Site and soil requirements
Do not replant an old asparagus bed with new asparagus plants. Choose fresh ground to avoid build-up of diseases. An open, sunny site is best, but asparagus will tolerate dappled shade.
Asparagus grows 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, so more acidic soils may need liming.
Clear the ground of perennial weeds and incorporate at least one bucketful of organic matter such as garden compost or well-rotted farmyard manure every square metre (yard).
Most gardeners plant one-year-old dormant plants called ‘crowns’ in March, although asparagus can also be raised from seed (see the propagation section below).
To plant asparagus crowns:
- Dig a trench 30cm (1ft) wide and 20cm (8in) deep. Work in well-rotted manure to the bottom of the trench and cover the base of the trench with a 5cm (2in) layer of the excavated soil
- Make a 10cm-high (4in) ridge of soil down the centre of the trench
- Place the crowns on top of this ridge, spacing them 30-45cm (1ft-18in) apart within the row. Spread the roots evenly and replace the rest of the soil, leaving the bud tips just visible
- Leave 45cm (18in) between rows and stagger the plants between adjacent rows
- Water in and mulch with 5cm (2in) of well-rotted manure or other weed free organic matter
Keep the asparagus bed weed free. Weed by hand rather than with a hoe, as the shallow roots of the asparagus plants are easily damaged.
Mulch the bed in late winter to discourage weeds and to retain moisture. Consider covering the bed from autumn to winter with an opaque weed mat to prevent annual weeds germinating.
In early spring, apply 100g per sq m (3oz per sq yd) of general fertilizer such as Growmore, or fish, blood and bone. If growth is weak, repeat this application once harvesting has finished.
Allow the foliage of your asparagus plants to yellow in autumn before cutting it down to soil level for the winter.
Asparagus plants are either male or female. Male plants produce more and better spears, so many modern cultivars are all-male. If any female plants do appear, they will be noticeable because they produce orange-red berries. If you are growing an all-male cultivar, you will need to remove any female plants as well as any seedlings that form.
Do not harvest for the first two years after planting. In the third year, harvest spears from mid-April for six weeks. In subsequent years you can harvest for eight weeks from mid-April.
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 the best quality spears.
Most people choose to grow asparagus from crowns bought from a garden centre or nursery, however, you can also grow from seed.
Choose an all-male F1 hybrid cultivar for best results. Non-hybrid seed produces female as well as male plants. Even F1 all-male seeds can produce the occassional female plant and therefore seedlings. These need to be removed to prevent competition with the existing crowns.
- Sow seeds singly into modules in February at 13-16°C (55-61°F), then harden them off and transplant them to their outside positions in early June
- Alternatively, sow seed in an outside seedbed in March-April, in drills 2.5cm (1in) deep and 30-45cm (1ft-18in) apart. Thin the seedlings to 15cm (6in) apart. Transplant them to their final positions the following March
Divide crowns in late winter or early spring, no more than every three years as asparagus can be slow to settle.
- Take strong grow from the edge of the crown, discarding any woody parts
- Ideally, prise sections apart though cutting may be unavoidable
- Ensure each section has several strong growing points
- Handle the roots very carefully
- Replant straight away with the buds still visible at the soil surface
Weak growth can be caused by inadequate moisture levels, over-cropping in the previous season, competition from weeds and asparagus seedlings (if female plants are present), or from defoliation by slugs, snails and the asparagus beetle.
When growth is weak, it is advisable to crop spears for a shorter period (four to six weeks) until plants mature again.
If mulching and hand weeding prove inadequate to keep the bed weed-free, then apply a systemic glyphosate-based weedkiller specifically to the leaves of the weeds, carefully avoiding the asparagus foliage.
Spears may lack flavour at the beginning of the growing season, especially if the soil is dry. Flavour should improve later in the season.
Late frosts can damage the growing tips, leading to distorted or dead spears. Remove any damaged growth and protect the bed with a double layer of horticultural fleece if frost is forecast. Slug grazing can cause similar damage.
Weedkillers for gardeners (Adobe Acrobat pdf document outlining weedkillers available to gardeners)