The Micro Revolution: Growing Hydroponic Microgreens
Succulent, slender but packing a powerful flavor punch, microgreens are increasing in popularity both on our plates and as a profitable hydroponic crop with just five days from seed to harvest for some species.
Succulent, slender but packing a powerful flavor punch, microgreens are increasing in popularity both on our plates and as a profitable hydroponic crop. While they may seem tender and delicate, most microgreens are easy to germinate and with just five days from seed to harvest for many quick-growing species, even the most impatient grower will be satisfied with the results.
Microgreens are also ideal for those with a limited indoor growing space – as little as a few square inches can grow a crop packed with the distinctive flavor of the mature plant but in a small, compact and highly nutritious package.
On a slightly larger scale, microgreens make a profitable commercial crop, well-suited to soilless production methods, hydroponic nutrition and a protected growing environment to give a high-quality, clean and grit-free product.
What are Microgreens?
Microgreens originated in California, where innovative chefs started using them in new dishes to incorporate color, flavor, texture and interest, a trend that rapidly spread worldwide. Microgreens are used as toppings, garnishes, flavorings in salads and features in many up-market dishes and are sold as a high-value product in produce stores and supermarkets.
Microgreens are larger than a sprout but smaller than a baby salad leaf and will usually have produced at least two true leaves after expansion of the seedling leaves or cotyledons. Because they are harvested at such an immature stage, seed is sown at a high density to maximize yields from each crop.
This also allows the developing seedlings to grow tall and straight with a tender, almost blanched stem and bright, well-developed leaves. While a diverse range of plant species are grown as microgreens, some are produced specifically for their healthy compounds and properties and these have found a niche market within the health food industry as well as being popular with home gardeners.
The most well-known of these is wheatgrass, which has been grown for many years as a health supplement. Other species such as flax, chia, broccoli and varieties of red brassica also have health compounds of interest. The darkly colored purple and red varieties of radish microgreens, which produce intensely colored young leaves, contain high levels of antioxidants shown to have cancer-fighting properties.
Commonly Grown Microgreens
Microgreens fall into four main categories. Shoots and tendrils such as pea, sunflower and corn shoots, are often used as garnishes, although they all have their own mild and somewhat surprising flavor. Spicy greens include arugula, radish, cress and mustards.
Micro herbs include those used not only as garnishes, but also for their characteristic flavor such as parsley, fennel, edible chrysanthemums, cilantro, basil, French sorrel, mint, dill, chives, onion and shisho (perilla).
Tender greens are highly diverse in flavor, leaf size, shape and color, and include red cabbage, broccoli, spinach, beet (red), tatsoi, mitzuna, amaranth, chard, kale, corn salad, endive, chicory, celery, carrot and lettuce.
Hydroponic Production of Microgreens
Hydroponic microgreens have a distinct advantage over those grown in trays of substrates or soil mixes in that no granular growing medium needs to be used. The high sowing rate and density of microgreens means small particles of substrate can end up in the foliage and since microgreens are not usually washed after harvest, this poses a risk of crunchy grit ending up in the final dish.
For this reason, hydroponic microgreens are best produced on a thin mat or capillary pad that holds the seed in place and retains some moisture for germination. Paper towel, hessian/burlap sheets, rockwool cubes or sheets, thin kitchen cloth and hydroponic microgreen pads can all be used to grow a clean, high-quality crop with little expense.
Seeds that have been specifically produced and packaged for sprout or microgreen production should be obtained, which means the seed will have a low percentage of foreign matter, will not have been treated with fungicides or other chemicals and will have been well cleaned.
This is particularly important when buying seeds for pea, corn or spinach microgreens, as the seeds from these species are often coated with fungicide. Seed companies have also introduced a range of specific microgreen cultivars that are a great improvement on standard varieties.
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Many of these feature intensely colored or modified first leaves, such as some of the radish microgreen species and those grown for pea shoots. Some microgreen varieties have seeds that are mucilaginous, meaning that once wetted, the seed forms a thick, gelatine-like layer that holds moisture.
Cress and basil are examples of mucilaginous seed and these seed types should not be pre-soaked before sowing. Larger seeds such as wheatgrass, corn and peas may be pre-soaked in warm water for 24 hours before sowing, although this step is not essential.
Hydroponic systems for microgreens can be as simple as a small, flat, hand-watered kitchen tray or as complex as an aeroponic or nutrient film technique system. Ideally, the growing system needs to have a flat, slightly sloping surface onto which the growing mat/pad paper or cloth can be laid out and wetted down.
The microgreen seed should then be weighed out and sown onto the wetted surface as evenly as possible. Use of seed shakers assists with this process. The correct seeding density depends somewhat on the species being grown.
As soon as the seeds germinate, microgreens require light and nutrients to produce the highest quality product. Artificial lighting doesn’t need to be intense and these young seedlings produce well under propagation lamps provided the lamps don’t produce too much heat, which may burn the tender young foliage.
Once the cotyledons (seedling leaves) are visible and are starting to develop chlorophyll, the seedling will have exhausted the reserves contained in the seed. At this stage, the young plant is starting to photosynthesize and produce its own assimilate and nutrient ions will be absorbed by the root system.
A general purpose vegetative or seedling nutrient formulation is usually sufficient for microgreen production, but wheatgrass has different nutritional requirements, including a higher EC level, for maximum harvest quality. EC levels are typically run at seedling strength for microgreens (0.5 – 1.0 mS cm -1), although they may be adjusted for season in a similar way to lettuce and herb crops.
There is some scope to manipulate growth of microgreens with use of EC – higher EC can be used to boost color development in red types during low winter light if necessary. Control of EC will also affect the shelf life of cut microgreens. Seedlings grown on dilute solutions or in media with a high water-holding capacity can develop softer tissue and a higher rate of water loss post-harvest than those that have received a higher EC and been slightly more hardened.
Nutrient solution needs to be applied regularly and carefully to developing microgreens to avoid flooding the microgreens and wetting the foliage, which encourages fungal diseases, and also to make sure fresh nutrient solution is flushed through the root zone, oxygenating and feeding the seedlings.
Most hydroponic systems used for microgreens don’t use a continual flow of nutrient solution, but intermittent application, followed by a period of drainage with the growing mat/pad or substrate holding sufficient moisture around the roots between waterings.
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Food Safety and Hygiene Considerations
Just as with sprout production, there are food safety requirements for microgreen production. Various fungal pathogens can develop and grow on sprouting seeds, particularly under the humid conditions and high densities microgreens are grown at, and these can pose a contamination risk. Generally, high-quality, viable seed, sown into a clean system at the correct temperature for germination and optimal moisture levels, will germinate rapidly with few problems.
Disease outbreaks are more common when old seeds are used or when temperatures are either too hot or cold for rapid germination. Over-watering also poses a risk as seeds may begin to rot before germination can occur. The water supply needs to be high quality – water can carry human and plant pathogens that contaminate a crop, although the municipal water supply is treated to prevent this risk.
There is an increasing concern and more regulations these days relating to food safety so commercial growers of microgreens need to be aware of the guidelines they must follow for fresh salad crops.
Cutting height is important, as high-quality microgreens need a good, clean portion of stem below the leaves, but should not be cut so low as to risk contamination with the growing media or material the seeds were sown into.
This becomes particularly important if a light, loose, granular media has been used for production, as particles can easily be picked up during the harvesting process and contaminate the product. Clean, sharp scissors are suitable for cutting microgreens on a small scale, while larger growers use mechanical harvesters.
During warm growing weather, microgreens, just as with herbs and lettuce, are best harvested early in the day when the foliage is coolest and most turgid. This will prolong the shelf life of the packaged product. Some microgreens are shipped out to customers while still growing in trays or cells, prolonging shelf life and allowing on site harvesting as required.
Problems with Microgreens
One of the most common problems seen in microgreen production is seedling rot and disease outbreaks caused by over-watering or high humidity levels, which create a saturated environment around the foliage.
Good levels of air movement (such as portable fans in small areas) help prevent issues of root rot, fungal and bacterial pathogens taking hold in the dense canopy of succulent young leaves. Over-mature microgreens may become too tall and flop over, making harvesting extremely difficult, so this is a crop that must be cut at just the right stage.
For the small-scale indoor gardener, microgreens are an ideal crop – fast, productive and requiring just a few inches of well-lit space, they can be easily grown on a warm, sunny windowsill, or incorporated into a high-tech hydroponic system. Their short shelf life makes them a good prospect for local markets and restaurants, as they are best used fresh within two or three days of harvest.
Although there is a degree of skill required in growing these seedlings at high densities and maintaining post-harvest quality, the wide range of species, diversity of uses and growing popularity makes them a great proposition for hydroponic growers.
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Written by Lynette Morgan | Author, Partner at SUNTEC International Hydroponic Consultants
Dr. Lynette Morgan holds a B. Hort. Tech. degree and a PhD in hydroponic greenhouse production from Massey University, New Zealand. A partner with SUNTEC International Hydroponic Consultants, Lynette is involved in remote and on-site consultancy services for new and existing commercial greenhouse growers worldwide as well as research trials and product development for manufacturers of hydroponic products. Lynette has authored five hydroponic technical books and is working on her sixth.