Growing asparagus in the home food garden

It can be a bit tricky growing asparagus in the garden, but the rewards make it well worth the effort. The main reason is that asparagus is one of the few truly perennial vegetables, with some varieties capable of continuous production for 30 years or more. This means that one of the first things to do is set up a dedicated perennial garden, since having asparagus in a normal vegetable garden will only interfere with your crop rotations.

Another problem is that much of what you may have read in books about growing asparagus is not entirely correct. For example, you will often hear it said that asparagus cannot be grown in acid soil and yet soil pH in some of the most important asparagus growing areas around Australia tends to range between 4.5 and 5.6 - way below the pH level of 7.0 that most textbooks call essential.

The answer to this contradiction seems to be that asparagus has two root systems. The shallow feeder roots are quite tolerant of pH over a wide range. It is the deeper storage root system that needs neutral soil and in all commercial asparagus growing areas it is the soil from about 1 metre below the surface that is always either neutral or alkaline.

In fact there is some evidence to show that moderately saline conditions can even be beneficial in suppressing soil-borne diseases like Pythium.

Health benefits of asparagus

This isn't really the topic of this website and I am certainly no expert, but it is certainly true that asparagus is one of our so-called "Super Foods". The health benefits in growing asparagus are quite substantial. It is high in saponins (glycosides) and so will have many of the same benefits as Ginseng. The trick is not to over-cook it. Stop cooking as soon as the spears turn a bright emerald green - if they turn a dull olive color, that's too long. In fact many asparagus growers even save the water the spears were cooked in and drink it as a herbal tea. Of course, the best thing is to eat asparagus raw in salads, etc.

Soils & Fertilizer for growing asparagus

Being such a long-lived plant, asparagus growing often seems to be more like managing an underground orchard than a vegetable garden. Clearly, everything depends on the soil, which must be free-draining, high in organic matter and moderately fertile.

In fact, the same is true for the other perennial vegetable, so when planning your perennial garden you should start with the 5-step process for improving soil. You may also find that a raised garden bed will also be an advantage.

Again, don't go by the pH, this just isn't a good indicator of soil suitability for growing asparagus.

You should also avoid using too much fertilizer - for asparagus more is definitely not better. This is because growth occurs in three distinct stages - harvest, fern and dormancy. While asparagus certainly responds well to applied Nitrogen, this is only in the fern stage. The crown stores all the energy and nutrition required for the other two, so the main objective is always to maintain the health and vigor of the crown.

One peculiarity of asparagus, however, is that although it is a long-lived perennial, ecologically it is also a 'Pioneer' species - preferring open areas with little competition. Like other plants that share these features, asparagus relies on "allelopathy" to reduce the amount of competition around it. Unfortunately, the main targets of this strategy are other asparagus plants.

Also, the compounds that inhibit the growth of other plants are released from the decomposition of dead roots. This is why it is important to have healthy soil with lots of organic matter and biological activity and also not to have asparagus planted too closely. Distance and biological activity in the soil will keep the levels of allelopathic compounds to a minimum.

Harvesting asparagus

Although there are different methods (cutting or snapping), the asparagus harvest is really all about timing. The spears are just unexpanded emerging leaves. Left uncut they will simply expand into fern. Early in Spring, the temptation may be to let the first one or two go and cut them later - when there are enough spears coming to make a meal. However, this is a mistake.

Similar mistakes also occur at the other end of the season, when cutting stops and fern growth is encouraged with the application of fertilizer. The longer you keep cutting, the more nourishment is drained from the crown and the less time remains for photosynthesis in the fern to replenish that nourishment before onset of dormancy.

There is no real indicator of when to stop harvesting, although many grower look for a reduction in the thickness of emergent spears. The main thing is to leave plenty of time for the fern stage.

The reason that timing errors at either end of the season can be so disastrous is that although removing spears takes something out of the crown, by far the greatest depletion of its reserves will begin the moment the spear begins to expand into fern.

Letting early spears keep growing will deplete the crown before the season has begun. And continuing the harvest to long may not leave enough left to support the fern later. In a well managed plantation, the fern should be dark green and standing 5 or 6 feet tall. Stunted and pale colored fern is a sign that errors of timing are being made.

A special case is "Summer-cut" asparagus. Here the fern is allowed to grow first (in Spring) and cut down in early or mid-Summer to stimulate a new flush of spears. This method is very useful for extending the availability of asparagus from the garden (Spring cut some and Summer cut others). However, many make the mistake of letting a second fern stage develop prior to dormancy. In this case, there is never enough time for photosynthesis to replenish the crown before dormancy sets in and plantings quickly decline. The key to summer cutting is to continue the harvest right into dormancy - no matter what. It feels wrong, but as long as you allow a good long fern stage in spring, the plants will be fine.

Pests and diseases

Unfortunately, there are now many pests and diseases of asparagus and, in parts of the World that are most affected, this has shortened the productive life of many varieties and increased the difficulty in growing asparagus. However, all is not lost, as most problems respond well to organic gardening methods - healthy soil, avoidance of over-using fertilizer, ensuring plenty of sunlight and good air flow around the plants and companion planting (such as asparagus and tomatoes).

Make sure you do not plant asparagus too close and give them lots of sun and ventilation (but of course no drafts). Fertilize only as much as may be needed to stimulate fern growth each year and maintain healthy, well-drained soil. With all this and good harvest timing, plant health will remain high and so to will there resistance to pests and disease.

Varieties to plant

After years of careful selection and breeding, the very best variety for growing asparagus remains the good old-faithful Mary Washington. This variety has only fallen short as production extended into warmer (and even tropical) areas. Varieties like Ida Lea seem to do better in Continental Climates (see our Garden Ecology page for more about this), but most commercial growers say that none of the new varieties has ever done better than Mary Washington.

The only other varieties of note are the Purple ones. These have more of the healthy components than green varieties (so long as they are not over cooked - turning green on cooking just like the rest).

Planting asparagus

When growing asparagus, the temptation is to put in a few more plants for greater yield, but this seldom works. Plants that are too close will suppress each others growth and reduced air flow will increase the incidence of disease. As a general rule, keep asparagus plants 2 to 3 feet apart - more space being better than less.

Also, while growing asparagus from seed is a good way to go (especially if seed is planted so it does not need to be moved later), there is a lot of bother with seed for relatively little gain. The best method of planting asparagus is to use 2 to 3 year old crowns (do not trim the roots off these) planted into the bottom of a 4 or 5 inch deep trench. No need to fuss over placement, just lay them flat in the trench and cover them over - they will sort themselves out as they grow.

A good idea is to soak the crowns for about half and hour or so in a weak seaweed solution (2 to 5 ml per litre) just before planting. This will ensure they well hydrated and the kelp will help protect them from early infections.

Further reading and useful links

Despite being a relatively small industry (Worldwide), there has still been a fair amount of research done on growing asparagus, so below are some links to help you find out more. You can also keep in touch with new developments by subscribing to our Free newsletter - The Food Gardener.