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Propagating rhodies

Q: What’s the best time of year to take cuttings to propagate rhododendrons, and are all rooting hormones created equal?

David Lawrence, Asheville, NC

A: Marlo Doherty, chief propagator at  Twombly's Nursery  in Monroe, Connecticut, replies: Rhododendrons are best propagated from cuttings taken in mid-September through November, usually after the first frost has occurred. The frost hardens up tissue and ensures that the stem cutting will not rot during the rooting process.

Cuttings should be 4 to 6 inches in length, and taken from the most recent growth. I have better success using side shoots as cuttings rather than terminal shoots that have set flower buds for next year. To help concentrate the cuttings’ energy on rooting instead of flowering, you’d have to pinch out the flower buds, and this often tends to slow the rooting process. I also only keep 4 to 5 leaves on each stem cutting to focus the stems’ energy on rooting.

Before dipping the stems in rooting hormone, use a small pocketknife to peel away a thin layer of bark on one side of the stem tip, or on opposite sides. Both ways are effective, so it’s basically a matter of preference. This wounding technique brings the rooting hormone into contact with the stem’s cambial layer. That’s where its natural hormones are centered, and where the rooting activity is going to happen.

I recommend using talc-based rooting hormones on semi-hardwood cuttings, and saving liquid hormone for hardwood cuttings. Buy a talc-based product that offers the highest percentage of IBA—a synthetic form of a natural plant hormone—because rhododendrons can be notoriously resistant. Only the tips of the cuttings should be dipped in the hormone, and then placed in a mix of one part perlite and one part peat.

At the nursery’s greenhouse, I have the luxury of a misting system to keep my cuttings consistently moist and warm. At home, I suggest a regimented watering schedule and an inverted plastic bag placed over the stems to keep the humidity level constant. A heating mat is a good way to provide constant warmth. Within six to eight weeks, your cuttings should develop roots. When they do, they will resist a gentle tug. 

From Fine Gardening 64, pp. 12