Propagation Prime Time

by Peter W. Gallagher

We often think of spring as the great season of renewal and rebirth in the garden, but autumn is truly the prime time for propagation of many of our favorite plants. Seed production is nature's way of assuring plant species' survival. Letting the last few flowers remain on the plant until seeds are ripe can provide a free source of new plants for the coming year. But what if the plant hasn't flowered, doesn't produce viable seed or won't come true from seed? In these cases, you may want to consider some form of vegetative or asexual propagation technique.

Don't overlook obvious sources of new plants from the garden. Seedling trees are often found popping up around the base of mature specimens. Tag them for transplant to a better location in November or December. Young seedlings can easily be transplanted during the winter with very little effort. Divide herbaceous perennials and spread out to increase their numbers as well. Root growth will occur over the winter months and they'll be off to a strong start in the spring! Watch for naturally occurring layered plants, which are stems that have come in contact with the ground and taken root. These can be severed from the mother plant and relocated at this time. Bulbs can often be lifted and divided in the fall. Spread them around and share with your friends!

Sexual (Seed) Propagation

Many flowering plants in the garden are most accommodating -- showing off their abundance of seeds or fruit ... just tempting us to harvest and plant the seed in an appropriate spot. Seed is a convenient propagation method for a number of tree species. Typical examples include the flowering dogwood (Cornus florida), southern magnolia (Magnolia grandiflora), redbud (Cercis canadensis) and several of the oaks (Quercus spp.). Seeds are likewise readily harvested from several common shrubs in the landscape. Herbaceous annuals and perennials generally produce abundant viable seeds that can be used for propagation.

Keep in mind, however, that specific varieties or cultivars may not come true from seed. The form, flower color or fruit characteristics will be subject to normal genetic variation when propagated by seed. Identical plants would be obtained by clonal or asexual propagation only.

Two seed treatments, stratification and scarification, are sometimes required for successful seed propagation. These procedures are employed to overcome natural barriers to seed germination.

Stratification usually consists of placing seeds in a cool, moist, well-aerated environment for a period of time (one to three months) to allow the seed to overcome a natural dormancy requirement. In nature, this may be a protective mechanism to keep seed from germinating until spring, when new plants will be less subject to winter freezes.

Scarification is a process by which very hard or impermeable seed coats are softened or cracked to allow for water penetration and actual germination of the seed embryo. This process may occur naturally over time in weathering the elements, but it may be done artificially with hot water, acid, sandpaper or a file. The hard seed coat is a physical barrier to germination, whereas the dormancy requirement is a physiological barrier.

Information as to specific seed treatment requirements is available from various references such as Seeds of Woody Plants in North America by James and Cheryl Young or Plant Propagation: Principles and Practices by Hartmann, Kester, Davies and Geneve. As a general rule of thumb, you might just take a cue from nature and plant the seeds in an outdoor bed or in containers outdoors. If a stratification period is required, let nature take its course and the seed will germinate in spring. Otherwise, germination will occur within a few weeks of planting.

Scarification can be performed on obviously hard seeds to improve upon uniformity of germination. Placing seeds into a bowl of very hot water and letting it sit and cool for a couple of hours prior to planting the seed will usually soften the seed coat. Otherwise, try using an abrasive material, such as a file or sandpaper, to scratch the surface, allowing for water penetration prior to planting.

Asexual [Vegetative] Propagation

At this time of the year, the best means of asexual propagation would involve the use of hardwood cuttings. This process is very simple in that there is no need for specialized greenhouse or misting equipment. Stem cuttings 6 to 8 inches in length can be removed from the tree or shrub and lined out in rows in a garden or field. The cuttings need to be oriented with the terminal portion of the stem up and the other end in a loose, moist medium, such as a sandy loam or mixture with compost often works well.

A rooting hormone such as Rootone or Hormodin may be helpful with this technique. Also, try not to let the area dry out or remain saturated. A well-drained, aerated and semi-moist medium is appropriate. Some plants that root successfully by hardwood cuttings include sycamore (Platanus spp.), crape myrtle (Lagerstroemia indica), poplar (Populus spp.), corkscrew willow (Salix matsudana 'Tortuosa'), butterfly bush (Buddleia davidii), forsythia (Forsythia spp.), althea (Hibiscus syriacus), St. John's wort (Hypericum patulum), bridal wreath (Spiraea x vanhouttei), weigela (Weigela spp.), privet (Ligustrum spp.) and clematis (Clematis spp.). You may try others if you have the time and space in the garden.

Peter Gallagher is a horticulture professor at Louisiana Tech University.


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