Herb Gardening Information at HerbGardening.com

Hydroponic Herb Gardening

by Harley N. Smith

Why Hydroponics?

Hydroponics is an ideal growing method for producing culinary and medicinal herbs.  Not only do hydroponic herbs grow faster, they have significantly more flavor and aroma than herbs grown in soil.  According to research performed at the University of Minnesota, it is a known fact that herbs grown hydroponically have 20-40% more aromatic oils than field grown.  Therefore, a small hydroponic herb garden can provide a continuous harvest of gourmet-quality produce in a relatively small space.

Many gourmet restaurants have already discovered the value of growing hydroponic herbs on site.  For example, the Golden Rose Restaurant in Mason, Michigan is a French restaurant with a 140 seating capacity.  Using only a 3 by 3 foot square hydroponic herb garden with a 400 watt metal halide lamp, the Golden Rose can produce enough basil to supply all of the restaurant’s needs.  Other restaurants with hydroponic herb gardens include the U.S. Senate Restaurant in Washington, DC, the restaurant at the World Bank in Washington, DC, and many other fine dining establishments around the country.  Hydroponically-grown herbs, cut fresh and added directly to signature dishes, add many subtle flavors and aromatic overtones to gourmet food that dried herbs cannot match!

In Europe, basil is the most popular herb for hydroponic production.  Basil accounts for about 50% of the total herb market in Europe, with the combination of all other herbs making up the other 50%.  Some of the most popular culinary herbs for hydroponic production include: basil (sweet basil, lettuce leaf basil, globe basil, Thai lemon basil, cinnamon basil, Siam Queen Basil, and purple/opal basils), rosemary, mint, lemon verbena, French tarragon, Italian flat-leafed parsley, baby dill, sage, oregano, thyme and cilantro.  Some medicinal herbs are also grown commercially in hydroponic systems, such as Echinacea, stevia rebaudiana, and several Native American herbs.

Many different styles of hydroponic systems with various growing mediums have been used successfully for herb production.  For example, NFT is excellent for growing top-quality basil in commercial greenhouses; drip systems using rockwool can produce six-foot hedges of rosemary; aeroponic systems are excellent for growing Echinacea and other medicinal root crops; and flood-and-drain systems with expanded clay pellets are extremely versatile for producing a wide variety of popular herbs.

For the home gardener, a small ebb-and-flow system is an excellent choice for a hydroponic herb garden.  In an ebb-and-flow system, the plants are usually held in plastic pots in a flood table, and the reservoir is underneath.  A timer clicks a submersible pump on, flooding the roots with nutrient rich water and expelling waste gasses.  When the timer clicks off, the water drains back to the reservoir, pulling fresh oxygen to the roots.  The timer is usually set to flood three to four times per day, with 15 a minute duration per flood.

Hydroponic Basil in a General Hydroponics AeroFlo aeroponic system.

Photo courtesy of the Progressive Gardening Trade Association

Since most herbs are vegetative, a good “grow formula” hydroponic nutrient with relatively high nitrogen to potassium ratio is an excellent choice for growing most herbs.  Low-to-medium Electrical Conductivity (EC), a measure of nutrient strength, is generally preferred, with a EC range of about 1.2-1.8, and a slightly acidic pH of about 5.8 to 6.4 will be ideal for most culinary herbs.  There are a few exceptions, so a little research and experimentation should be done before launching into full-scale production.  For example, parsley and watercress grow well in milder nutrient formulas with an EC closer to that of lettuce (1.0 EC), while English Spinach grows better at much higher EC’s (2.5-3.5 EC).

Most herbs grown indoors do best with full spectrum light with plenty of the blue end of the spectrum.  Metal halide lights or T-5 high-output fluorescent fixtures with 6500K tubes are excellent choices for a hydroponic herb garden.  A 400 watt metal halide lamp will cover about a 4 X 4 foot square area, and a 1000 watt lamp will cover up to 6 X 6 feet square. For best results keep the light about a foot to a foot and a half above the growing tips and raise the lamp as the plants grow.  High output fluorescent lamps, such as T-5, run cooler and can be hung closer to the plants, usually about 6 to 12 above the growing tips.

Adding an oscillating fan to an herb garden is also a good idea.  Good air movement keeps the plants cool while providing a constant supply of fresh carbon dioxide for photosynthesis.  Air temperature of 70-75 degrees is best, with a relative humidity of about 40-60%.  But many herbs are quite hardy and can handle less than ideal conditions from time to time.  In fact, some think that if the fertilization and environmental conditions for herb growing are “too perfect”, the plants will be weaker and not as pungent.  In other words, a little stress is good, as long as the plants are not damaged in the process!

Harley Smith is the Executive Director of Pro-HydroTech, a hydroponic consulting firm with a focus on education with his wife, Sue Smith.