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How To Propagate Rhododendron

Growing Rhododendron from Seed

  • Required
    • Seeds
    • Containers: 
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2” or 4” pots
  • Medium: 
    • 50% Peat, 50% Pearlite, pre-wetted
    • 100% Peat, pre-wetted
  • Plastic wrap or plastic bags
  • Labels & marker
  • Spray bottle
  • Fertilizer (Miracid or similar)
  • Optional
    • Fungicide

    Principles of Growing from Seed:

    1)      Seed Storage:  Seed should be stored dry, in a plastic container in the refrigerator or freezer.  Kept this way, it will stay viable for years.

    2)      Cleanliness:  Use new or cleaned pots or containers.  Clean other equipment before use.  Use sterilized water (boiling for 20 minutes will do) or well water to wet the media.  Work on a clean surface or newspaper.

    3)      Water Quality:  Water used to water the seedlings when inside the house must have a low salt content.  Softened water should never be used.  Some wells contain a high level of sodium (salt).  Over time this may build up in the medium and damage/stunt the seedlings.  Best free source of water:  rainwater via a barrel under a downspout.

    4)      Light:  Rhododendron seeds need light to germinate.  They must be sown on the surface and kept in fairly bright light.

    5)      Seedling Hydration:  Medium must be kept moist, but not soggy both before and after the seeds germinate. The medium cannot be allowed to dry out completely or the seedlings may be lost.  The smaller the seedlings, the more vulnerable to dehydration. They should be checked at least every 2-3 days.  Keeping the humidity high around them helps.  Plastic tents can be constructed to help with this.

    6)      Warmth:  The warmer they are kept, the faster the seeds will germinate, and the faster the seedlings will grow.  The ideal is between 70 F and 80 F.  Near a wood stove or radiator, the top of a refrigerator or other such areas work well.  Seeds will germinate, and the seedlings will grow at lower temperatures, however, just not as quickly.

    7)      Safety

    a)      Chemicals:  Fungicide can help keep mold under control, but is not generally needed.  Use with caution.

    b)      Peat:  Wear a dust mask when handling dry peat and mixing it with perlite.  On rare occasions, it has been known to cause a lung infection when inhaled.  Wear gloves if you have cuts on your hands.  On rare occasions, infections have resulted from bacterial spores contained in wetted peat.




    1)      The best time to plant, in my experience, is fall – October to November.  This allows for almost a full year of growing time before the seedlings have to face their first winter outside.

    2)      Prepare medium by pre-wetting the peat until well moistened, but not soggy.  Kneading in warmed water in a large tub is the fastest method.  Put aside ¼ of the wetted peat.  Add an equal part pearlite to the remaining peat.  Add additional water to the mixture, as needed.

    3)      Fill clean containers ¾ full with peat/pearlite mix.

    4)      Fill remainder with pure peat.

    5)      Spread seed thinly on the surface.

    6)      Using a spray bottle, “spray in” the seeds with sterilized water, well water, or optionally, a solution of fungicide.  This causes the seeds to make good contact with the medium.

    7)      Cover containers with plastic wrap or place in an air-tight transparent bag or container.

    8)      Place in a warm, well lit location.  If available, use fluorescent lighting for 18 hours/day.  Alternatively, place in or near a window where they will get bright light, but not a lot of direct sun.

    9)      Germination will begin in 2-4 weeks, depending on temperature and seed type.

    10)  As the seedlings develop their first true leaves, begin poking holes in the plastic to acclimate them to room humidity.  Gradually remove the plastic.  At about the same time, begin bottom watering with very dilute fertilizer (1/4 tsp./gallon.)

    11)  Thin seedlings.  Remove weaklings and space out remainder, leaving best 20 – 40 plantlets.

    12)  When they have 3-5 true leaves, transplant into individual 2” pots.

    13)  Continue to provide a warm, lighted environment.

    14)  When seedlings begin to crowd each other in the 2” pots, transplant each into a 4” pot.

    15)  Place pots outside in a shaded location after danger of frost has passed (around mid-May).  They should not be placed in direct mid-day sun.  The north side of the house, or under high shade is good.

    16)  Continue to lightly fertilize, but not after July 1.  A small amount of time-release fertilizer formulated for acid-loving plants can be placed on the surface when the plants are first placed outside.  None should be added thereafter.

    17)  Hardier species and hybrids can be lined out directly into a nursery in the early fall. They need not be given any winter protection.  The nursery should be in an area with a considerable amount of shade, and without root competition from trees with aggressive root systems, such as maples.  In a border along the north side of a building works well for small numbers of plants.

    18)  More tender species and hybrids should be given some degree of protection the first winter.  Below are suggestions:

    a)      Place in a cold-frame.

    b)      Plant in or on the ground and cover with a wooden box between December and March.

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