it down to soil level, resources will be transferred to the roots to improve spring cropping.
Weed your asparagus bed by hand, rather than using a hoe, as the shallow roots are easily damaged. Mulch the bed in late winter with weed-free compost to keep down weeds and to retain moisture. Alternatively you could cover the bed from autumn to winter with an opaque weed mat to prevent annual weeds germinating.
Every spring, apply 100g per sq m of general fertiliser such as fish, blood and bone or Growmore, repeating after harvest if growth is weak.
Asparagus plants are either male or female, though male plants produce more and better spears, so many modern cultivars are all-male. Female plants produce orange-red berries, so if you are growing an all-male cultivar, you will need to remove any female plants as well as any seedlings that appear.
Although some modern varieties have been bred for earlier cropping, most asparagus plants are ready for harvesting after two years. In the third year, harvest spears from mid-April for six weeks, and in subsequent years from mid-April for eight weeks. To harvest, cut individual spears with a sharp knife 2.5cm below the soil when they are no more than 18cm tall. In warm weather, harvest every two to three days for the best quality spears.
Some recommended varieties are:-
‘Backlim’ F1 AGM: An all-male plant with good yields and fat spears.
‘Gijnlim’ F1 AGM: An all-male plant with a high yield of thin spears.
‘Lucullus’ F1 AGM: An all-male plant with a later cropping yield of medium size spears.
'Guelph Millennium’ F1 (AGM) All-male, matures later, useful in frosty gardens.