One of the most sought-after vegetables, asparagus is not difficult to grow if kept well fed and weed free
The delicately flavoured young shoots of asparagus are one of the great luxuries of the vegetable plot. Much of the mystique surrounding their cultivation is unwarranted.
Most modern types are all-male F1 cultivars; these are more vigorous than older, open-pollinated cultivars and do not self-seed. Although plants require space, once established they should crop for up to 20 years, each crown yielding nine to 12 spears per cutting season.
Choosing a site
Avoid frost pockets and exposed areas. Do not replant on an old asparagus bed as diseases may be a problem.
Asparagus will grow on most soil types provided they are well drained. On heavy soils consider creating raised beds. A pH of 6.5-7.5 is ideal but more acidic soils may need liming. As with any long-term crop, soil preparation is essential. Clear the ground of perennial weeds. On heavily compacted soils consider double-digging, otherwise cultivate to a spade’s depth, incorporating well-rotted farmyard manure.
Planting
Asparagus can be raised from seed or young dormant plants - crowns - can be purchased. Sow seeds of an all-male F1 hybrid singly into modules in February and transplant in early June. Most gardeners choose one-year-old crowns, planting in March or April.
Fork over the prepared area and dig a trench 30cm (12in) wide and 20cm (8in) deep. Work in well-rotted manure in the bottom, cover with 5cm (2in) of the excavated soil and make a 10cm-high (4in) ridge down the centre of the trench. Place the crowns on top, spacing them 30-45cm (12-18in) apart (right). Leave 45cm (18in) between rows and stagger the plants. Spread the roots evenly and fill in the trench, leaving the bud tips just visible. Water in and mulch with 5cm (2in) of well-rotted manure.
Cultivation
Asparagus beds must be kept weed free - best done by hand as the shallow roots are easily damaged by hoeing. Mulching discourages weeds and retains moisture. Apply 100g per sq m (3oz per sq yd) of a general fertiliser in early spring and repeat once harvesting has finished.
To avoid top-growth breaking off in wind and damaging the crown, use canes and twine either side of the row for support. Remove any female plants (those bearing orange-red berries) and any seedlings.
Allow the foliage to yellow in autumn before cutting it down to 2.5cm (1in).
Harvesting
To harvest, cut individual spears with a sharp knife 2.5cm (1in) below the soil when they are no more than 18cm (7in) tall. In warm weather, harvest every two to three days for best quality spears.
Do not harvest for the first two years. In the third year, pick from mid-April for six weeks, and in subsequent years for eight weeks.
Cultivars
Asparagus cultivars with the RHS Award of Garden Merit:
‘Backlim’ F1: has good yields and fat spears
‘Gijnlim’ F1: produces a higher yield, but of thinner spears
‘Connover’s Colossal’: a traditional cultivar with good yields from selected crowns
Troubleshooting
If mulching and hand weeding prove inadequate, apply the systemic herbicide glyphosate to the leaves of perennial weeds, carefully avoiding the asparagus foliage. In the dormant season, when no asparagus foliage is showing above ground, a contact herbicide such as Weedol 2 will control non-persistent weeds.
The main pests to affect asparagus are slugs and snails, and the larvae and adults of the asparagus beetle.
Thin, spindly shoots may be due to inadequate moisture, especially with young crowns. In established beds the cause is more likely to be overcropping or competition from weeds. Late frosts will cause distorted growth: protect with a double layer of fleece.
Tony Dickerson