Welcome to the top of the crops! Today you will learn how to grow asparagus in a polytunnel.
Asparagus is one of the best known perennial vegetables that you might grow in a polytunnel garden, or in a vegetable patch. Asparagus is a healthy addition to a homegrown diet.
Growing perennials like this one allows us to enjoy yields from our gardens over multiple years, without having to sow crops anew each year. So they can be a good choice for a sustainable and relatively low-maintenance space.
Asparagus is a plant that can live for over 20 years. Planting some in your garden is a long-term investment – one that will reap higher dividends as time goes by.
Asparagus plants provide a yield of young shoots, or spears, which are produced for a period of around 8 weeks in spring and early in the summer.
Over the summer, the plants will then grow into tall feathery or fern-like plants. They can then be cut down to the ground in autumn or very early spring and they will sprout once more in spring the following year.
Asparagus has been cultivated as a vegetable since at least ancient Egyptian times, and is still seen as a delicacy. Since it has a relatively short harvesting season, it is viewed as a seasonal treat and highly prized. Asparagus season is often highlighted on local food calendars – something to look forward to each spring.
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It is very important, if you want to grow asparagus at home, to choose the right location for this crop. Remember, these are long-term additions to your garden and they will remain in the same location for many years.
Asparagus will grow best if it is provided with the right growing conditions. This means choosing a location that has:
Full sun, or light/ dappled shade.
Plenty of space and not too much competition. (A dedicated bed with only asparagus is sometimes recommended, but careful companion planting may be beneficial.)
A fertile, free-draining soil rich in organic matter, with a pH of 6.5- 7.5.
Asparagus is typically grown from 'crowns', often bare-root plants purchased during the dormant period. Crowns can be purchased online from a number of online retailers, and from many local garden centres and plant nurseries. However, though this takes more work, asparagus can also be grown from seed.
If you would like to grow asparagus then of course, as well as deciding whether to grow from crowns or from seed, you will need to select a variety or varieties to grow.
Asparagus plants are either male or female – the males produce more and better spears and so many modern varieties are all-male F1 hybrids. However, note that even these can sometimes produce female plants that need to be removed for best results.
To help you choose the right variety of asparagus, there are some suggestions towards the end of this guide.
Remember, in order to grow this perennial plant successfully over many years, you need to provide a site with suitable growing conditions.
If you have a heavy soil, or one that is overly acidic, then it is a good idea to create a raised bed for your asparagus crop. Make sure you any planting area is rich in organic matter and weed free before planting out asparagus in your garden.
If you have chosen to take the more challenging but cheaper option of growing asparagus from seed, you should sow seeds indoors in February, maintaining temperatures of between 13 and 16 degrees for successful germination. The seedlings are grown on in warm conditions and watered well, before being hardened off and planted out in around early June.
Alternatively, sow the seeds outside in March or April. Typically the seeds are sown into drills that are 2.5cm deep, and 30-45cm apart. Seedlings are thinned to around 15cm apart and then moved into their final growing positions around a year later.
Usually people will choose to grow asparagus from crowns – one year old dormant plants – rather than growing them from seed. Ideally these should be ordered and planted out as soon as they arrive – ideally in March. This planting process is also that which should be followed for the plants you have grown from seed.
Asparagus is best grown traditionally, using the trench system. First of all, you should
dig a trench around 30cm wide and 20cm deep.
Add organic matter such as compost or well-rotted manure into the base of the trench,
and cover this with some of the soil removed from the trench to make a ridge along the base of the trench around 10cm high.
Put the crowns atop this ridge with the new shoots upwards, spacing them around 30-45cm apart. Gently spread out their roots.
Refill the trench around the crowns with a mix of soil and compost or other organic matter, so that the bud tips are just visible above the surface of the soil.
If you are making multiple trenched rows, these should be around 45cm apart, with the plants in staggered rows.
Water the plants in well, then mulch with around 5cm of organic mulch to conserve soil moisture and suppress weeds.
Asparagus, once fully mature and established, really won't require all that much care. However, these are plants that will need some care and attention during the earlier stages of their growth.
As mentioned above, individual Asparagus plants are either male or female. The males are considered to be more productive than the females, producing more spears of a better quality.
Older varieties produce both male and female plants, while more modern cultivars are often mostly male. However, even with modern varieties, female plants can sometimes appear. These should be weeded out if you want to achieve the best possible yields.
It is also important to weed out any other unwanted plants from an asparagus bed as these are not plants that can cope well with too much competition.
Asparagus plants are fairly drought tolerant once established, but of course you will still need to provide water when they are grown undercover, and you should also water outdoor plants consistently through dry spells for their first summer.
Replenish the organic mulch around asparagus plants each autumn or early winter, taking care to make sure that there is good coverage and that weeds cannot get established in the growing area.
It is a good idea, in addition to feeding with an organic mulch, to add a high potassium organic fertiliser in early spring and, if growth is poor, again after the harvesting period is over.
Since asparagus grows into a tall plant over the summer, it can be a good idea to provide support to avoid any damage to the crown due to stems breaking in windy conditions. You can simply use natural branches and twine to create barrier fences on either side of an asparagus row.
Asparagus, once established, can be propagated by division. Divide established crowns in the late winter or early spring, but avoid doing this too frequently as it will weaken the plant and plants that are divided can be slow to return to full form.
You need to have patience and think of asparagus as a long-term investment rather than something that can deliver quick results. It is best not to harvest any spears at all in the first year, and to hold off again in the second, and even in the third year when you have grown from seed. This gives the plants the time they need to establish themselves properly.
When it is finally time to harvest, harvest from mid-April but stop harvesting after around 6 weeks. In following years you can then continue to harvest over 8 weeks.
To harvest asparagus, cut individual spears with a sharp knife around 2.5cm below the surface of the soil, when they reach around 18cm tall and are around the width of a finger. Harvest regularly, every 2-3 days, so spears don't turn woody and so more spears will be produced. Over the 8 week harvesting period, an established plant should produce around 10 spears.
The varieties ‘Backlim', ‘Connover's Colossal’, ‘Gijnlim’ and ‘Guelph Millennium’ have awards of garden merit from the RHS and are recommended for UK growers.
Asparagus is generally largely a trouble free plant once established when it is grown in a suitable location with the right growing conditions.
However, certain pests can pose a problem, including slugs and snails, and asparagus beetles. Manage these through increasing biodiversity in your garden to encourage natural predation of these pests, and with careful companion planting.
Beetles can also be picked off the plants by hand in the case of a severe infestation, though small populations can be tolerated without too much harm to your asparagus crop.
A polytunnel can be particularly useful to those in wetter and cooler regions who wish to grow this crop. Asparagus particularly enjoys the warm soil conditions, and shelter from frosts and wind that a polytunnel can provide. And the control you can have in a polytunnel over watering means that waterlogging is less likely to become an issue.
Create a perennial bed in a polytunnel and it can provide food for many years to come rather than just a single season. Asparagus is just one of many perennial vegetables that you might consider growing within the space.
Be very careful with companion planting since asparagus is intolerant of too much competition. But consider intercropping annuals like tomatoes or peppers in the early years to make the most of the space, and consider using basil, parsley and marigolds as companion plants too.
The tomato plant may also help to repel the asparagus beetle. And asparagus is said to help tomato plants too, by repelling certain root nematodes.
Basil is great planted near asparagus because it helps to repel a common asparagus pest – asparagus beetle. Adding parsley to the mix can also help in this regard. Marigolds are well known as a companion plant for a wide range of crops.
Asparagus should not be planted near potatoes or other vigorous tubers or root crops, nor near alliums (members of the onion family) which may stunt its growth.
Felton, K., (2022) 9 Health Benefits of Asparagus. health.com. [online] available at: https://www.health.com/nutrition/asparagus-health-benefits
