A New Approach to Seedling Culture
Dr. Mark Konrad
Sewickley, Pennsylvania

In recent years I have become quite envious of the culture found in greenhouses, i.e., producing liners for flowers and vegetables. It all seems so simple: remove the plug and plant directly into the soil, usually with good results.

The questions I asked myself: could this technique be adapted to rhododendron seedling culture? Would it be a major benefit to take a plug and plant directly into the soil?

The first order of business was to analyze why the method is successful. Two things stood out immediately. First, the use of highly specialized professional mixes have the right pH, consistency, and added nutrients. Second, I feel, the type of container used is of prime importance. Generally, they are deep and narrow, which tends to trap and conserve moisture. Usually, they are also small, which allows for the plant to become easily root bound, thereby making for an easy transfer to the soil. On these basic premises, I decided to see if the results could be duplicated with rhododendron seedlings.

The first task was to acquire the right professional mix suited to rhododendron seedling culture. Premier Horticulture of Red Hill, Pennsylvania, supplied me the technical data on their PRO-MIXES as follows: PRO-MIX 'BX', a general purpose growing medium, is a peat-based professional growing medium designed for the cultivation of horticultural greenhouse plants. It is lightweight, uniform, and blended to the highest degree of consistency to provide conditions necessary to establish plant growth. It is suitable for a wide variety of plant species. It is composed of 75-85 percent sphagnum peat moss (by volume) and horticultural grade perlite and vermiculite. It contains macronutrients, micronutrients, dolomitic limestone, calcitic limestone, and a wetting agent. Its pH range is 5.5-6.5. It is essential to begin fertilization within seven days after planting and maintain fertilizer applications throughout.

PRO-MIX 'PGX', a plug and germination growing medium, is a peat-based growing medium formulated for germinating and growing on of ornamental and vegetable seeds with plug systems. Seedlings produced in plugs are grown under high density conditions, which means developing root systems are limited to the confines of the plug cell. This type of culture requires a highly specialized medium that can provide optimum physical and chemical properties to promote seedling growth. It is composed of 65-75 percent sphagnum peat moss (by volume) and finely granulated horticultural vermiculite. It contains macro-nutrients, micronutrients, dolomitic limestone, calcitic limestone, and a wetting agent. Its pH range is 5.5-6.5. Additional nutrients must be applied at the grower's discretion to insure plants receive proper nutrition. The main difference between the two is the lower fertilizer charge and the deletion of the perlite from the plug and germination growing medium (PGX).

The usual pH range of Canadian peat is between 3.8 and 4.3. The corrected range is quite satisfactory for rhododendron culture with the following caveat: water with bicarbonate and carbonate content should be avoided to prevent an unacceptable increase in the pH. The easiest solution is to use rainwater. The PRO-MIX 'BX' even though slightly stronger has worked well with my seedlings.

Figure 1. Varying sizes of plugs may be used. Photo by Mark Konrad

The following has been my approach to the so-called plug method. Small cell paks were obtained in the following sizes: ¾" x ¾" x 1¼" is ideal for the first transplanting. This was followed by transplanting into the 1½" x 1½: x 2" size. Transplanting was surprisingly easy with a pointed pencil used as a dibble. This was done after the seedlings developed several true leaves using milled sphagnum moss. The cell paks are easily cut into any size with the use of scissors. Many different sizes and shapes are available (see Figure 1). An option for transplanting is the placement of newly germinated seeds into each cell module with the use of a toothpick.

Figure 2. Hanging basket pots with 36 plugs, 3/4" x 3/4" x 1 1/4" Photo by Mark Konrad

The following method was used indoors under fluorescent lighting. Cell paks are cut to size to fit into the bottom of 10-inch white hanging basket pots (see Figure 2). The pot can hold thirty-six of the smaller cells and twelve of the larger ones if the corners are trimmed. Plastic sheeting (3 mil) is laid over the pots which affords ideal moisture and temperature control (see Figure 3). Twenty-four-hour lighting gives faster growth.

Figure 3. Pots with plastic sheeting in place. Photo by Mark Konrad

One advantage with rhododendrons is the lower need for light intensity. Bedding plants will only do well with heavy light intensity which can only be supplied by a greenhouse. The disadvantage is the slow growth compared with bedding plants.

Another advantage with the method is the luxury of watering from below. A very small amount of liquid fertilizer and garden sulfur can be added as needed. All nutrients must be replaced as used. Sulfur is essential to plant life and may not be readily available under highly artificial conditions. It is interesting to note that Premier Horticulture has other specialized media, one of which is PRO-MIX 'BX with MYCORISE'. Unfortunately, it is not specific for rhododendron culture.

I would like to speculate that the cell paks have certain advantages:
1.  Having the plants become root bound early might help with the nutrient availability.
2. The plastic is very flexible which may help with the formation of air pockets and the availability of oxygen along the sides.

At this point, I would like to refer the reader to Premier Horticulture technical information that could be helpful to rhododendron growers as shown below.

Summary
The results have given exceptional improvement. The method allows for better control and organization, easier transplanting and more rapid and uninterrupted growth. Many variables are eliminated.