When to plant asparagus

Asparagus is a long-lived perennial. You plant it once and, if the bed is well made, pick from it for well over a decade. It is grown from crowns, the dormant root clusters of one-year-old plants, rather than sown in place.

Plant crowns in March, as early in spring as the soil can be worked. In Germany, Austria and Switzerland that can be from late February in mild areas; the end of March is the latest good time. Growing from seed is possible, but it adds a year before the first harvest.

Take your time over the site: this bed will stay put for many years. Choose full sun and loose, deep, free-draining soil; sandy soil is ideal. On heavy soil, build a raised bed.

Clear the ground of perennial weeds first; they are almost impossible to remove from an established bed. Dig a trench about 8–12 in deep and 12–16 in wide and work a generous amount of compost or well-rotted manure into the bottom.

Make a low ridge of soil about 4 in high along the bottom of the trench. Sit the crowns on top, 12–20 in apart, and spread their roots down both sides like a spider's legs. Cover them so that the bud tips are just at or below the surface, and water well.

As the shoots grow through the summer, gradually fill in the trench until it is level with the surrounding ground, by the time the shoots are about 12 in tall.

Space rows about 18 in apart if you plant a staggered double row, or more for single rows. In a 120 × 80 cm raised bed you can fit about six crowns in two staggered rows. That gives a small harvest, and the bed must be deep enough for the long roots. If you want asparagus in quantity, it belongs in open ground.

Weed by hand; a hoe easily damages the roots and buds. A mulch of about 2 in of compost in spring helps keep weeds down.

After harvest, let the spears grow up into tall, feathery ferns. They feed the roots for next year's crop, so never cut them in summer. In windy spots, give them a simple fence of stakes and twine. When the foliage turns yellow in autumn, cut it down to the ground.

Feed lightly: a layer of compost each year, about 1 to 2 litres per square metre, is usually enough. If growth is weak, add a general fertiliser in early spring.

Plant all-male varieties if you can. Asparagus has separate male and female plants; female plants spend energy on red berries, and their seedlings come up as weeds in the bed. Pull out any seedlings you find.

Don't cut anything in the first two years after planting. The plants need that time to build up strong roots. In the third year, cut for a short season of about six weeks from mid-April; from the fourth year on, you can harvest for about eight weeks.

Cut green spears when they are about 7 in tall and at least as thick as a finger, a little below the soil surface. In warm weather check every two or three days, because spears shoot up fast.

Stop by mid to late June. In German-speaking countries the rule is never after St John's Day, 24 June. After that the plant needs its ferns to recharge the roots for next year; cut too long and the bed weakens.

A well-kept bed crops for about 10 to 20 years before yields fall off.

White and green asparagus are the same plant, grown differently. White asparagus grows under ridges of soil that are heaped over the rows each spring, so the spears never see light; you dig them out just before they break the surface.

For a home garden, green asparagus is the easier choice. It needs no ridging, is quicker to harvest, and many gardeners find it tastier. Rows for green asparagus can also sit a little closer together.

The asparagus beetle is the pest you will see most: small red-and-black beetles and their grey larvae strip the ferns and chew the spears. Pick them off as soon as you see them. The asparagus fly also attacks the spears.

Slugs damage emerging spears, and late frosts can blacken them. Asparagus rust shows as rusty spots on the ferns. Fusarium root rot weakens plants, and root rots in general are worst on wet, badly drained soil.

Asparagus belongs to its own family, the Asparagaceae, so it doesn't compete with vegetables in a rotation. The rule that matters: never plant a new asparagus bed where asparagus grew before. Fusarium and other soil problems build up over the years, and German advice recommends a break of as much as 20 years before asparagus returns to the same spot.

Tomatoes, parsley and basil are often listed as good asparagus companions, sometimes with claims about deterring asparagus beetle. We found no solid evidence for this; it is tradition.

In practice, keep other crops well away from the young bed so they don't compete, and don't let tall neighbours shade the ferns after harvest.