JARS v55n3 - How to Grow Rhododendrons from Seed...The Easy Way

How to Grow Rhododendrons from Seed...The Easy Way
William Wilgenhof
Antigonish, Nova Scotia, Canada

Many articles have been written about growing rhododendrons from seed - some excellent, some very basic and some confusing to the layman. Here is yet another we would like to call "The Willow Garden Method."

Our aim is to try to grow hardier and more beautiful plants than are presently available in this area (Atlantic Canada). We obtain most of our seed from the ARS Seed Exchange each year. The packets are stored in an airtight jar in the refrigerator from spring (when the seed is delivered) until fall.

We start our rhododendrons during the first two weeks of October. Our growing area is the basement of our home where we have two growing cabinets each with four shelves. A two-tube 48-inch (120 cm) fluorescent fixture with cool-white tubes is suspended and about 1 foot (30 cm) above the level of the trays. Each shelf will accommodate five propagation trays.

Our growing medium:
• 20 liters (5 US gal) peat moss
• 20 liters (5 US gal) old, coarse sawdust or bark, sifted through a ½-inch (1.25 cm) sieve
• 15 milliliters (1 Tbs) superphosphate (0-40-0)

1. Dissolve the superphosphate (make a slurry) several days before adding it to the above medium. Make sure it is evenly mixed.

2. Moisten the above mix with rainwater to which has been added instant dissolving fertilizer (15-30-15 or equivalent) at a rate of 15 ml per 20 liters (1Tbs/5 US gal) of water.

3. Fill 4-6-inch (10-15 cm) pots with the mix and tamp down to remove most of the air pockets.

4. Mark each pot with the date, plus the source and the seed-lot number.

5. Sow seed as thinly as possible on the soil surface - do not cover.

6. Gently spray the surface with full strength No-Damp, being careful not to disturb the tiny seeds.

7. Place pots in tray with water/fertilizer mix and allow to absorb moisture.

8. If medium becomes too wet place pots on absorbent paper (newspaper) to wick away excess moisture.

9. Place pots in individual plastic food storage bags (not freezer bags) and tie with a twist tie.

Place pots in trays under fluorescent lights, on a windowsill or near another light source. Cool white fluorescent lights with a timer set for fourteen hours light works great. Keep artificial light 12-16 inches (30-40 cm) away from top of the pots. Maintain a temperature of 20°C (70°F) during the day with a slightly cooler nighttime temperature. If using a windowsill, protect from direct sunlight, which can cook seedlings in a hurry! The 100 percent humidity needed for germination is easily maintained by the plastic, which will show beads of moisture inside the bag. Germination will occur in ten to twenty days, and by about eight to ten weeks most seedlings will have developed their first true leaves. During this time nothing much needs to be done except to perhaps pluck out bits of seed debris that may show signs of mold or to thin seedlings, if sown too thick. Inspect periodically, and if necessary gently water and then reseal the bags.

When most seedlings have their first true leaves start poking a few holes in the bags to allow air in and to reduce the humidity. This will prepare them for the "outside" world. Make the holes bigger over a week or two until you can finally open the bags completely. The humidity in our growing area is seldom higher than 50 percent.

Transplant the seedlings into 6-cell packs, using the same mix and carefully labeling each seed lot. Prick out plants carefully, grasping by the true leaf and place one plant per cell. Place packs in trays with water/fertilizer mix as in step 2. Drain and place back under lights for fourteen hours a day. We seldom mist the newly transplanted seedlings.

Do not let the plants dry out. Root systems are very shallow at this stage and drying can be fatal. We water every four to five days with fertilizer/rainwater (1 Tbs/10 US gal water). The well water here has a pH 8.3 so rainwater has a more desirable pH or 5.5. If possible use unchlorinated water and water only from the bottom.

Towards the end of April or early May the plants should be 1-4 inches (2.5-10 cm) tall and ready to be hardened off. This is done following much the same procedure as would be done with most garden plants except for minimal exposure to sun. If time or other circumstances do not allow hardening off, simply wait until outside temperatures are about the same as inside and transfer directly to the prepared nursery bed. The preparation of the nursery bed involves soil amendment with a generous addition or organic material. Place plants in rows about a foot (30 cm) apart with 6-8 inches (15-20 cm) between plants. Label both ends of the row of similar seed lots. We use strips of vinyl siding, about 8 inches (20 cm) long, labeled with a permanent marker on both ends. The end in the soil will be protected from fading, which means you may actually be able to identify plants later on!

Water thoroughly and protect from sun. Fifty percent shade cloth is ideal, but lattice, snow fence or brush will work. Ensure that the little plants don't dry out during the summer. A moderate layer of mulch should be added. We use rotted sawdust/horse manure as both a soil amendment and mulch.

Remove the shade cloth by early to mid-September. Protect the plants from winter wind by erecting a 2-foot (0.5 m) high shelter (of plywood, jute or plastic) around the perimeter of the bed. We now hope for snow!

Seedlings planted out at the Wilgenhof garden
Seedlings planted out at the Wilgenhof garden near Antigonish, Nova, Scotia.
Photo by William Wilgenhof

Come spring some of the plants will be dead, some will look very "ratty" and some will look perfectly okay. This of course depends upon the parentage of the plants. Our observations here at The Willow Garden indicate that the dead and ratty ones are always from a cross having one parent that would be less than hardy for this area. This is, of course, a good way to cull the less hardy or those with poor root systems. In recent years we have had very little snow with many alternating freeze-thaw cycles. This is par for the course in our part of the Maritimes (about USDA Zone 5). Grow the plants for a few years and rogue out the less desirable, keeping only the best to transplant to a larger area.

How many plants to grow?

• Seed from species that are self pollinated will produce plants that are more or less the same as the parents...twenty-five or so would be plenty.

• Seed from a species/species cross will give a fair number of variable seedlings...perhaps 100 plants.

• A species/hybrid cross will give a large variation in plants...perhaps 100-500 plants.

•A hybrid/hybrid cross will give potentially huge variations...anything is possible...500-1000 plants perhaps!

You don't have to be discouraged by the large numbers. It is somewhat like a lottery, but who knows, you could hit the jackpot with only a few plants!

We asked a well-known hybridizer how many plants from one cross to grow in order to have a fair chance at a plant better than either parent...the answer was "only one, except you have to know which one."

How lucky are we? We have grown 10,000-plus seedlings over the last twelve years and have several beautiful plants. It is our opinion that these plants should have a rating of at least 4/4/4 before we will consider them for registration. The effort to get one "better" plant out of every 10,000 is the challenge. The dependable nature of the many species rhododendrons is more than balanced by the undependable nature of the many crosses made by enthusiasts from around the world.

Much of the "fun" is the growing process.

Good luck!

Bill Wilgenhof is a member of the Atlantic Chapter.