Paradise Garden

Gardening Articles written by Experts


Asparagus

by Mark Rowland
asparagus cultivation asparagus varieties asparagus faqs
asparagus beetle asparagus diary asparagus links

Introduction

The asparagus that we eat is the species Asparagus officinalis; there are many other species, many of which are cultivated as ornamentals. Edible asparagus has ben cultivated for thousands of years, initially as a medicine, and more recently as a food. Advances in modern breeding have transformed the crop, improving both yield and quality almost beyond recognition.

Asparagus is rightly regarded as a gourmet vegetable and commands high prices in the shops. It is available all the year round from Peru, while domestic UK production is concentrated mainly in the spring. It is expensive to buy, and is at its best when freshly harvested; for these reasons it has become a popular grow-your-own crop. Growing asparagus is not really difficult, but to get the highest yields and the best quality you do need the best advice. Unfortunately, much of the advice given in books and gardening magazines is outdated and counter productive. This article is based on scientific trials carried out in asparagus growing areas around the world, as well as personal experience.

Open Pollinated Varieties

Traditionally, asparagus has been produced from open-pollinated varieties such as Connovers Colossal in the UK and Mary Washington in the USA. These varieties are still widely available but do not compare in yield with modern hybrid varieties. In part this is due to the fact that asparagus is a dioecious crop, that is plants are either male or female. Seed raised open pollinated varieties will always give approximately equal numbers of male and female plants, and male plants give considerably higher yields of edible spears. The seed of open pollinated asparagus varieties tends to have a relatively low germination rate.

Males and Females

The significance of all-male hybrid asparagus is clear if we consider the differences between male and female plants. Male plants give much higher yields, are less susceptible to disease, and are longer lived. Female plants put much of their energy into producing copious quantities of seed which falls and germinates within the asparagus bed. These seedlings are difficult to weed out without damaging the established crowns, and the bed can easily become choked, dramatically reducing its productive capacity.

Hybrid Asparagus

Some modern asparagus hybrids are all male, but it is important to realise that others are conventional F1 hybrids and give 50% male and 50% female plants. Conventional hybrids are derived from a cross between a male asparagus plant and a female, and suffer from some of the disadvantages as an open pollinated variety. All male hybrids, on the other hand, are created by using a 'supermale' parent, and, as the name implies, give 100% male plants. There are active breeding programs in many parts of the world, so it is possible to find varieties which have been bred to crop successfully under a wide range of climatic conditions and soil types. Hybrid asparagus seed has a much higher germination rate than open pollinated varieties.

Males and Supermales

Male plants carry both the male (M) and female (F) genes, while female plants carry only the female gene. Thus in normal sexual reproduction, a seed can inherit either gene from its male parent, but always a female gene from its female parent. Occasionally, however, a male asparagus plant will produce a hermaphrodite flower and from this the genes can recombine in three ways, MM (supermale), MF (male) or FF (female). When a supermale is crossed with a female, the progeny must always inherit a M gene from the supermale and an F gene from the female giving 100% MF male plants. A picture of male berries can be seen on the asparagus cultivation page.

Choosing an Asparagus Variety

Although quality and yield are the most obvious considerations when choosing which asparagus variety to grow, there are others which are no less important. Organic gardeners will benefit from the greater disease resistance of varieties with more upright foliage, while compact foliage will be much easier to manage for those intending to grow asparagus in a glasshouse or polytunnel. Some varieties are particularly sensitive to a high water table, others perform well under particularly warm or cool climatic conditions. Growing both early and late varieties of asparagus will give a welcome extension to the harvest period, but harvesting in hot weather needs to be very frequent if quality is to be maintained. The choice of variety is critical, so refer to our page describing the most widely available open pollinated and hybrid asparagus varieties.

Planting

Asparagus is usually grown from crowns, but there are advantages in raising your own plants from seed. You can be confidant that they start off free of the common asparagus pests and diseases, be easier to plant, and, of course, they will be much cheaper. Losses from planting crowns is normally about 10% for one year old crowns, rising to 20% for 2 year olds, whereas losses from planting pot grown asparagus seedlings should be virtually zero. We have added a short illustrated guide on how to grow asparagus from seed.

Asparagus needs deep, rich, light, well drained soil, and the crowns should be several inches deep to encourage the production of thick, heavy spears, although planting too deeply will reduce yields. A planting depth of about eight inches, 20cm, is considered optimal for white asparagus production, but may be difficult for the amateur to achieve. Building up the asparagus bed with light sandy soil in the late autumn after the old fern has been cut away will help increase subsequent spear size. This operation can also be combined with a top dressing of fertilizer to keep the crop healthy and productive. Asparagus is a hungry crop and responds well to generous treatment.

Support

Asparagus is a vigorous crop and produces tall "fern" which needs sturdy support. A rigid system is much better than string or thin plastic mesh, but whatever is used, the posts must be anchored firmly in the ground. A solid support system is particularly important in exposed situations or if you choose to grow purple asparagus which has less fibre in the stem.

White, Green and Purple Asparagus

White asparagus is green asparagus which has been grown in the dark, usually by mounding the beds to cover the crowns with some eight to twelve inches of light sandy soil so as to exclude light from the developing spears. It is highly esteemed in continental Europe. White asparagus is generally peeled before being eaten as it has been grown underground, but with purple asparagus varieties this is not necessary. Optimum planting density is a little lower for white asparagus than for green.

Purple asparagus is a variant of green which originated in Italy. It turns green if boiled, but retains its colour if steamed. Purple varieties of asparagus have a sweeter flavour and are less fibrous making them superior to green cultivars for eating raw. The original purple variety gave low yields and was very susceptible to disease, but modern purple hybrids have gone some way towards addressing these problems. It should be stressed that the lack of fibre in the stems means that purple asparagus is less weatherproof and should be avoided in exposed situations. Purple asparagus is particularly suitable for eating raw.

Harvesting Asparagus Spears

Harvesting asparagus is something of a balancing act. Cutting a spear has two effects; good - it stimulates the crown to produce more shoots, and bad - it robs the crown of a source of energy. Commercial growers try take a light cut from a new asparagus bed in the year after planting in order to get more spears in subsequent years. The success of this technique depends on early planting, good establishment and vigorous growth in the first year.

Asparagus spears are normally cut at or just below the soil surface when the emerged spear is about seven or eight inches long. This gives the normal green asparagus which is favoured in most parts of the world. In hot weather the spears start to fern out earlier and need to be cut at around five inches to maintain quality.

Asparagus spears can be either cut or snapped. Cutting is usual, but care has to be taken to avoid damaging emerging spears. The base of a cut spear may have started to become tough; if the spear is snapped, it will naturally tend to break at the junction between the fleshy and the woody parts of the spear. Snapping is faster than cutting, but requires greater skill.

Cropping Season

The traditional season for UK asparagus starts on St George's Day, 23rd April, and ends on 21st June. The season can be extended earlier by growing forcing varieties under protection, and later by planting suitable varieties. Under cold glass, early asparagus varieties can be cropped from mid March.

Quality

The quality of an asparagus spear hinges on two factors - the stage at which it was harvested, and the time elapsed since harvest. A top quality spear is 'tight', once the small triangular bracts start to open out quality is dropping rapidly. A spear which has started 'ferning out' is best used for soup as most of the stem will be tough and fibrous. Once cut, the spears should be kept cool and eaten as soon as possible. Bought asparagus can never match home grown for harvest to table interval. The thickness of the spear is not in itself a factor, although thick spears look better and generally sell at a premium.

Asparagus Pests

While soil pests like slugs and cutworms can be a problem, the main enemy is the Asparagus Beetle, Crioceris asparagi. The Asparagus Beetle page has photographs of the adult beetle and its larvae as well as notes on its life cycle.


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