Above ground container production

The use of containers to grow nursery stock is generally recognized as first beginning in the early 1950's (1). Nurserymen in southern California, who had considerable experience in growing plants inside greenhouses, were first credited with taking plants outdoors in containers (2). By having plants with foliage, and flowers, in a neat "package" that was easily transportable, the sale of nursery stock could expand beyond the typical dormant season. With the advent of the Federal Highway system, and the expanding use of truck transportation, growers were able to distribute their stock all over the United States. The first containers used were recycled metal food containers, which were often scrounged from schools and restaurants (3). With their sharp metal edges, and straight sides, these early pots were difficult to work with.

Dwarf Hinoki cypress (Chamaecyparis obtusa 'Nana Gracilis') being raised in decorative wooden boxes for the retail garden center trade

Container culture expands
During the early 1960's nurserymen started to explore all of the factors associated with growing plants in pots. Studies looked at using native soil (as opposed to modern container mixes), the time the plants could be left in the containers, and the growth characteristics of the contained plants. Today 80-90% of woody ornamentals produced in the southern states of California, Texas, and Florida are grown in containers. Plants that have small root systems and fast growth rates are ideal candidates for containers. As for terminology, there are different types of container plants:

• Potted stock is used describe seedlings, rooted cuttings, or grafted plants that are first started in small containers before they are put out in the field as liner stock or re-potted into larger containers. Examples would include flowering perennials, and rhododendrons.
• Container-grown stock refers to plants spend their entire lives in pots, up to the time of sale. Examples would include many of the various types of spring bedding plants, or vegetable transplants.
• Containerized stock is used to describe the result of taking smaller field stock (including trees and shrubs) and placing it into pots. Examples would include global arborvitae and Japanese maples.

Wholesale Containers
The nursery industry has a vast multitude of different containers to choose from. Propagators will consider the following factors in determining what type to select:

• Round or square shape
• Durability
• Size
• Impact on root growth
• Cost

Plastic nursery containers come in vast array of sizes. This nursery has ordered them by the pallet load. They should be kept shrink-wrapped untill they are needed so that the wind does not carry them away.

Plastic predominates
As for durability most nurserymen use what are known as blow-molded plastic pots (made from polyethylene, polypropylene, or polystyrene resins). Blow molded pots are used for outdoor conditions as they last longer, have greater strength, and can be easy stacked together (3). For plants in the greenhouse, injection molded plastic pots are more common. While there are versions that can stand shipping, their lighter weight generally precludes their use to propagation. Most pots come in either black or green. Black pots can heat up excessively on hot days, which can harm root systems. Larger blow-molded pots (#5's and greater) can be purchased with handles. Very large pots (#45's and greater) can be ordered with forklift slots.

Nursery pots are available in a multitude of different sizes. In the same volume class, deeper sizes are best for aeration and drainage (4). The size of the plant determines the size of the pot it should be raised in. Tables have been developed by the American Association of Nurserymen suggesting the size of pot to use for different sized stock (1). For ornamental trees over the height of 4' a 5 to 15 gallon pot will be needed. Most shrubs will remain in 1-2 gallon pots. For globe narrow-leafed conifers a 2-5 gallon pot is required. For a complete listing of the different sized pots turn to the Nursery Supplies Incorporated web page.

Fiber pots
Fiber pots are made from biodegradable wood fiber. As a rule fiber pots promote healthier root systems as the roots with can breathe. They also impart a cooling effect on root systems. Fiber pots are known to increase rooting depth over that of plastic pots. When the insides of treated fiber pots where treated with a water-based latex coating containing copper hydroxide coating (Spin Out, Griffin Corporation) root circling was reduced. Ruter (5) found that copper treated fiber pots (from Western Pulp Products) could be shipped across the United States successfully. Fiber pots appeal to retail consumers who would rather purchase a biodegradable product as opposed to one made from plastic.

One gallon pots
The most common size at the garden center is a 1 gallon pot, referred to as a #1. In general round pots prevail over square Plant material consisting of flowers, trees, and shrubs from liners, cuttings, and flats typically go into #1's. Pot terminology is unique to the nursery trade. A 1 gallon pot only holds 3 quarts of soil in terms of liquid measure, but is still referred to as a #1. As for dimensions a #1 pot measures 6" wide by 7" deep and has a capacity of 183 cubic inches (6). Once the plant has been put into the pot, and some space has been left at the top for water, ten #1 pots can be filled with a cubic foot of soil. With 27 cubic feet of soil in 1 cubic yard, a nursery can pot up 270 #1 pots with a cubic yard of either soil or container mix. If #1's are placed pot-to-pot, in rows 12 pots wide, with 2.5 foot alleys between, approximately 2,700 #1's can be set out on 1,000 sq.ft. of ground.

Herbaceous perennial producers often use 3-6" square pots for smaller plants. Square pots are more space efficient that round. If perennials are to be sold in #1's they will need to transplanted from liner beds 6 months prior to sale in the spring.

Few tree seedlings are put into #1s as there is very little demand for such a small tree. The only exception would be the multitude of grafted Japanese maples. In the Pacific Northwest these small trees are a common sale item at spring garden fairs.

#2s, #3s, and #4s
These larger pots are especially useful for herbaceous perennials, and hybrid roses. Retail growers enjoy the sales of bedding plants grown in 2-3 gallon fiber pots, which are sold as color pots in the trade. Buy purchasing wholesale annuals in plug sizes, and fiber pots and potting soil from a nursery supply house, a part-time income can be generated during the months of March-May. By using an array of annuals grouped together in wide, relatively shallow container a grower create an excellent retail product for backyard decks and apartment complexes, with very little investment. When a fiber pot is selected with a wire hanger, growers can capitalize on the growing interest in hanging baskets filled with bountiful spring and summer annuals. Other common uses for 2-3 gallon fiber pots include the sale of budded hybrid T roses during the months of February-May.

#5s, through #15s and larger
A #5 container is the generally the minimum size for the sale of woody ornamentals at retail garden centers. This is also the minimum size for commercial landscape construction. Tree seedling liners are often put into #5s without first going into #1s. Larger liners look better from a retailer's viewpoint in a #5 container, as opposed to a smaller pot. A #5 container filled with mix and a plant weighs approximately 25-30 lbs., thus making it is still manageable by retail customers.

A #15 container can be an excellent choice for the smaller independent retailer looking to capture the higher end of the specialty garden trees such as Japanese maples, or weeping conifers. If growers have the time to allow these plants to attain a larger size they will be rewarded with a very high profit potential. Conifer growers who have dug their trees as B&B will often pot up any stock that did not sell in the spring into #15s for sale during the summer. A #15 pot, filled with a plant and soil will often weigh more than 90 lbs. so that a dolly and extra help will be needed for loading. Landscapers prefer #15s as they need at least 1" caliper trees for their installation projects. By having access to these larger container sizes they can plant good sized trees (6' or more) all year long, as opposed to field grown bare root material in the spring only. A complete guide to growing trees in #15s and larger trees is available from North Carolina State University (7).

Above ground, open weave containers
In the last 10 years there has been a lot of interest in alternatives to hard-sided plastic containers. The inherent tendency towards circling roots in plastic pots has sparked investigations into engineered porous fabric and plastic weaves that stop root by air pruning. The RootMaker Products Company (8) has developed 10-30 gallon white sided soft containers, referred to as RootTrappers®, that can used above ground. Smaller woody stock is planted into these white-sided containers and allowed to grow. Root death due to sun exposure is minimized as the containers stay relatively cool. With time the expanding roots fill the RootTrapper® tubs with a thick dense mat of fibrous roots. Roots can penetrate the base of the RootTrapper® containers so that they can take up some water from the soil (9). By having roots anchored in the surrounding soil these above ground fabric bags are less prone to blowing over. The resultant trees can be planted all year long.

This company also markets a product known as RootBuilder®, which is a product that growers build themselves out of a roll of porous semi-rigid plastic that is 11"-28" wide. When cut to length and stapled tubs of 12-1200 gallons can be built. This product is designed to be holding unit for either container or field stock, prior to landscape installation. Once again, by having a porous weave, air pruning prevents circling root formation.

Another new above ground poly bag has been introduced by Nursery Supplies Inc. Called the Plant Pouch these bags are made from durable polyethelyene-covered mesh fabric. Available in sizes from #15 through #200, these bags are designed for above ground use. Bags can be ordered with drainage holes in the bottom or sides. Custom ordered bags can made with air-pruning holes. In addition Plant Pouch bags can be ordered in 3 different colors: black, white, or green, and custom labeling is available.

Tree boxes
The latest container developed by the nursery trade is known as a tree box. By using these larger containers growers can either raise very large trees for latter transplanting, or use them as permanent, above ground planter boxes. Typically consisting of plastic, these collapsible, square-sided units come in sizes of 35 gallons (24" wide), 75 gallons (31" wide), 125 (38" wide), and even 130 gallons (10). These containers are designed for use with either high-end nursery stock sales at independent garden centers, or for commercial and municipal applications where they can be used for portable traffic barriers. They come ready to be moved with forklifts. They are built with molded sidewalls that prevent root circling, as well as with elevated bases to stimulate air root pruning. Plastic versions can be disassembled for transport (11). For the homeowner that wants instant gratification these products can be a potentially lucrative item for the discriminating buyer.

References
1. Nursery Management: Administration and Culture, 3rd Edition. 1994. Harold Davidson, Roy Mecklenburg, and Curtis Peterson. Prentice Hall, Inc.

2. Production of Landscape Plants II (in the field). 2001. Carl Whitcomb, Lacebark Inc., Stillwater, OK.

3. So You Want to Start a Nursery. 2003. Tony Avent. Timber Press, Portland, OR.

4. Containers and media for the nursery. 1991. Michael Schnelle, and Janet Henderson. Oklahoma Cooperative Extension.

5. Cross-country containers. John Rutter. American Nurseryman, Feb.1, 2000.

6. Growing Profits: How to Start and Operate a Backyard Nursery. 2000. Michael and Linda Harlan. Moneta Publications, Citrus Heights, CA. Distributed by Chelsa Green Publishing, White River Junction, Vermont.

7. What's quality for a plant grown in a big container? Tom Bilderback. North Carolina State University, Department of Horticulture.

8. RootMaker® Products Company, Huntsville, Alabama.

9. Focusing on fabrics. David Morgan, senior editor with American Nurseryman magazine. May 15, 2003.

10. Worthington Farms, Greenville, North Carolina.

11. Amaroo Enterprises, San Diego, CA.

First posted: December, 2004.