Fresh asparagus spears are one of the earliest delights of spring. While asparagus is often shipped long distances from faraway lands to provide a year-round supply of this delicacy, nothing beats locally harvested spears—particularly those grown in your own hydroponic garden.
Asparagus spears are the succulent, tender new shoots cut from the plant before the foliage unfurls. They have a high rate of respiration and a short shelf life. Asparagus can be refrigerated and transported to distant markets, but it is never at its best after several days. The texture and flavor of spears are best when eaten within an hour of harvest. But fresh, locally grown asparagus doesn’t need to be a seasonal treat; it can be grown year-round provided the right conditions and plant management techniques are applied.
Along with the common green form of this vegetable, white and purple asparagus spears are relatively easy to grow. White asparagus is the blanched (grown in the absence of light) version of the green form, while purple asparagus cultivars produce sweet spears with a deep coloration. Indoor gardening offers great opportunities for producing all three asparagus types for a year-round harvest.
The asparagus plant is fascinating to grow. Often considered a temperate-zone crop that becomes dormant through winter, the plant doesn’t actually require cold conditions as part of its life cycle. If sufficient warmth is provided, the plant won’t need to go into dormancy to survive a cold winter and will produce spears that develop into a ferny foliage year-round. A warm indoor hydroponic garden is a perfect environment for small-scale asparagus production, as the plants can be easily grown in bags, buckets or beds of hydroponic substrates such as coconut fiber, perlite or rockwool.
Asparagus Planting Stock: Seeds or Crowns
When starting out with hydroponic asparagus, there are two options for obtaining planting stock: seeds or crowns. Asparagus crowns are usually sold as planting stock consisting of a central fleshy crown and root system. While crowns give a quicker harvest, they are often produced in the field and could carry soil and soil-borne disease into the hydroponic garden. Fortunately, growing asparagus from seed with plenty of warmth and good light doesn’t mean a three-year wait for that highly anticipated first spear harvest. Modern hybrid plants will produce the first few edible spears within 10 to 12 months if well grown, and will be in full production within 18 months. Plants should then continue to crop for many years provided they are not overharvested and are able to produce sufficient assimilate from mature fern growth.
Asparagus seeds typically germinate within 10 to 14 days under ideal temperatures between 71 to 79°F. The seedlings are fairly hardy and although lower temperatures will slow growth, the plants can handle cool conditions and are tolerant of high salinity. As with many crops grown hydroponically, genetics are vital to ensuring a good crop and high yields of edible asparagus.
Older varieties of asparagus are much slower to start cropping than modern hybrids, are lower yielding and prone to a number of root rot diseases and also produce both male and the less desirable female plants.
Most commercial hybrid asparagus varieties produce all-male plants, as female plants will at some stage flower and set fruit (berries), which drains reserves from the plant crown and lowers the yield of spears. Female plants, if they do occur in asparagus plantings, are typically culled as soon as they flower and are replaced with male plants. Good hybrids of asparagus that can be grown from seed include Jersey Supreme F1, Jersey Knight F, UC157 and the purple cultivar Purple Passion F1.
After germination, young plants can be potted into progressively larger containers as they grow and the root system develops. At this stage, warm conditions between 75°F to 86°F speeds up the rate of development of the fern (foliage) and the young crown. Light levels can be increased as the plants mature with intensities similar to those provided to capsicum or tomato crops applied.
At around 10 months, the plants should be settled into a deep grow bed or container at least 1 foot deep, with the young crown positioned 2 or 3-inches beneath the surface of the substrate. A well-balanced vegetative nutrient solution can be applied throughout the active growing stage.
Recommended EC levels are in the 2.4 to 3.0 range for mature plants, although higher levels can be run for this salinity-tolerant crop. Asparagus fern can grow quite tall and may need staking to keep it contained in small spaces. The older yellow fern can be cut and removed as it ages to help maintain air flow around the base of the plants.
As the plants mature, the spears (new shoots) sent up from the crown developing beneath the substrate will gradually increase in diameter until they are of edible size. Although the thin spears of young asparagus plants can be harvested, it’s best to leave these to develop into foliage, which in turn produces assimilate for plant growth. Try to refrain from harvesting until the plant is more mature and spear diameter has increased.
Asparagus Plant Management
Outdoors in temperate climates, the normal cycle of asparagus crop production is to allow the plants to grow only fern through summer, building up carbohydrate reserves in the crown. During fall and early winter, the plants stop producing fern and the foliage dies back as the crown goes into dormancy.
Once conditions begin to warm up in spring, the crowns re-activate and start to send up new spears that are harvested for a period of six to eight weeks, and then plants are permitted to go to fern again for the summer. With a heated indoor garden, there are several ways the plants can be managed to either produce spears year-round, as they do outdoors in tropical areas, or as a managed system of forcing spears out of season.
Since asparagus can be grown in a pot or container, these can be moved outdoors for some of the year and shifted back into the heated indoor garden once dormant during winter to force early spear growth long before any local outdoor crops are ready for harvest. The application of heat and moisture will start the dormant, crown-producing spears relatively quickly once indoors.
At this stage, green asparagus can be produced if the plants are positioned under artificial lighting. If white asparagus is being produced, a dark room or lightproof covering over the dormant crowns will ensure the spears become blanched with no green colouration. Once spears have been harvested for around eight to 10 weeks, the grower should stop cutting new shoots so the plant can generate sufficient foliage to replenish the reserves in the crowns that support spear growth.
For a year-round asparagus production system, plants need to be managed a little differently. There is a trade-off between harvesting spears and future yields, or strength, of the plant. Harvested spears don’t get a chance to develop into fern and produce assimilate for the plant, so over-harvesting can weaken the plant to some degree. For this reason there has to be a balance between the amounts of shoots harvested as spears, and those allowed to fern to keep the plant healthy and growing.
When the plant does not become dormant at any stage, the mother fern or mother stalk method of harvesting can to be used. In this case, the plants first produce some foliage (at least four or five large shoots of fern), then successive spears are harvested (usually around 6- to 8-inches long) for a period of time, then a further three or four shoots of fern are permitted to develop, then further harvesting of spears.
Older foliage is removed as it ages and turns yellow so that the plant always maintains four or five healthy shoots. The mother fern method allows the mature foliage left on the plants to provide assimilate for the spears that are cut for eating, thus preventing the plant from becoming weakened by continual harvesting.
This means there are always some mature ferns on the plants that have to be carefully parted during the harvesting phase to find and cut the new spears as they emerge. While this is a little more time consuming than cutting from crowns as they come out of dormancy (and don’t have any canopy of foliage to negotiate), it does ensure year-round cropping that vastly extends the traditional eight-week spring harvest period.
Mature plants can always be forced into dormancy if they need to be shifted out of the indoor garden for a period of time, or as another method of managing spear growth and harvesting. This is done by inducing drought conditions by withdrawing irrigation for a month, which has the same effect on the plant as when temperatures drop in fall and winter outdoors. Plants can then have the dying foliage cut back are kept in this dormant state until growth needs to be resumed with warmth, nutrients and moisture.
Commercially, all these methods of asparagus production are carried out in different climates, although early spring spears produced from plants coming out of winter dormancy are often considered of higher quality than those grown in tropical environments where spear growth is more rapid and may be thinner and more spindly.
With an indoor garden, where temperatures can be easily controlled, the problems of excessive heat as experienced by asparagus in tropical growing regions and cold winter dormancy can be avoided and top-quality spears produced.
As an added bonus, hydroponic spears grown with advanced nutrition; control over moisture, light and temperatures; a clean, grit free medium (coco fiber is excellent); and a protected environment are extra tender and succulent as compared to those grown outdoors.