QBARS - v35n2 Propagation of Rhododendrons

Melvin C. Allen, Santa Cruz, CA
De Anza Chapter

Principles in plant propagation:
1. Age or condition of cutting wood
2. Taking the cutting
3. Wounding
4. Hormone treatment
5. Rooting Medium
6. Inserting the cutting in the rooting medium
7. Air temperature and humidity
8. Light
9. Rooting and transplanting
10. Sanitation

1. Time Spring (June, July) Fall (Oct., Nov.). Calendar only a guide. Condition of wood is the important factor. Current year's growth with terminal bud completely developed. Leaves full grown and about the same color as old leaves. Stem firm, not fleshy.

2. Taking the Cutting . Preferably small slender shoots 3 to 4 inches long, taken from the inside or underside of the plant. Take cutting in the cool of the morning, immerse in water preferably with one tbsp. Captan per gallon. When the cuttings have been taken from the plant, take them to a cool, shady place with a clean table surface wiped down with Captan. Use a sharp knife to cut off the base of the shoot at an angle just below a bud to make a cutting 3½ - 4" long. Remove all the leaves but the top whirl. On large leaf varieties, cut the remaining leaves in half.

3. Wounding the Cutting . On large leaf varieties, cut the bark down to the cambium layer on each side of the cutting, one inch from the bottom. On small leaf varieties, wound only on one side. Try to avoid cutting off a bud. After the cutting is made and wounded, dip in Captan solution. Drain.

4. Hormone Treatment . After the excess water is drained from the cuttings (up to 100) they are ready for "sticking" (planting in the rooting medium). First, dip the cut end in a hormone powder. For soft-wood cuttings, Hormex #3 (0.3% Indole Butyric Acid). For hard-wood cuttings, Hormex #8 is about right. These hormones stimulate root growth, but if too strong, may burn the stem tissue.

5. Rooting medium. The rooting medium should be prepared well in advance so that it can be moist and aerated. I prefer 50% Perlite and 50% coarse Canadian Peat Moss. However, equal parts of coarse sand and peat moss or vermiculite may be used.  The rooting medium should be from 4 - 5" deep. In the nursery, we use bulk beds with electric heating cables in the bottom, with fine mesh screening 3" above the cable, suspended on 2 X 3's, with 4 - 5" of cutting medium on the screening. This gives excellent drainage and aeration as well as good distribution of basal heat. We also use flats made of 4" bender board which will take 100 large leaf cuttings. Each system has its advantages.

6. Sticking the cuttings. After the hormone powder has been applied, tap off the excess, make a small hole in the cutting medium and insert the cutting 1 ½ - 2" into the medium. Then press the medium firmly down around the base of the cutting and water till the water flows out of the bottom of the container.

7. Temperature and humidity . The basal temperature should be held as close to 70° as possible and the surface temp. somewhat less. This promotes root growth without stimulating top growth. Humidity may be controlled in a closed system or by misting.

8. Light . The cutting should be stored in as much light as possible, with no direct sun. A grow-light over radiant heated floor would be ideal.

9. Rooting and transplanting . Most broad-leafed rhododendrons and deciduous azaleas will root in 2 - 3 months. Small leafed rhododendrons and evergreen azaleas in 6 - 8 weeks. The first two weeks are most critical. If the cutting is still turgid, has not dropped its leaves, or turned black at the base in two to three weeks, its chances are good.

This evening we are going to demonstrate the planting of individual cuttings in foam cups, each enclosed in a plastic bag. Leave the bag closed tightly for 3 months. Then remove the bag and gently feel of the cutting. If it appears to be rooted, leave bag off, keep the medium moist for a couple of weeks, then transplant into a 6" pot or a gallon can of high organic matter and moist soil (75% peat or compost and 25% soil). Leave it in a protected area until the weather moderates in the spring. (Any cuttings not rooted in 3 months could be resealed immediately and checked again in 30 to 60 days).

10. Sanitation . All through the process of making and rooting cuttings, sanitation is very important. Dip your hands, clippers and knives in a solution of Captan. Wipe off table tops, flats, etc., with Captan solution. Never re-use cutting medium for cuttings.

Note: If you are really interested in propagating all kinds of plants, make a small cold frame with a thermostatic heating cable. A 4' X 6' frame shouldn't cost over $25. and you could grow $100, or more worth of plants every year.