How and When Herbaceous Cuttings are Stuck Influences Winter Survival

Richard E. Bir
North Carolina State University

Nature of Work: Herbaceous perennials are frequently purchased as dormant, leafless rooted cuttings or liners for potting into larger containers for spring and summer sales. Occasionally these liners produce no new top growth yet roots appear to be healthy.

The objective of this study was to investigate whether the time of propagation or the depth of cutting penetration into rooting media has any effect on survival and subsequent growth of plants reported with this problem. Test plants were Caryopteris divaricata, Monarda 'Raspberry Wine' and Phlox paniculata 'Robert Poore'. Cuttings from container grown stock plants at the Mountain Horticultural Crops Research Station, Fletcher, NC, were stuck in mid June, mid August and early September 1997 except for Phlox 'Robert Poore' where only June and September cuttings were stuck due to a lack of sufficient propagation material in mid August.

Cuttings were treated with 1250 ppm IBA (C-Mone) quick dip and stuck into a 1:1 sphagnum peat:perlite (v/v) rooting medium in 60 cell flats and placed under intermittent mist until rooted. Half of the cuttings were stuck so that a node was at least 0.5 in. beneath the surface of the rooting medium (+) while the others were stuck so that no buds were beneath the surface of the rooting media (-). The number of cuttings stuck per treatment depended upon availability but no fewer than 18 cuttings per date and location (+,-) treatment were stuck in 3 replicates for any test plant.

Once rooted, all cuttings were transplanted to quart pots in standard MHCREC potting media (8 pine bark/1 spagnum peat to which 7 lbs dolomitic limestone and 3.0 lbs Esmigran was added per cu. yd.), topdress fertilized with 0.25 tsp/qt. Wilbro (Polyon) 12-6-6 Nursery Special for the June cuttings or with Peters 20-20-20 at 100 ppm N weekly until October 1 for the summer rooted cuttings. Plants were placed under overhead irrigation at the MHCREC container research facility where they were exposed to ambient temperatures until late November when they were moved to an unheated white plastic covered overwintering structure. On February 2, 1998 all liners were moved to a heated greenhouse to encourage vegetative growth. Percent survival was determined on March 13, 1998. Those showing no vegetative growth were determined not to have survived.

Results and Discussion: All cuttings rooted in high (over 90%) numbers and produced mostly vigorous liners. Cuttings with limited vigor were not kept as part of this study, i.e., all liners were alive and vigorous entering winter.

All cuttings were exposed to twelve sub-freezing (lowest temperature 18 F.) nights before being moved to the overwintering structure. Leaves had been killed and most had abscised. However, stems were not cut back until after new growth appeared in the greenhouse. Percent survival for all plants is shown in Table 1.

Table 1. Percentage survival Monarda 'Raspberry Wine,' Carypoteris divaricata and Phlox 'Robert Poore' following winter as affected by date and method of propagation.

Survival of Monarda 'Raspberry Wine' was excellent. All treatments survived at 100% except the September cuttings without nodes beneath the propagating medium (-). Therefore, placement of cuttings for this cultivar only seems important for late stuck cuttings.

Caryopteris divaricata survived in commercially acceptable percentages only when cuttings were stuck August 13 or earlier and only when nodes were stuck below the surface of the rooting medium (+). Cuttings stuck in September did not survive in acceptable percentages regardless of node location.

Phlox paniculata 'Robert Poore' survived in commercially acceptable percentages only when nodes were below the surface of the rooting medium (+).Surival percentages were 100% on both propagation dates when nodes were beneath the propagating medium.

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North Carolina State University