JARS v51n1 - Tips for Beginners, Hybridizing Notes: Planting the Seed

Tips for Beginners, Hybridizing Notes: Planting the Seed
Jim Barlup
Bellevue, Washington

The purpose of this article is to describe my method for planting seed from the rhododendron crosses that I make using minimal equipment and a small amount of space in my basement. My goal is to plant 52 crosses each year with the expectation that 40 of them will germinate and provide 750-1,000 seedlings to transplant outdoors in the spring. You can reduce or expand this scale to meet your needs.

I determine the time for planting the seed according to when I expect to transplant the seedlings outside. Since I don't have a greenhouse, it's too cold to transplant before the beginning of May, but I must move the seedlings outside before they outgrow the planting case or become too leggy. I found that the beginning of January is the optimum time for me.

Planting case and equipment

A 2' x 4' x 14" plywood box on 12-inch legs is adequate for 104 4-inch pots. It has a ¼-inch screen in the bottom with braces below the screen. There is a 1-inch Styrofoam pad on the screen with holes in various places for drainage. I place a plastic tarp under the planting box and attach it to each leg to form a big trough to catch the excess water. The cover is a frame with glass that fits over the top. A high quality heat cable is a worthwhile investment. It helps in the germination by keeping the temperature at a constant 70°F.

I place a heavy duty shop light with two 48-inch bulbs directly on the case and attach it to a timer. There are several types of bulbs that can be used in various combinations. Any of the following will work: Gro-Lux (becoming difficult to find), Vita Lite, different kinds of wide spectrum bulbs, and a combination of warm white and cool white fluorescent bulbs. In my attempt to create compact plants and at the same time to accelerate growth, I find that one Gro-Lux and one high quality wide spectrum bulb work best.

I started my first seed in two pots in 1975. The pots were covered with Saran Wrap secured with a rubber band. They were kept indoors by a west window with no additional light. The seed sprouted and the plants grew. I had very little idea of what I was doing, but it worked. I was on my way! You may want to start on a small scale without a planting case, but lights are helpful in speeding up the growth.

Preparing the case:

Fill a container half full of peat moss, add water and mix to a wet but not dripping consistency. Pour the damp peat moss into the planting case to a depth of 4 inches. Place the heat cable on top of the peat and then add another 2-inch layer. The 6 inches will settle to about 5 inches. This 5-inch depth of peat holds moisture well and elevates the pots to within 5-6 inches of the lights. Initially there may be water coming through from the weight of the peat. After the first few days this seepage will subside and the plastic can be removed. The peat will stay moist through July. Give the top of the peat a good dousing with Captan to keep down the green mold.


The most difficult part of the whole process comes next. I must pick only 52 crosses from the more than 200 seed packets. Each one tempts me to plant it, but I must be selective and eliminate almost 75 percent of the crosses. Knowing that I can plant the seed another year makes the task easier. I start with 52 4-inch pots under one light and expand to 104 under two lights when I repot. Fill a bucket ¾ full with a good grade of peat and fine vermiculite. Half of each seems to be a good ratio, but this can be varied anywhere from 100 percent vermiculite to 100 percent peat. Mix with water to a damp consistency. I generally work with 10 pots at a time. Fill the pots with the mixture to within 1 inch of the top, and then add ½ inch of No Damp Off sphagnum moss. Work with each pot separately so you don't contaminate the seed. Open a seed packet and tap the seeds evenly across the top. Excess seed can be stored for future use in the event that the cross turns out to be exceptionally good. A sprinkling of dry No Damp Off on the top provides a rooting base. Place a marker showing the cross in each pot. Spray the pot with water to moisten. Set this pot aside and follow the same process with the next one. Place the pots in four rows in the center of the case. Dust lightly with Captan and mist with water. Place the cover and light on the case and set the timer for 18 hours on and 6 hours off.

Caring for the seedlings:

Gently mist the seedlings every three to four days with water. Watch for fungus growth and spray with a fungicide in the same way at the first indication of fungus. Continue this process until the seedlings are repotted.


After approximately two months, the seedlings will develop two or more leaves and some of the pots will be crowded. It is now time to begin repotting. I generally use the same mix as for starting the seed but without the No Damp Off. This year I used a high grade pre-mix. The seedlings grew very well, but when I transplanted them outside I found that they hadn't rooted well. I was surprised that a plant that looked so good had so few roots. The right mix is very important.

Some pots may not germinate at all, some may have only a few seedlings, and others may have a hundred or more. I place up to 20 seedlings in a 4-inch pot. If the cross looks good, I may plant two or three more pots of the same cross. What an emotional task it is to discard excess seedlings! I can expand the number of pots to a maximum of 104 in one case. Add a second light if you have more than 52 pots. Turn off the heat cable at this time.

Care of the seedlings after repotting:

Begin to fertilize immediately after repotting. I fertilize and water simultaneously once or twice a week using a 13-ounce rubber bulb syringe with a spray nozzle. Fill with water and add 7 drops of Schultz Instant Fertilizer. This is more than the recommended amount but it works without burning the plants. Miracle Gro at half strength also works well. Be careful that the peat mix doesn't dry out. Otherwise it is difficult for it to absorb moisture. You can detect dryness from the weight of the pot. If it does dry out, you may be able to resurrect the seedlings by soaking the pots in a container of water until they become moist throughout.

The first week of May I transplant the stronger seedlings and place them in a cold frame outside. The indoor feeding and watering continue for the weaker plants until the beginning of July when they are all transplanted outside.

Addendum: Sometimes things just don't go right. I thought if I share the mistakes I made last year you won't have to make them yourself. Five weeks into germination, the heat cable died of old age. At this point, everything slowed down, but it was too late to replace it. After the sixth week, I found that my lights were quite weak. Then at the two month repotting stage, I inadvertently sprayed all my new seedlings with carpet cleaner. That made them mad and I wasn't sure that they were going to forgive me. (I no longer store the plant chemicals with the housecleaning products.) I proceeded with the fertilizing for about a month before I realized there was a big chunk of solid material at the bottom of the bottle. Was it the nitrogen or what? The seedlings hadn't grown a bit during the past six weeks. From now on, I'll use fresh fertilizer each year. As of Sept. 1, 1995, I had 1,000 unusually small seedlings in the cold frame. Thus, an unexpected experiment: planting ¼-inch seedlings in September. I was relieved to find how tough little rhodies can be. The 11°F winter temperature didn't phase them one bit! Although my plants survived all this mistreatment, they deserved better. I'm paying a lot more attention to details now!