ATTRA - National Sustainable Agriculture Information Service
PO Box 3657
Fayetteville, AR  72702
Phone: 1-800-346-9140 --- FAX: (479) 442-9842

By Steve Diver and Lane Greer NCAT Agriculture Specialists May 2000 (Revised Nov. 2001)

The PDF version of this document is available at {pdf lcoation} 9 pages — 287 kb

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Container Production

A wide selection of ornamentals is produced in containers. Homeowners usually prefer to buy containerized plants because they are easier to transport and transplant than B&B plants. The following section will summarize some important container production practices as well as address sustainable nursery management issues like recycling of plastics, weed control, and fertilization.

The advantages of containerized production include:

The disadvantages are also numerous:

Certified organic nursery stock (intended for sale to vineyards, berry farms, and orchards raising organic produce) is a niche market that requires special attention. Restricted products include many of the common ingredients in conventional nursery production, such as chemical fertilizers, wetting agents, herbicides, and synthetic insecticides and fungicides. For more information, refer to ATTRA's publication on organic certification.

 

Containers

There are several factors to keep in mind when deciding which containers to use: cost, design features that control root growth, how the container affects growing medium moisture content and temperature, availability, how the container suits the particular needs of the nursery, durability, and shipping capacity.

Round black-plastic pots are the norm, but they can cause root constriction, leading to plants with poorly developed root systems. There are other kinds of containers that promote better root systems. For instance, copper-lined, white, and light-colored containers produce more root growth, and square and stair-step pots help keep plants from becoming root-bound (1). Pots and containers designed for enhanced root growth have become such an important feature in containerized nursery production that a few examples are provided below.

 

Copper

Foresters first discovered that copper could be used to control root growth. Copper kills root tips that come in contact with it; this forces roots to branch within the root ball, instead of circling around it.

Not only are plants grown in copper-treated containers less root-bound, they also are taller, have higher transplant survival rates, and have increased nitrogen recovery (so they require fewer applications of nitrogenous fertilizer). More than 120 species have been shown to perform better in copper-treated containers than in untreated ones. Also, there is very little or no leaching from copper-treated pots into groundwater or soil.

Copper-treated fiber pots, made from recycled paper, are one option. Fiber pots are biodegradable and can even be composted; the main problem with them is that they can degrade too quickly. Research at Ohio State University showed that incorporating copper into fiber pots can increase their longevity (12). Dr. John Ruter at the University of Georgia found that copper-treated fiber pots keep roots
cooler in summer, increase root dry weight and shoot dry weight of several species, and can well withstand shipping (13).

Griffin LLC in Valdosta, Georgia, offers a product called Spin Out™, a copper paint registered by the EPA. Root Right™ pots are round black-plastic pots that have been manufactured with Spin Out as a component of the container walls. For more information on Spin Out and Root Right pots, contact the Lerio Corporation (14).

 

Bottomless Pots

Another way to prevent root circling is to use air root pruning. This works in much the same way as copper: root tips that come in contact with air are killed and the root system branches out within the root ball.

Growing tree seedlings in bottomless milk cartons is one way to use air root pruning. The milk carton, when folded open, creates a long square bottomless container. These containers are placed in a wooden flat with a wire-screen bottom and then filled with a soilless nursery mix. (Another option is to place them in plastic milk crates.) When the seedling germinates, the taproot grows downward and out through the bottom of the container where the root tip is exposed to the air, becomes desiccated, and dies back. Repeated air root pruning stimulates lateral branching and results in a fibrous root system as opposed to a strong taproot system. The benefit to the tree is rapid establishment in the field or landscape with increased scaffold branching and top growth. Nursery stock production by the milk carton method is especially useful for on-farm tree production and can be used in the propagation of a wide range of woody plants, including strong tap-rooted species such as black walnut and pecan, as well as pines for Christmas trees.

There are other types of containers that promote excellent root branching and discourage root circling. One of these products is RootMaker™, developed by Dr. Carl Whitcomb at Lacebark, Inc. (Dr. Whitcomb, formerly head of the nursery research program at Oklahoma State University, is well known for his numerous innovative approaches to unusual container systems.) RootMaker pots have staggered walls and a staggered bottom, which prevent root circling and direct roots toward the many holes in the walls and bottom of the pots

 

Tubes

Growing plants in long bottomless tubes is another production system that uses air root pruning. The tubes are generally made of plastic or styrofoam, and can be single tubes or imbedded in a flat. Tube plants range in size from large plugs (sold as nursery liners) to seedling trees grown in long, narrow pots (sold directly to consumers). Regardless, tubes are popular because massive quantities can be quickly grown in a small area. Tubes are particularly adaptable to small-scale nursery production and to specialized stock like perennials and tree seedlings.

For more information on containers, see The Container Tree Nursery Manual, Volume 2, by Landis et al., listed in the Resources: Publications section. For suppliers of unusual pots, see the Resource: Suppliers section.

 

Pot-in-Pot System

The pot-in-pot method of production was developed to alleviate some of the problems associated with container production, such as blowover and moisture loss (15). This system involves burying a holder pot, or moat pot, into the ground, and placing a containerized plant inside this pot. The greatest drawback to this system is the high initial cost of the moat pot. But the moat pot is a long-term investment, since it will last for 15 years or more. For more information on pot-in-pot systems, see the articles listed in the Resources section.

 

 

Recycling Plastic

Most nurseries use lots of plastic, in the form of pots, flats, hanging baskets, and greenhouse film. While some of these can be re-used within the nursery, it's important to have a recycling system in place. Buying multi-year, ultraviolet-stabilized greenhouse film will decrease the amount of sheet plastic used each year, but this kind of film is very expensive and not always readily available (20).

Fortunately, there are a number of recyclers around the country who accept nursery plastic. The American Plastics Council website <http://www.plasticsresource.com > provides a wealth of helpful information on recycling of plastics, and maintains the United States & Canada Recycled Plastic Markets Database <http://markets.plasticsresource.com > with contact data for plastic recycling centers on a state-by-state basis.

 

Recyclers that accept agricultural plastics often have certain restrictions. They may require that sheet plastic be clean, which often means washing it before storage. It must also be stored properly, indoors. Most hard plastics (plug trays, flats, pots, hanging baskets) are either No. 6 polystyrene or No. 2 high-density polyethylene. This distinction is important to some recyclers (20).

Many recyclers require that a certain amount of plastic be gathered, before it is worth their time to send a truck to pick it up. Smaller nurseries may have trouble storing that amount of plastic. A way to get around this is to combine plastic waste with other growers in the community. Some recyclers will not pay the grower for the plastic, but neither will they charge transportation costs, which are often high.

 

Weed Control

Weed control is extremely important in container production. Weeds not only compete for water and nutrients, but they can also hinder sales of nursery stock. Weed control efforts should focus on two areas: in the pot and under the pot.

When deciding how to treat a weed problem, the grower should ask herself several questions: Are the weeds mostly annuals or perennials? Are they easy to pull? Does the problem exist only in a small group of plants or is it widespread? Are the weeds monocots (grasses) or dicots (broadleaf)? What time of year is most effective for controlling these weeds?

Sanitation is the least costly and most effective method for controlling weeds. To prevent weed seeds from blowing into pots, attention to a vegetation-free zone—both on and surrounding the production bed—is critical. To keep weeds from growing under the pot, more and more growers are placing the containers on geotextile weed barriers (often called fabric weed barrier or landscape cloth). Modern landscape cloths are durable and long-lasting—they can last for 10 to 12 years in full sun. They do an excellent job of controlling weeds, yet they are permeable to water from irrigation and rainfall, so drainage is not a problem. Although the initial cost is high, the expense can be pro-rated as an annual weed control investment.

Hand weeding is costly, but it may be appropriate in a small nursery setting. Weeds must be removed when they are still small, since large quantities of media are lost when big weeds are pulled out of containers.

Herbicides, on the other hand, are widely used in container nursery production. Even though weed-free media is used to establish nursery plants, wind and birds and surface irrigation water are sources of weed seeds that get deposited onto the pot surface. Broadleaf and grassy weeds love to get a free ride into the container nursery, because the growing conditions in a media-rich pot are absolutely perfect. Thus, pre- and post-emergent herbicides are commonly used in commercial nursery production to control these free-loaders.

In 1991, Monrovia Nursery compared hand weeding to spraying herbicides and found that a combination of the two was the least costly method (21). (See Figure 1 below.) When they used no pre-emergent herbicides, it took workers 10 hours of hand weeding per acre, performed 10 times a year. By using a pre-emergent once in the spring and once in the fall, the workers were able to perform hand weeding only seven times a year for one hour
per acre. Other pertinent information: the workers were paid $8 an hour; the cost of herbicide was $200 per acre per application and it took two hours to apply it.

More information on weed control is presented in the Field Production section below.

 

Alternatives to Herbicides

Fabric weed barrier disks can be used to control weeds in containers. The fabric disks are pre-slitted and fit on top of the pot around the stem. They prevent weeds from growing in the containers by excluding sunlight and inhibiting weed germination. The disks are permeable to air and water but prevent germination of troublesome container-nursery weeds like oxalis. An added benefit of the disks is that they reduce evaporation.

 

Tex-R Geodiscs® are fabric disks that have been treated with Spin Out™ and work the same as the disks mentioned above. They prevent weed growth by excluding light and pruning the roots of weed seeds that are blown onto the fabric. They provide effective weed control for up to three years and can be moved from pot to pot. For distributors, contact Texel USA (22).

Professor Bonnie Appleton at Virginia Tech recently conducted research using Geodiscs on container-grown willow oaks (23). The Geodiscs suppressed all weeds completely. Trees grown in the pots with Geodiscs had higher top dry weights and root dry weights than both those grown without any form of weed control and those sprayed with a conventional herbicide.

 

Bioherbicides

A recently introduced organic weed control is corn gluten meal (CGM), a by-product of corn syrup processing. CGM is a pre-emergent herbicide, applied in early spring. It works best when applied to the top ¼ inch of soil. It has no carry-over into the second year of growth, so it must be applied every year. CGM contains 10% nitrogen and acts as a slow-release fertilizer for the crop. CGM has been patented and is currently being sold as a herbicide (see the Suppliers section for sources), but treating a large area can be quite expensive. Wheat gluten meal has many of the same effects as CGM, but it has not been patented and so may be more affordable.

More recent research has revealed that corn gluten hydrosylate (CGH), which is made from corn gluten meal, is more effective than corn gluten meal for controlling weeds (24) and could be applied at less than half the rate for effective weed control. Iowa State Univer-sity—patent holder of corn gluten meal as a natural herbicide—maintains a web list of licensed suppliers for this product (see Resources: Suppliers).

There are some new environmentally friendly contact herbicides that break down quickly and provide options for weed control in container nurseries around irrigation risers and perimeter areas, as well as for general use in field nursery production. One class of products—Weed Eraser™, Scythe™ —is made from pelargonic acid, a fatty acid found in plants and animals. They work by rapidly lowering the pH of any plant sprayed, which weakens the cell walls and kills the weed, usually within two hours. A second class of products—Nature's Glory™, Burnout™, and Bioganic™—contain acetic acid (vinegar), lemon juice, eugengol, thyme oil, orange oil, and other natural ingredients. All of these products work as contact herbicides and control, with varying degrees of success, broadleaf and grassy weeds. Application to nursery plants should be avoided, and several applications may be necessary to kill perennial weeds.

 

Fertilization

Container nursery production has become a huge success largely due to advances in media and fertilizer combinations. This has resulted from several decades of research collaborations between land-grant universities, commercial nurseries, and the fertilizer industry. Commercial synthetic fertilizers (including slow-release and liquid fertilizers) have played a key role in this picture. Detailed information on commercial nursery mixes and fertilizer systems is widely available through the Cooperative Extension Service.

As organic production becomes standardized under the new federal Rule, more and more nursery growers are looking at organic fertilizers and wondering how they can be used. Unlike synthetic greenhouse fertilizers, organic fertilizers have been given little research to support their use in a nursery mix recipe. Most of the following material will focus on organic fertilizers for container nursery production. Regardless of fertilizer type—whether the source is synyhetic or organic—in sustainable nursery production the emphasis is on zero runoff. Excessive nitrates and phosphorus are the most common problems in runoff water (25).

There are four basic ways to fertilize containerized plants: incorporate, topdress, liquid feed, and foliar feed. In a nursery container, fertilizer incorporation in the mix combined with liquid feeding should provide sufficient nutrition.

Organic fertilizers that can be incorporated to provide nitrogen include alfalfa meal, blood meal, cottonseed meal, feather meal, hoof and horn meal, soybean meal, and animal manures, among others. Materials that provide phosphorus include oak leaves, bone meal, shrimp wastes, residues from raw sugar, and various forms of rock phosphate. Greensand, granite meal, soybean meal, ash from orange and potato skins, unleached wood ashes, K-Mag™, and tobacco (stems, leaves, and stalks) all provide potassium. Table 3 is not exhaustive, but it provides analyses of some popular organic and synthetic slow-release fertilizers.

 

 

Adequate levels of nutrients must be maintained in the container medium for optimum growth of woody ornamentals. The levels of soluble nutrients in containers can be significantly reduced after 3 or 4 irrigations because of limited container volume and frequent application of water. To overcome this problem, two fertilizing systems are used: slow-release and liquid.

Organic or synthetic slow-release fertilizers help cut down levels of nitrates in runoff water (1). Slow-release and controlled-release synthetic fertilizers, such as Nitroform™ and Osmocote™, are becoming more common in container production systems. For best results, they should be incorporated into the growing media, rather than topdressed. Slow-release fertilizers are often used in combination with liquid fertilization.

Nitrogen is the main nutrient supplied through liquid feeding (fertigation). Organic liquid fertilizers include fish emulsion, fish powder, blood meal, bat guano, seabird guano, worm castings, and composted manure teas. Some forms of organic fertilizers are more amenable to low-volume irrigation systems (drip or trickle). A 1992 study found that the spray-dried fertilizers fish protein and poultry protein did not clog drip emitters and microsprinklers (26). Fish protein, blood protein, poultry protein, and brewers yeast are all available as spray-dried materials. Table 4 was compiled by Amigo Cantisano, an organic agriculture consultant in California (27).

Foliar feeding can be used to supplement soil and liquid fertilization, especially where certain nutrients are deficient and must be incorporated into the plant quickly. Filtered solutions of manure, seaweed, fish powder, and fish emulsion can be used. Seaweed is an excellent foliar material because it contains growth hormones (auxins, gibberellins, and cytokinins) as well as trace elements. Research suggests that foliar feeding programs enhance plant resistance to pest and disease attack. Compost teas are gaining popularity as a foliar feed primarily for their disease-suppressive characteristics. For more information, request the ATTRA publication Compost Teas for Plant Disease Control.

For more information on alternative fertilizers, request the ATTRA publications Alternative Soil Amendments and Sources for Organic Fertilizers and Amendments. Another useful resource is Fertile Soil by Robert Parnes (28), an indepth publication on organic fertilizers. Parnes's book provides detailed tables on the nutrient content of various manures and plant and animal by-products.

 

Potting Media

Field soil is sometimes used in container mixes (10%_30% by volume), but soil is heavy and requires the additional step of pasteurization to eliminate diseases and weed seeds. The standard replacement for soil is peat moss, but there has been concern over the past few years that peat is a non-renewable resource. Consequently, research is being conducted to determine what materials can be used to replace peat. Most of the products being tested are some form of waste. For example, pine bark (a by-product of the lumber industry) is an excellent medium for containerized plants, once it has been composted. Mixes containing more than 20% composted pine bark support a significant level of suppression of Pythium damping-off (30). Other alternatives are coir, spent mushroom compost, paper mill sludge, apple pomace, shredded newspaper, compost, processed alfalfa, processed kenaf, recycled cardboard, and composted municipal yard waste. Most studies have shown that these alternative products should not compose more than 25_50% of the mix. For in-depth information on these topics, ask for ATTRA's publications Organic Potting Mixes and Disease Suppressive Potting Mixes.

 

 

Mycorrhizae are soil fungi that form beneficial associations with plant roots. They enable plant roots to do a better job of gaining nutrients and water. Mycorrhizae can be used in field or container production; growers have been able to get better stand establishment, cut down on fertilizer, and inoculate bareroot seedlings. There are now commercially available mycorrhizae that stimulate the roots of almost all tree and shrub species. For a listing of suppliers of mycorrhizae, see ATTRA's Sources for Organic Fertilizers and Amendments.

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ATTRA is the national sustainable agriculture information service operated by the National Center for Appropriate Technology under a grant from the Rural Business-Cooperative Service, U.S. Department of Agriculture. These organizations do not recommend or endorse products, companies, or individuals. NCAT has offices in Fayetteville, Arkansas (P.O. Box 3657, Fayetteville, AR 72702), Butte, Montana, and Davis, California.