Source: JARS V48:No.4:p201:y1994
Tips for Beginners: Success with Rooted Cuttings
These written instructions are directed primarily toward those members who have not tried their hand at rooting cuttings and who have only limited facilities. If you have a heated greenhouse or a propagating frame but have never attempted to root rhododendron cuttings, you would be well advised to refer to one of the many books on rhododendrons where precise instructions are given
What materials do you need? You will need a clean plastic pot. A round 6-inch pot will hold half a dozen small cuttings. The shallow azalea pots are probably the more suitable, but the taller ones will do. The mix: a 50 / 50 (by dry volume) mixture of peat moss (the ordinary sphagnum peat) and perlite. You will need a rooting hormone formulated for hardwood cuttings - the dry powder form is most often used, or there are others, including the liquid forms, such as Dip 'n Grow. You will need a clean plastic bag big enough to hold the pot filled with cuttings, a rubber band, and plant markers for each variety of cutting. Tools? Just a sharp cutting implement. Sharp is the key word. A very, very sharp knife (the ones with the snap-off blades are good) or a razor blade.
Now we turn to selecting the cuttings. Choose easy to root kinds, either species or hybrids. As a general rule, small leafed ones are easier than ones like 'Anna Rose Whitney' or 'Trude Webster'. Rhododendron williamsianum crosses are usually cooperative - 'Bow Bells', Dormouse Group and 'Cowslip' - and those with R. forrestii Repens Group blood in them -'Elizabeth', Carmen Group, and 'Ruby Hart'. There are, of course, scores of other such hybrids. Rhododendron impeditum is probably the easiest among the species. Not only are the smaller varieties easier to root, but they take up less space. You can try five or six different ones in a single pot.
It is important that cuttings be taken when they are turgid - filled with moisture. That's not usually a problem, but if there has been a dry spell, water the plants well the day before you take the cuttings. The cuttings should be of the current year's growth, and they should be pliable enough that you can bend them to a 90-degree angle. If they are so stiff that they cannot bend without breaking, they are probably too "ripe" and will not root. Another indicator (but not an infallible one) is the color of the stem. As the wood ripens it becomes darker, usually brown. The wood of the cutting should be pale, usually white or light green.
Try to take the cuttings from an upright growing branch. Ones from the side won't stand upright in the pot, and the foliage may touch the surface of the medium, creating disease problems. Cuttings from young plants are likely to root more readily than those from old plants. If you don't plan on potting the cuttings immediately, put them in a plastic bag and pop them in the fridge, or simply put them in a container of water as you would a bunch of flowers. Don't forget to identify them, either by writing the name on a lower leaf or by attaching a plastic pot marker.
To prepare the medium, simply mix the dry ingredients and add water, preferably warm, until you have a moist but somewhat crumbly mixture. Put the mix in the plastic pot and firm it (but not too firmly) so that its surface is flat and within an inch or so of the rim.
To prepare the cuttings, make a right angled cut immediately below the leaf node. (That's where a leaf has grown out from the stem, or where there is a little protrusion.) Then use your sharp knife to remove a narrow strip of bark from about half an inch above the cut running down the cut. The purpose is to expose a large area of the cambium layer where rooting occurs. Remove all but the top four leaves. If the leaves are damaged or large or are likely to touch the medium, cut them back by up to a third of their area. If your cutting has flower buds, remove them. If the cutting is dirty, clean it off in water.
Now put some of your rooting hormone into the cap of the container and dip the cut area of the cutting into the hormone. (It's better not to dip the cutting into the hormone container; you may bring the dirt into the powder.) Shake off any loose powder. Dibble a hole in the medium and push in the cutting. (If you don't make a hole, you probably will brush off the hormone powder.) Firm the mix around the cutting. Put in as many cuttings as the pot will hold, bearing in mind that you must leave room for roots to form.
Water well. The inexpensive spray bottles are good for this and for many other gardening jobs. Now put the pot in a clean plastic bag and fasten the bag above the cuttings with a twist tie. Put a rubber band around the rim of the pot to keep the moisture from running down the side.
Where are you going to put your pot of cuttings? Ideally, they should be in a good light but not direct sun, and the soil should be about 70°F. An east or west facing window sill in a heated room would, at first blush, appear ideal. Unfortunately, most window sills are much colder than room temperature. Heat outlets are often under windows; perhaps you can place the pot above an outlet so that the top of the pot is about at window level. It would be better still to place the pot under a fluorescent light, ideally leaving about a foot between the light and the top of the foliage.
The cuttings need virtually no care until they have rooted. This may take three or four months. Look occasionally to see if any mold is forming or if any of the cuttings are dying. Remove any dead cuttings, and if there is mold, act quickly by spraying with benomyl or a similar fungicide.
Don't be too excited if you see new green shoots-it doesn't necessarily mean that roots are forming. Wait at least two months before checking for root growth. You can do this by gently tugging at the cutting, or you can poke around the stem to look for roots. In either case, be careful not to cause damage. If you have been successful, cover up the pot again (it may need watering) and leave it until about the end of February. You can then put the rooted cuttings in individual pots using the same 50 / 50 mix. Water with quarter strength liquid fertilizer, cover with plastic bags (but leave the pots a little open) and wait until the weather warms up enough to put the plants outside.