JARS v54n2 - Controlled Nutrient System and Rhododendron Culture

Controlled Nutrient System and Rhododendron Culture
Dr. Mark Konrad
Sewickley, Pennsylvania

In recent years, I have become fascinated with plant growth using a hydroponic system. Hydroponics is a specialized method of growing plants in a nutrient solution (soilless culture). Media can be used which are usually nutrient free. With the usual arrangement, the solution is constantly recycled by a circulating pump and refurbished as needed. Appropriate solutions are designed for plant growth and development. The method consistently enjoys some popularity.

I think we are all trying to get rhododendron plants to grow more quickly and to bloom earlier. With this in mind, I decided to experiment with a method which somewhat approximates the hydroponic procedure.

Two and a half-inch peat pots are used which are filled with equal parts of perlite, Canadian peat, and shredded pine bark. Premier PRO-Mix 'BX' combined with shredded pine bark has worked extremely well (see Winter 2000 issue). The pots are placed on plastic trays which allows the solution to circulate around the bases of the pots. The peat pots serve as a wick to draw up the solution. The moisture seems ideally controlled. The solution is added only as needed depending on how heavy the tray seems, indicating the amount of moisture, and how moist the pots feel to the touch.

The solution I designed is as follows:
To a 2-liter bottle of rain water the following is added: several drops of Liquid Miracle-Gro, African Violet Food (which can be increased as plants grow larger) along with small amounts of garden sulfur and fritted trace elements (less than a milligram of each).

Sulfur is essential to life and many plants use almost as much as phosphorus. It is usually absorbed from soil in the form of a sulfate ion. Sulfur comes from many different sources, including soil, rain, fertilizers, insecticides, fungicides, and the atmosphere ( Soil , the 1957 Yearbook of Agriculture, US Department of Agriculture). It becomes part of some proteins, oils, and chlorophyll. Under highly artificial conditions, sulfur may not be readily available to plants.

The method has given greatly improved plant growth, but I am not entirely sure why. I would like to offer the following speculation:
1.  The moisture level is kept under critical control which is very important for proper plant growth.
2.  A steady supply of nutrients is supplied from below which makes sub-irrigation a very important way to stimulate continuous growth.
3.  Supplemental watering is done from above with rain water which may back-flush nutrients.
4.  The peat pots offer a buffering zone and the walls are kept cool through evaporation. Root tips thrive best when kept cool.
5.  The method may simulate natural soil planting in regard to water hydraulics.
6.  Peat pots afford maximum aeration.

Rhododendron seedlings growing in plastic lined flats
Rhododendron seedlings growing in plastic lined flats.
Photo by Mark Konrad

The method has been so successful that I am now using many flats lined with 3-mil plastic. Originally, I got started by placing 2¼-inch peat pots in the lower trough of 10-inch white hanging flower containers. Each can hold five. Because of the simplicity, I would recommend this to anyone on a trial basis.

During the last growing season, newly rooted azalea cuttings grew more than 12 inches (30 cm) on outdoor benches.

Because of the low nutrient need of rhododendrons, it might be difficult to adapt to a true hydroponic method; however, it might be an avenue worth pursuing in an attempt to supply a phosphorus level conducive to early bud formation.

A nutrient system has been described which has the advantage of significantly accelerating the growth of seedlings and rooted cuttings.

Dr. Konrad is a member of the Great Lakes Chapter.