Nursery Propagation: Root Cuttings
Plant propagation plays a vital role in the profitability of most commercial horticultural operations. While today’s comments are slanted toward traditional nursery operations specializing in woody plant species (trees, shrubs, etc.), most of the principles cross over to herbaceous plant species that bedding plant growers and perennial plant growers produce ). In order of discussion this article will cover the stock or mother plant, cutting, rooting medium, propagation environment, and rooted cutting fertilization.
Basic rules of plant propagation apply similarly across a wide range of horticultural crops requiring asexual propagation. Whether terminal stem cuttings are being used to propagate a poinsettia or geranium, immature wood cuttings for an ornamental tree or shrub, or root divisions for a herbaceous perennial, growers committed to clean, sound propagation practices are likely to experience higher yields and quality of rooted cuttings.
Start with healthy, clean asexual propagules whether they are stem cuttings, leaf cuttings, grafts, or root divisions. Stem cuttings will be referenced in the remainder of this discussion for brevity. Complementing asexual cutting propagation is sexual propagation involving flower pollination, seed production, seed sowing, and germination. Good propagation practices are nearly identical for asexual (cutting) and sexual (seed) propagation alike.
Cutting uniformity is a requirement that ripples through the entire propagation cycle and subsequent production cycle. Start by harvesting and sticking uniform terminal stem cuttings if terminal cuttings are preferred. Occasionally, cutting numbers may fall short of what’s scheduled to be stuck. In order to harvest and stick enough cuttings it might be necessary to harvest some sub-terminal cuttings. When this practice becomes necessary additional effort is required.
Isolate sub-terminal cuttings in separate plug trays because they likely will require more time to root given their older age and maturity. Having terminal and sub-terminal cuttings in a single plug tray may result in removal of the tray from mist earlier or later than desired as one of the two cutting types will influence the decision process. Also, subsequent growth and development of the young plants can be quite different. A terminal cutting begins growth as an un-pinched plant whereas a sub-terminal cutting, by definition, has already been pinched. Growing these plants side by side may create challenges in maintaining crop uniformity.
Rooting medium
Growers often find this area of propagation to be unnecessarily challenging. There are many growing mixes on the market which can be confusing. The primary characteristics of a quality propagation medium is that it be balanced regarding drainage and water holding capacity. Secondary inputs include nutritional amendments, growth promoting compounds (hormones, microbial products), pest control inputs (fungicide, beneficial microbes), and others.
These secondary inputs add complexity to the decision of which mix to purchase. In a future article proper design and execution of product trials will present effective ways growers can conduct their own on-site trials to help make decisions on which product upgrades enhance profitability and which are not worth the investment.
Propagation environment
The basic factors affecting plant growth in general also play a role in propagation; temperature, water, light, and nutrition. The first two factors, temperature and water, play a primary role during the rooting phase of a cutting. Light and nutrition can be considered secondary, to a degree, until roots have initiated.
Both temperature and water are needed in higher amounts during propagation than during finished crop production. The rooting of cuttings usually requires a warmer temperature than the growth of the finished crop. Generally, the temperature of the rooting medium should be in the low seventy degree F range with 68-74 degrees being an acceptable target. Air temperature, particularly night temperature, does not have to be as high which meshes nicely with a bottom heat system of delivery.
Heat delivered to the root zone from below the plug tray is transferred to the plug tray and root medium. Some heat continues to be transferred from the plug tray and root medium to the air above the cuttings, keeping them warm but not as warm as the medium which is ideal. If the air temperature around the cuttings is too high stress can be created; before the cuttings root they may lose water faster than it can be replaced.
Water has two dimensions in a propagation discussion, root medium moisture content and relative humidity of the air. It is common to apply intermittent mist during cutting propagation to maintain relative humidity in the micro climate surrounding the cuttings. This moist air helps the cutting avoid desiccation via its leaves, petioles, and stems. Misting of cuttings usually applies enough water in the form of a fine mist to settle on the surface of the root medium and percolate through the root zone profile, thereby maintaining moisture throughout the root cube.
Once cuttings form roots they are weaned off of mist to reduce disease incidence. Simultaneously, traditional irrigation commences to deliver water to the rooted cutting as it’s needed. Hand watering, boom watering, overhead sprinklers, and flood and drain benches are common irrigation systems in propagation areas. At this time, rooted cuttings are usually moved from the warm, moist propagation environment to a hardening off area where root development continues and the young plant’s shoot resumes active growth.
Light and fertilizer are not needed in high amounts during rooting. Shade is often used during high light periods and supplemental light is sometimes used during low light periods. Supplemental light may not be required during the early stage of rooting but has been shown to be beneficial once the rooted cuttings resume active shoot growth. An exception is photoperiod control where certain species may require day length manipulation. This area of light management is separate from photosynthetic light management.
Fertilizer
Until a cutting forms roots there is little need for applying fertilizer during propagation. Doing so can have the opposite effect as fertilizer salts in the root medium could damage the fresh wound at the base of the cutting. Once a cutting roots and the shoot resumes active growth fertilization can commence (Figure 5).
Depending on crop species the resumption of fertilization often begins at a lower than final rate and gradually increases. Young rooted cuttings coming out of a warm propagation area may not be ready for full fertilization until they are acclimated (hardened off). The practice of hardening cuttings, also known as toning, will be discussed in a later article.