2.3 Containerized Production
2_Growing-Methods-for-Nursery-Production
6b - Planting and Establishing Trees COPYRIGHTED
6b - Strategies for Growing COPYRIGHTED
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Growing Methods for Nursery Production
Overview
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Introduction
Lesson Objectives
Compare various growing methods for nursery production.
Describe ball-and-burlap, bare root, pot-in-pot, and in-ground fabric container growing methods, and identify advantages and disadvantages of the various growing methods for nursery production.
Explain the impact of growing method on transplant success.
Key Terms
ball-and-burlap - plants are grown to marketable size in fields and harvested by digging the root ball from the soil, wrapping in burlap, and securing within a wire basket
bare root - plants are harvested during the dormant season; roots are cleaned of soil then wrapped in a moist protective material prior to delivery to the customer
containerized production - involves growing plants directly in a container (typically plastic pots)
pot-in-pot production - a form of containerized production where the planted container is placed inside an in-ground permanent container
Introduction
There are several common methods for producing woody plants in a nursery, including ball-and-burlap, bare root, traditional containerized production, pot-in-pot, and in-ground fabric container systems. At the end of this section, you will be able to describe each production technique, identify advantages and disadvantages of the various growing methods for nursery production, and explain the impact of growing method on transplant success.
Field Production
In field production, young nursery-grown plants (also called “liners”) are planted directly in the soil. Rows of these small plants are arranged in blocks based on species, date of planting, and expected date of harvest, as well as other factors. It is important to properly space liners in the field to limit competition between plants and to allow plenty of space for healthy development and harvesting (McMahon, 2020).
Blocks and rows must also allow space for any tractors and trucks that will require access for maintenance and harvesting. Some growers may space liners somewhat close together with the intention of harvesting plants at different times. For example, liners may be spaced close enough to allow every other tree in a row to be harvested at an early date. After the first harvest, the remaining trees will have more room to grow, allowing them to be harvested at a larger size at a later date. However, there are some problems with this method of harvesting every other tree. If consumer demand does not meet grower expectations, the first crop may need to be culled (removed) to prevent competition between trees and to allow the remaining trees to develop a healthy branching structure (McMahon, 2020).
Digging from the field can be a traumatic event for a tree. More than 80 percent of roots are usually left behind in the soil (McMahon, 2020). Some growers practice root pruning as a way to encourage more root growth in the area that will be harvested. The grower will slice through lateral roots around a tree 5 years and again 2 years before harvest, leaving the tree in the soil. New roots will branch from where the root was severed, promoting more dense rooting within the root ball (Watson, 1986).
Field-grown trees are typically harvested during the dormant season, either in the fall between when leaves drop and the soil freezes, or in the spring after the soil thaws and before budbreak. Waiting until plants are fully dormant reduces demands on the plant for water and nutrients and limits stress on the plant.
Historically, field-grown trees have been harvested by hand-digging around the root ball of the plant. Digging woody plants by hand (especially large trees) is both physically demanding and time-intensive. Fortunately, tree harvesting technology continues to improve. One common method of harvesting is to use a mechanized tree spade. Each blade of the spade is spaced around the tree trunk. Hydraulic pressure forces the blades into the soil, slicing through plant roots (Figure 6.2.1). An air spade is another useful tool for harvesting field-grown trees. The mechanical spade forces pressurized air into the soil while the tree is being dug. This air removes soil from around the roots, limiting damage during harvest. Field-grown trees may be produced as either ball-and-burlap or bare root plants (McMahon, 2020).
Ball-and-Burlap
In ball-and-burlap production, the root ball with soil is lifted from the earth and placed in a wire basket that is lined with burlap (Figure 6.2.2). Ball-and-burlap techniques allow larger trees to be harvested more easily than other nursery methods. If ball-and-burlap trees or shrubs are planted relatively soon after harvest, their roots should establish well in the new soil environment. If plants are stored for an extended period of time, it is possible for roots to begin circling around the root ball. If these circling roots are not removed before planting, they will eventually girdle the tree by cutting off circulation between the roots and the stem (Gilman, 2017).
While ball-and-burlap trees will need to be watered regularly to prevent the root ball from drying out, they may not need to be watered as frequently as container-grown plants because the roots are held in natural soil. However, natural soil does weigh more than the materials used to wrap bare root plants or the soilless mixtures used in container production. This added weight increases the cost of transporting ball-and-burlap trees. Workers must also take special care to prevent injuries caused by lifting and moving heavy ball-and-burlap plants.
Bare Root
In bare root production, soil is removed from the plant’s roots before delivery to the customer. While soil can be removed by hand, this practice can result in damage to the roots. Many growers use water to gently wash soil from roots or air to blow soil from roots. As soon as the soil is removed, the roots must be protected from drying out. Growers may wrap the roots in damp sphagnum moss, shredded newspaper, moisture-retentive gel packs, or some other moist material (McMahon, 2020).
Bare root trees are usually harvested as soon as possible before packaging and shipping. The longer these trees are stored outside of the soil, the greater the risk of damage from the roots drying out. Harvesting bare-root trees often uses less expensive equipment than ball-and burlap. Growers also save money in comparison to container production because there is no need for plastic containers, growing media, and labor associated with potting up plants. Because there isn’t any weight from soil or bulk from containers, shipping bare root plants is less expensive than other production methods. Bare root production is usually more successful with young trees rather than larger trees.
While the majority of bare root plants are grown in traditional field production, new methods are growing in popularity. The Missouri Gravel Bed System is a new production method that allows bare root trees to be harvested and planted locally at any time of year. If bare root plants need to be packaged and shipped, they should be harvested when dormant (Starbuck, 2005).
First, a planting bed is prepared with a rubber liner, then filled with a mixture of pea gravel, sand, and calcined clay. Bare root trees are planted during the dormant season. Liners can be spaced more closely than in a Missouri Gravel Bed system than in traditional field production. Consistent, daily irrigation is required to ensure roots remain moist in the free-draining sand and gravel mixture. A drip irrigation system is installed with emitters spaced over the root zone of the young trees. Slow-release fertilizer can be applied at planting, or a liquid fertigation system can be connected to the drip irrigation system (DeMaris, 2015).
When it is time to harvest, trees can be lifted relatively easily from the sand and gravel mixture and the roots can be cleaned by hand or with a water hose (Figure 6.2.3). If the trees are to be planted locally soon after harvest, the roots can be wrapped in a tarp to conserve moisture and for ease of transportation. If the trees are to be shipped, the roots should be wrapped in damp sphagnum moss, shredded newspaper, moisture-retentive gel packs, or some other moist material. While bare root plant production tends to be limited to smaller plant material, larger caliper trees can be produced in a Missouri Gravel Bed system.
Containerized Production
Growing both herbaceous and woody species of plants in plastic containers that sit above the ground is one of the most popular forms of plant production (Figure 6.2.4). Containerized production is very different from field-grown production and presents its own unique set of challenges and benefits.
Growing Medium
Plants are grown in plastic containers that are filled with growing medium, which is a mixture that often contains soilless products, such as peat moss, coconut coir, composted bark, composted sawdust, vermiculite, perlite, parboiled rice hulls, or sand ( McMahon, 2020). The grower will often create a custom mixture for the species grown, which allows for better consistency for the root environment between plants and more control for the grower. This increased control over the growing environment can speed up production time for container nurseries when compared to traditional field-grown production. Growing media is usually more lightweight than native soil, which allows for easier and less expensive transportation than ball-and-burlap plants.
Roots of container plants are completely contained within a plastic pot, which prevents damage and root loss common when harvesting field-grown trees. Container plants can be grown much closer together than they would in field production, which allows for higher yields of harvestable plants.
Managing Wind Tipping
Plants grown in containers are prone to blowing over in the wind. Correcting plants that have blown over can be a labor-intensive part of container production. Containers must be upright to receive rainwater or sprinkler irrigation. Plants that have blown onto their side may become drought stressed. Top-dressed fertilizer and some of the growing media may be lost when pots blow over, causing inconsistent growth between plants (Mathers, 2003).
Container nurseries often plant trees quite deeply in the pot in an effort to prevent blowing over. While this practice doesn’t usually negatively impact plant health in the nursery, it can ultimately lead serious damage for some species of trees in the landscape. Landscapers are encouraged to remove the top layer of growing medium at planting and identify the root flare. The root flare is the area where the first woody roots emerge from the trunk or stem. This root-shoot junction should be level with the soil at planting (Gilman, 2017). Some growers limit blow-over by staking rows or running pipes tightly alongside containers between rows (Figure 6.2.5).
Managing Circling Roots
While “plantable” and compostable nursery pots are available to growers, black plastic is by-far the most common material used for container production, which has some limitations (Nambuthiri, 2015). When plant roots touch the smooth sides of plastic pots, they begin to grow in a circular fashion around the outside of the root ball. Plants with circling roots take longer to anchor into soil after planting as well as establish in the landscape. As the plant increases in size, circling roots can develop into girdling roots, which will cut off circulation between the root system belowground and the stem and leaves aboveground, eventually killing the plant (Figure 6.2.6). Such plants are also more susceptible to wind damage in the landscape due to poor root colonization and lack of anchorage (Gilman, 2017).
Simply teasing or breaking these circling roots at planting is not enough to prevent future girdling. The outer inch of all sides of the root ball must be sliced with a sharp spade before planting to completely remove all circling roots. While this practice may seem severe, it is best for the long-term health of the tree (Gilman, 2017).
Typically, plants in larger containers can be sold for higher prices than plants in smaller containers. As plants increase in size, they are often moved from smaller to larger pot sizes. When nursery plants with circling roots are moved to a larger pot size, there may be a layer of circling roots within the larger root ball that customers may not be able to see. These circling roots also have the potential to grow into girdling roots. Nurseries should pot on plants before they become rootbound, remove circling roots before increasing pot size, or use “root pruning” containers (Gilman, 2009).
There are a variety of root pruning containers available to growers. Some nurseries may coat the interior of a standard plastic container with a root pruning chemical (typically a copper agent). When roots come into contact with the side of the pot, the root tip will die back rather than turning to grow along the side of the pot, promoting more lateral rooting within the root ball. Other growers may opt for air pruning containers that have small holes or slits along the sides of the pot (Figure 6.2.7). Root tips will die back when exposed to air outside of the pot, promoting more lateral rooting within the container (Stuewe, 2006).
Moderating Container Temperature
Moderating the temperature of the root zone of container-grown plants is a major concern for growers. In the heat of summer, black plastic pots can become extremely hot, reaching temperatures up to 137 degrees Fahrenheit in some Southern states. In the cold of winter, the roots of container plants aren’t insulated by soil as with field-grown production and can be exposed to extremely cold temperatures before plants have completely acclimated for winter. Both extremes in temperature can damage or kill plant roots, which in turn will damage or kill the plants themselves (Mathers, 2003).
Tennessee nursery growers tend to rely on high tunnels (or “poly houses”) to protect container plants from extreme temperatures. When temperatures dip in the fall, a single layer of polyethylene plastic film is stretched over a hoop frame (Figure 6.2.8). The growing environment within the polyhouse is warmed by the radiant energy of the sun during the day and holds warmer temperatures into part of the evening. Even though a polyhouse will lose a great deal of heat on cold days, temperatures within high tunnels are usually several degrees warmer than outside. Growers in northern climates may need to provide additional insulation around container plants by layering straw, mulch, or poly-coated plant foam around pots. The sides of the polyhouse may need to be raised on warm, sunny days to ventilate hot air and to allow plants to acclimate to outside temperatures. When temperatures begin to rise in the spring, growers often remove the polyethylene covering and may replace it with shade cloth (Figure 6.2.9) (McMahon, 2020).
Pot-in-Pot Production
In pot-in-pot production, container plants are placed within a slightly larger plastic pot that is buried at ground level. This production system was developed in the South as a way to help moderate extremely warm temperatures in the root zone during the heat of summer. The insulating qualities of the surrounding soil in pot-in-pot production can help prevent cold damage in northern climates as well (Mathers, 2003). In fact, plants that are grown in a pot-in-pot system typically don’t require protection from insulation or a high tunnel (McMahon, 2020).
Pot-in-pot production completely removes the challenges associated with pot blow-over in normal container production. Supplemental watering is often tailored for individual plants, with each pot connected to a drip irrigation emitter or next to a micro-irrigation spray stake. In addition to using more water-efficient irrigation techniques, plants grown in a pot-in-pot system tend to lose less water due to runoff and evaporation. Harvesting is as simple as lifting the potted plant from the larger in-ground container (McMahon, 2020).
In-Ground Fabric Containers
Some nurseries produce woody material by planting liners in fabric rather than plastic containers. Native soil is used to fill the containers rather than soilless growing media common in other forms of container production. These containers are then planted in-ground where plants are left to grow and develop (Fabric, 2020).
Plants grown using in-ground fabric containers can develop the same amount or more roots than plants grown in ball-and-burlap production. However, the root ball of plants grown in fabric containers will have a smaller volume than those grown using ball-and-burlap methods, which makes the plant easier to handle (Gilman, 1996). This is important, as root loss caused by harvesting field-grown plants or removing circling roots of container plants is typically the primary cause of transplant failure. Root loss affects hormone synthesis which regulates shoot growth (Vandergriff, 200).
Some fabric containers prevent large roots from developing outside of the container. This promotes more fibrous rooting and can make harvesting somewhat easier. Other systems allow larger roots to explore the soil beyond the fabric container. Because their root growth is restricted to the container, plants grown in fabric bags require more supplemental irrigation than those grown using traditional field production methods (Fabric, 2020).
When it is time to harvest, the grower digs around the fabric bag, cuts any roots that have grown through, and lifts the plant and bag from the soil. Plants may be sold in their fabric bags (usually to landscapers) or potted up in a plastic container prior to sale. Plants grown in fabric containers must be “hardened off” for several weeks between harvesting and planting. This hardening off period is crucial to allow plants to recover from the loss of roots growing beyond the bag (Fabric, 2020).
Dig Deeper
"Layout and Design Considerations for a Wholesale Container Nursery" by T.H. Yeager & D.L. Ingram, University of Florida. Copyright © University of Florida IFAS Extension. Used with permission.
"Planting and Establishing Trees" by E.F. Gilman & L Sadowski, University of Florida IFAS Extension. Copyright © University of Florida IFAS Extension. Used with permission.
"Strategies for Growing a High‐Quality Root System, Trunk, and Crown in a Container Nursery" by E.F. Gilman & B. Kempf, Urban Tree Foundation. Copyright © University of Florida IFAS Extension. Used with permission.
Attribution and References
Attribution
Title image "20170831-OSEC-LSC-0145" by USDAgov is licensed under CC PDM 1.0
References
DeMaris, S., & Shervey, M. (2015). Missouri gravel bed. North Dakota Urban and Community Forestry Association. Retrieved November 2021 from, https://nducfa.org/wp-content/uploads/2015/09/Missouri-Gravel-Bed.pdf
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Gilman, E.F. & Kempf, B. (2009). Strategies for growing a high-quality root system, trunk, and crown in a container nursery. University of Florida IFAS. Retrieved June 2021 from, https://hort.ifas.ufl.edu/woody/documents/BMP-container-production.pdf
Gilman, E. F. & Sadowski, L. (2017). Planting and establishing trees. University of Florida IFAS. Retrieved June 2021 from, https://edis.ifas.ufl.edu/pdf%5CEP%5CEP31400.pdf
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Vandergriff, D.S. & Clatterbuck, W.K. (2000). Transplanting Trees. University of Tennessee Extension. Retrieved June 2021 from https://extension.tennessee.edu/publications/Documents/sp572.pdf
Watson, G.W. (1986). Cultural practices can influence root development for better transplanting success. Journal of Environmental Horticulture, 4(1): 32-34.