A Hobbyist's Method of Growing Rhododendrons From Seed
Molly Grothaus, Lake Oswego, Oregon
We start our rhododendrons from seed in a hobby greenhouse which we try to keep several degrees above freezing. Our heating equipment will only hold out 15° of frost, and several times during January and February the temperature in the greenhouse may slide a few degrees below freezing during the night.
Seed planting is delayed until mid February so that the rhododendrons emerge as the light hours are lengthening and outdoor temperatures rising. The small plants experience no slowing of growth and February started seeds have produced for us a higher percentage of sturdy plants with fewer losses than have seeds planted in November, December or January.
The major problem facing the amateur growing rhododendrons from seed is the control of the post emergent diseases, in particular damping-off which can kill a stand of seedlings in a few days. Some rhododendron seed is truly irreplaceable, but at the very least the loss of a seedling batch is the loss of a year's time.
In the past eight years I have raised more than a hundred different rhododendrons from seed and have found that the following method of growing them gives complete control of damping-off under our growing conditions, and should be equally effective for growing rhododendrons from seed in a cold frame.
The seed envelopes, as soon as received, are stored in a screw top jar on the bottom shelf of the refrigerator which offers cool convenient storage at a constant temperature. A pinch of seed in a 3 inch pot will provide all the rhododendrons that the average amateur wants; in fact he may find 30 to 50 rhododendrons of one variety an embarrassment of riches. A quarter of an inch or more of space around each seed will allow the development of adequate roots for transplanting and allow air circulation between the seedlings, lack of which encourages damping-off to develop. It is important not to crowd the seed and better to regard a little seed left in the envelope as insurance against the dropped pot or other unforeseen calamity.
The planting mix consists of ⅓ chopped baled peat moss, ⅓ compost or loam and ⅓ sand. Unless we have glacial sand available, both the sand and compost are sterilized by steaming. A container which will fit on a rack over boiling water in a larger covered pan works well as a steamer for the small amounts of sand and compost needed. Steaming is continued for an hour after the material reaches 180°. If a candy thermometer isn't available to plunge half the depth of the soil being steamed, a wall thermometer with the bulb end wrapped in aluminum foil is satisfactory.
Three inch pots are scrubbed and boiled twenty minutes and enough ½ inch rock to provide the drainage needed in the seed pots is also boiled. The pots with a layer of rock on the bottom are filled to an inch of the top with the completely cooled sterilized sand and compost to which the peat moss has been added. The pot contents are leveled gently and topped with half an inch of chopped sterile sphagnum moss which has previously been dampened. (The sphagnum has particular value in preventing the development of soil fungi.) The rhododendron seed, spaced on this surface, are lightly dusted with more sphagnum to barely cover. The seed pots are set in the greenhouse and protected from direct sunlight, but are not covered. Because the sphagnum topping and the compost beneath absorb water at different rates, it is better to water the pots from the top. A rubber bulb hand sprinkler can gently flood the surface without disturbance and the seedlings will send strong roots into the mix below. Bottom watering is apt to drown the compost by the time the sphagnum is sufficiently dampened and the excessive water in the compost may have an adverse effect on root development. This is avoided by continuing to water from the top.
If the seed has not been crowded, the seedlings can be grown on and fertilized in the seed pot until they reach a convenient size (½ inch) for transplanting. The seedlings are transplanted into a greenhouse bed and grown there until fall, or held until spring if their size would require too much watching to protect against heaving, or washing out, in an outdoor nursery bed.