Hydroponic Leeks: A Not so Dirty Business After All

The first job I obtained after graduating from the University of Nottingham in 1957 was to work for a large market garden some 30 miles south of London. F A Secrett Ltd supplied Covent Garden with a range of fresh vegetables year-round, but their main production was oriented toward early lettuce and cauliflower production—and with this in mind they required a regular labor force, which they retained during the winter months by growing large quantities of leeks.

First Stages of Growing Leeks

Early vegetable production required a sandy soil and this—provided irrigation is available—is also ideal for growing leeks. It was my experience that even with a sandy soil, however, harvesting and washing of leeks for market was a very demanding task. It was therefore of great interest to me in 2011 to listen to Dr. Erik van Os presenting a paper entitled Recent Developments in the Netherlands towards the Development of Hydroponic Leeks at the soilless culture and hydroponic symposium held in Puebla, Mexico in May of that year.

Traditionally, leeks are grown from seed sown in nursery beds in the field and the young plants are then transplanted into the field during the summer. As the white blanched stem is the usable product, it is normal to mold the plants up with soil as they grow—because of this, the between-the-row spacing is normally up to about three feet and the in-the-row spacing is normally six to eight inches.

With adequate nutrition, irrigation and weed control, yields tend to be about 30 t/ha. Some efforts have been made to direct-drill leeks in situ at much higher plant densities by markedly increasing the between-the-row spacing, but although this will produce a blanched stem the diameter of the leeks is still normally considered to be too thin for market.

Some Disadvantages of Growing Leeks Conventionally

A major disadvantage of conventional production is that even when a sandy soil is used there is a huge amount of labor (over 600 hours/ha) required to harvest, wash and trim the crop for market. Large quantities of water are also required to wash the leeks—a major environmental problem with all root vegetables.

Harvesting is usually a late summer, autumn, winter and early spring operation, as it is impossible to plant seedling leeks outside too early in the spring since they might go to seed and overwintering leeks tend to go to seed as soon as the weather warms up in the spring.

This is why there is always a two to three month period when leeks are not available in the late spring or early summer. Growing hydroponic leeks might open up the opportunity of producing ‘out of season’ leeks by using a greenhouse.

Harvesting in the field is a dirty operation, as it involves digging leeks either by hand or machine, trimming off the roots and tops and then transferring them to a pack house for washing, final trimming and packing. Some of the outer leaves will almost certainly have to be removed to produce a nice white blanched stem.

It is normal to produce only a single leek crop per year, requiring some eight weeks in the seed bed and at least three months in the field—probably longer if the crop is maturing during the winter months.

Growing leeks hydroponically is a totally different operation—for a start, the time in the nursery is greatly extended in order to produce a larger and taller plant, while the time from transplanting to harvest is significantly reduced.

The Research and Methodology to Growing Leeks

Experience in The Netherlands suggests that from sowing to planting out averages 87 days, while the time from transplanting to harvest averages only 48 days, except during the winter months. Leeks grow much faster in a hydroponic system than in the soil, probably because the nutrition and watering they receive are so much superior to what they get in a soil-based system.

There is still a considerable way to go until the best methodology for producing hydroponic leeks is determined. For example, there is some debate regarding the most effective method for growing the seedling transplants—should they be produced in rockwool, or some other media?

Certainly they need to be grown at high density in order to get a good stem length prior to transplanting, but how can this best be achieved? There is also some doubt concerning the most efficient hydroponic system to use for growing the plants to maturity—clearly it must be a soilless (and medium-free) system, but whether this should be a deep-flow or an NFT system has yet to be determined.

What is clear, however, is that the plants should be placed in eight-inch long pipes (to obtain a blanched stem) and that the pipes should be held in place by the NFT or the deep-flow cover.

Of course productivity is the name of the game and because it is no longer required to mold up the leeks they can be grown much closer together between the rows. In fact, there appears to be no reduction in yield per plant up to a density of 10 plants per square foot, which gives us in some 40 days a yield of close to 80 t/ha.

This compares more than favorably with the 30 to 40 t/ha achieved in the field in a year, with the added advantage that harvesting simply involves pulling a nice clean leek plant from the hydroponic system, cutting off the roots and top and possibly peeling off one or two leaves.

The question in my mind is: do we need to grow this crop outside, or would it be better to grow it year-round in a greenhouse?

If we are able to grow an 80 t/ha crop in 40 days in the field, then presumably we should be able to produce a similar yield in a slightly shorter time in a greenhouse—so it should not be impossible to achieve 10 or even 12 crops per year using such a system.

At 80 t/ha from each cycle, we would be looking at something in the region of an amazing 900 to 1,000 t/ha of quality leeks per year.