Tame the growth

Spacing plays an important role in quality and plant form.

Overgrown, leggy and disproportionate woody ornamentals lose value at retail and decrease profits for growers. It is important to keep growth controlled in production to ensure plants are at their peak when they go to market. Growers have a number of options available for growth regulation, including biological, physical, environmental and chemical controls. Such control tactics help keep woody ornamentals, shrubs and trees compact and visually appealing.

Biological growth controls
As a preventive measure, growers can choose genetics that are less likely to cause problems in production with overgrowth. Choosing less vigorous or shorter cultivars can reduce costs. Physical controls, such as pruning and mechanical shearing increase labor costs, and chemical controls can also contribute to production expenses.

Physical growth controls
Spacing of plants in production can make a large difference with quality and plant form (upright and columnar versus v-shaped or rounded). Tighter spacing can cause woody plants, especially young tree saplings, to grow upward with little branching resulting in weak and leggy plants. Selective pruning of shoots may be all that is required to create a plant proportionate to the container. Shearing can increase lateral branch production and help shape ornamentals. For large operations, a retrofitted lawnmower raised to a desired height above the pot rim is sometimes used to trim liners or smaller containerized shrubs with prolific shoot production.

Growers can restrict growth of woody ornamentals by maintaining plants in small containers for extended periods of time. Unfortunately, prolonged periods in these containers can cause “root wrapping.” Transplanting or “stepping” plants up to larger containers can make plants look more proportionate to the container.

Environmental growth controls
Temperature, light and humidity also affect growth. Negative environmental outcomes from being too cool or too warm in a greenhouse or on a nursery pad can cause delayed or leggy plants. If woody plants are grown in the greenhouse, temperatures should be regulated to encourage growth yet avoid excessive stem elongation. A typical setting for day/night temperatures would be 70/60°F. Most woody ornamental growers use slow release fertilizers that contain a balance of ammonium and nitrate nitrogen. If temperatures are too warm (greater than 80°F), the fertilizer prills can release nutrients too quick and provide above average levels of fertility. Undesired growth is sometimes the result of this quick release of nutrients. Fertilizer applications should be timed to coincide with plant growth cycles and temperature variations.

Chemical growth controls
Plant Growth Regulators (PGRs) can be used to limit stretch of woody ornamentals. Important factors include timing of application, specific tissue to be targeted, dosage, application techniques and environment conditions.

PGRs with consistent formulations are ideal, so growers can depend on the same performance every time. Generic formulations can sometimes be unreliable, so they should be used with caution.

Growers should also watch out for bark when using PGRs. Bark, the predominant mix component in woody production, has shown to reduce efficacy of some PGRs when used as media drenches. Growers should adjust rates accordingly, especially in more southern locales.

Do not settle for unmarketable, overgrown plants. Controlling plant growth and using plant growth regulators is just as much of an art as it is a science, but these tips can help you maintain compact, beautiful plants.

Jamie Gibson leads the Syngenta Technical Services team; james.gibson@syngenta.com.