The list of crops which can be grown hydroponically is endless. Theoretically, any plant that can be grown in soil can also be grown in a soilless system. Most hobbyists and commercial growers tend to focus on practical or high-value crops that are strongly suited to hydroponics. There are of course some unconventional crops, which can also be profitably grown in a soilless set-up, however impractical they may be. While these crops might not be ideally suited to hydroponics, the applications are sometimes worth pursuing.
By CLIF DROKE
One such crop is sorghum (Sorghum bicolor). Sorghum is in the grass family and is an extremely versatile crop. It can be easily germinated from seed and the plant will tolerate a variety of growing conditions. One of sorghum’s most attractive traits is its tolerance to drought. But while sorghum grows well in arid growing conditions, it absolutely thrives in a hydroponic system in which water and nutrients are constantly available.
There are 25 species of sorghum worldwide, 17 of which are native to Australia. One species is grown for grain while many others are used for animal fodder. The heads of grain sorghum plants can be used to make flour or a nutritious hot cereal, which is valued in less developed countries. Certain types of sorghum seeds can even be popped like popcorn. The plant has also been used in the production of biofuels such as ethanol, and as a sweetener.
My interest in sorghum began with a winter trip to Florida. While driving across the central part of the state just south of Lake Okeechobee, I had the pleasure of witnessing the state’s vast sugar cane fields. Each year I make it my goal to grow something challenging or unusual in one of my hydroponic set ups. This year I was inspired to try sugar cane.
I typically start my plants from seed and was disappointed to find that sugar cane (Saccharum officinarum) is propagated via ‘seed cane’, which involves taking a cut section of the fully mature stalk and planting it in the ground. I was determined to harvest my own sugar cane juice and was upset at the prospects of having to abandon my plans. That’s when I discovered sorghum.
One of the most popular uses of the plant is the ‘sweet cane’ variety of sorghum, which is a close relative of sugar cane. Unlike sugar cane, sweet sorghum can be grown from seed and reaches heights comparable to that of sugar cane, which is anywhere from 12-14 feet at maturity. At harvest, the cane is crushed and yields a juice virtually identical to sugar cane juice in taste and appearance. A little known benefit of sweet cane juice is that it is nature’s perfect energy drink, full of vitamins, minerals, enzymes, and chlorophyll. The natural sugar contained in sweet cane is both delicious and energising. What’s more, it contains less than 15% of the sugar found in the refined version of table sugar we’re all familiar with. Cane juice can also be boiled down into a tasty syrup, known as sorghum molasses.
You might be thinking how cumbersome it would be trying to grow a tall cane-type plant in an outdoor soilless system, let alone an indoor hydroponic system. This doesn’t have to be the case, however. My experiments growing sorghum were surprisingly well suited for both indoor and outdoor crop development. I also found that sorghum plants grew thicker canes and developed faster when grown in soilless media such as rockwool or hydroton than when grown in soil. I also discovered that by harvesting the plants before full maturity I could eliminate the need for using an expensive crushing machine to extract the juice.
I further discovered that by growing certain varieties of sweet sorghum one can even grow the plants indoors under artificial lights from seed to harvest. The ‘Black Amber’ variety is a dwarf-type sorghum, which typically attains a height of six to eight feet, making it practical for greenhouse trials and small-scale indoor applications. It was grown by early American colonists and is still prized today for its sweet golden syrup.
The type I decided to grow was ‘Red Sweet’ sorghum. I planted the seeds in both Jiffy peat pellets and rockwool cubes. My germination success rate was about 75%. The plants seeded in Jiffy develop faster in the initial stage of growth, but the rockwool seeded plants outperformed in the latter stages of development. Germination occurred in two to four days for most seeds.
One advantage of sprouting sorghum in soilless media is that it can be started several weeks before seeds are usually sown in spring. Consequently, indoor-grown sorghum germinated in the winter will mature months before the sorghum traditionally sown outdoors in spring.
My seeds were germinated in January (our winter here in the US) in my kitchen using a heat mat and a plastic dome tray. Once the seedlings were established I transplanted them either to a soilless potting mix in plastic pots (for the Jiffy pellets) or to larger rockwool cubes. The plants were nursed under 23-Watt CFL lights for several weeks before finally being moved under HID lighting after six weeks. Sorghum can be theoretically grown with full-spectrum fluorescent lights, provided that coverage is adequate for the length of the plant (e.g. three-foot T5 strip lights, both vertical and overhead). Metal halide lamps are the preferred light source, however.
For the nutrient solution I used water-soluble MaxiGro fertiliser (NPK analysis 10-5-14) from General Hydroponics. The solution can be pH adjusted to a 6.5 level of acidity, which allows for maximum uptake of all macro and micro nutrients in sorghum. As the sorghum plant grows it can be transplanted to a bigger pot. Sorghum can be grown to full maturity in a three-gallon (11.4L) plastic pot using a rockwool/hydroton media combination.
The nutrient solution can be hand watered or delivered via automatic irrigation tubes at timed intervals. As sorghum is a very hardy and forgiving plant, the leaves will curl inward when the root zone goes dry, allowing the plant to survive several days before dying. In my experience, a once-daily application of nutrient solution that completely soaks the root zone of a three-gallon pot is all that is required to keep the plants healthy.
Sorghum grown for syrup should be harvested when the seeds are fully in the dough stage, which is usually about five-and-a-half to six months after seeding. Outdoor harvesting should be done before a killing frost if possible; if not, the crop should be harvested immediately after the freeze. Leaves should be stripped off before the freeze to lessen the damage. Stalks can be juiced using a traditional sugar cane juicer or, if the stalks aren’t too thick, a roller-type pasta maker can be used to extract juice from the cane. The resulting juice can be consumed raw as a refreshing beverage or boiled down further into syrup.
Sweet sorghum is a valuable but overlooked crop in many Western nations, especially the US. While it was once prized by farmers for its hardiness and variety of uses, it has since been supplanted by sugar cane which is valuable only when grown in extremely high volumes. Sorghum is far easier to grow and should be a staple of every serious gardener’s crop lineup. For the hydroponics enthusiast, sorghum offers endless possibilities for experimenting with the technique and viability of soilless production.
About the Author
Clif Droke lives in Topsail Beach, North Carolina, USA, where he has been involved in hydroponics for 13 years. He is the author of the book, Year ‘Round Micro Gardening (ISBN 097925727). Ω
PH&G July 2016 / Issue 169