Asparagus

Light and exposure

Asparagus wants full sun, with no shade cast by a nearby tree or hedge. Since the bed stays put for fifteen years or more, choose the spot carefully from the start: an open square, away from trees whose roots would compete with the crowns underground.

A slightly separate corner of the vegetable patch works well, since asparagus does not like being disturbed by the rotation of neighbouring crops. Give it plenty of room: a planting trench takes up more space than you expect, and the crowns spread out over the years.

Watering

Young crowns planted at the start of the growing season need regular watering right through that first season, while they build the deep roots that will make them self-sufficient later. On sandy or sandy-loam ground, which drains fast and holds little water, that first year calls for particular attention: a young row left to dry out is hard to bring back once the roots have shrivelled.

After that, asparagus becomes one of the most frugal plants in the kitchen garden: roots that plunge a metre or more from the third year onwards go looking for water long after the surface mulch has dried. During harvest, generous but well spaced watering, rather than small frequent doses, gives plump spears, above all in a dry spring, which is becoming more frequent in temperate climates.

Avoid any standing water around the crown, though, whether it comes from badly aimed watering or from soil that has compacted: it is the leading cause of crown rot, well ahead of a shortage of water. At the end of the season, let the foliage yellow without watering more: the plant goes naturally dormant and no longer needs moisture in the soil.

Soil and repotting

Asparagus wants deep, light and above all perfectly drained soil. A sandy or sandy-loam soil suits it beautifully; heavy clay that holds water over winter rots the crowns and can doom the bed before it even gets going.

Clear the plot of stones and perennial weed roots before digging the planting trench: the spears, which push their way up through the soil, get deformed by the smallest obstacle. A generous helping of well-rotted compost at the bottom of the trench, worked in at planting time, gives the crop a strong start for the fifteen years ahead.

Temperature and humidity

Asparagus is a fully hardy plant: crowns in the ground fear neither frost nor snow, and they come through the winters of USDA zones 3 to 8 with no protection to plan for. The foliage yellows and dries at the end of the growing season, the plant goes dormant and comes back on its own the following spring.

The real threshold to watch is in the soil, not the air: spears start to push as soon as the ground reaches 8 to 10 degrees at depth, but growth only turns genuinely fast above 18 to 20 degrees, where a spear can gain several centimetres in a single fine day. Market growers aim at that threshold by stretching black plastic film over the ridge a few weeks before growth restarts, which brings the harvest forward by two to three weeks, a trick within reach of any home gardener with a plain black tarpaulin.

The only remaining risk is late frost, which blackens young spears already up out of the ground after an early spring, without ever threatening the plant itself or the harvests that follow.

Pruning and upkeep

Asparagus is not pruned in the usual sense. The key task is to stop harvesting spears at midsummer, around the end of June, and let the stems grow on freely from then. These feathery stems, a metre and a half tall or more, use photosynthesis to build the reserves the crown will store for next year.

In autumn, once the foliage has properly yellowed and dried out after the first frosts, cut the stems back to a few centimetres above the ground. Never cut them while still green: it is precisely while they are yellowing that the plant is moving its sugars down into the roots.

Pests and diseases

The asparagus beetle is the most visible pest: this small red-and-black beetle with a metallic sheen lays its eggs on the stems, and its grey larvae can strip the foliage within days if left unchecked. Picking them off by hand as soon as you spot the first ones, early in the morning while they are still sluggish with cold, remains the most effective method in a home garden.

Asparagus rust forms orange, later brown, pustules on the stems in late summer, encouraged by stagnant humidity between rows planted too close together. The asparagus fly, for its part, lays eggs in the young spears, and its larvae tunnel through them, making them unfit for harvest. Spacing out the plants and cleaning up the foliage thoroughly in autumn keeps all three problems in check at once.

Propagation

Asparagus is almost always propagated from crowns, those fleshy, star-shaped roots you buy ready to plant in spring from a nursery or garden centre. This is by far the simplest and quickest route for a home gardener, since it skips the two to three years of growing from seed needed before you even get a plantable crown.

Growing from seed is possible for the curious, sown in pots at the end of winter, but takes even more patience: it needs about a year of nursery growing before you get a crown worth planting out for good. It is also worth knowing that asparagus is dioecious, with separate male and female plants: the modern varieties sold as crowns are almost all male hybrids, which are more productive, since female plants spend energy producing decorative but useless red berries instead of spears.

Month-by-month care calendar

Green Hub tips

• Be patient: do not harvest anything for the first two years after planting the crowns, let the plant build up its reserves before you touch a single spear.
• Always stop harvesting at midsummer, end of June, even if spears are still coming up: a harvest stretched too long exhausts the crown for the following years.
• Waterlogged winter soil kills crowns more reliably than frost does: if your soil is heavy, build the trench up as a well-drained ridge with coarse sand.
• Watch for the asparagus beetle from spring onward: pick it off by hand early in the morning, while it is still sluggish with cold and easy to catch.