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Root Control Bags

Nursery crop growers continue to explore alternatives to the conventional in-ground and container production methods. Field production suffers from either having to dig smaller, bare-root stock or having to use expensive tree spades for balled and burlap stock. In the case of above-ground container production, plant roots are susceptible to heat stress during the summer, cold injury during the winter, wind blow-over, and development of circling root systems. The pot-in-pot (PNP) system was developed to bridge the gap between field and container production (1) by burying socket pots in the ground into which liner pots would be inserted to grow the plant. While the PNP system has gained wide-spread acceptance it still has the disadvantage of typically consisting of hard sided plastic containers that can encourage root spiraling. In addition, there are still problems with vigorous plants that will root out of the liner pots (2), through the socket pots and into the surrounding soil. As a result, at harvest time, it may be difficult to remove the liner pot from the socket pot (3). The PNP system remains the most expensive production system for woody ornamental nursery stock.

Fabric containers
Root control bags combine the advantages of growing plants in the ground, with the root containment properties of a container. The first work on root control bags began in the early 1980's. Using various types of porous polypropylene weave, the aim was to develop a flexible container that could be placed into the ground, filled with native soil and a plant, and then dug out at harvest. The currently available knit fabric containers work on the principal of allowing water, nutrients, and air to pass freely through the fabric, but restrict the passage of roots (4, 5). As roots grow from the base of the plant stem they intercept the sidewalls of the fabric container. The root tips will be able to partially grow through the sides of the container, but will not be able to expand given the small size (2-3 mm in diameter depending upon the manufacturer) of the openings. The root tips loose their apical dominance when they encounter the fabric encouraging them to branch laterally, and thus are essentially pruned back. Root balls of field-grown trees contain the same number of roots as from a fabric, root-control bag (6), but the fabric bag has half the volume.

Unlike roots that contact a hard plastic container wall and start circling, roots in fabric knit bags form a dense fibrous clump. Upon examination, the root tips will appear swollen. Root tips will accumulate carbohydrates from aerial portions of plant. Once the tree or shrub has been removed from the bag and planted into the field (or into a container initially prior to sale) the swollen tips will commence new growth and spread out (7). The bags have non-porous bottoms which will not allow roots to escape out the bags, thus in theory easing bag removal at harvest. The proponents of fabric bags state that more than 75% of the plant's root system is retained at harvest thus greatly reducing transplanting shock when the tree or shrub is planted.

Only the best quality liners should be used in fabric bags. Liners with thick fibrous root systems will make excellent candidates for a knit container. Any liners with spiraling root systems should be avoided as they will simply not straighten out in a fabric bag. Liners grown in air-pruned pots (4) will continue to grow very well in root control bags.

Growth requirements
Fabric bags are filled with native soil prior to being planted with a liner. This practice will reduce the establishment cost associated of having to purchase composted bark and peat moss. By using the native soil in the containers, as opposed to container mix, the developing plant roots will have less chance of drying out on hot days. Trees grown in root control bags require less irrigation than those grown in either PNP or above ground containers. Under the hot conditions of Florida the plants required 25% less water (8). By growing plants in the ground there will be less winter freeze and summer heat damage. Blow-over problems will be less of a problem. Nutritional needs will be reduced as the roots can tap into the native soil.

Bag sizes
Bags come in various sizes. The desirable tree diameter at harvest dictates the size bag that should be used. The American Nursery and Landscape Association has set forth guidelines for the relationship between tree height, caliper, and root ball size. Known as the American Standard for Nursery Stock (ANSI Z60.1), these recommendations suggest the minimum size of fabric container to support different tree calipers at transplanting. For instance, the company Root Control Inc. (9) suggests that an 18" diameter bag be used for 2"-3" caliper stock. Similarly, RootMaker Products Company (4) suggests an 18" bag for 2.5"-3" caliper trees.

Root control bags are often compared to balled and burlap (B&B) stock as both involve in-ground root systems. A 24" Vermeer tree spade (10) is the appropriate sized implement for a 2" caliper shade tree dug up, put in a wire basket, and sold as B&B stock. The cone shape of soil ball will consist of approximately 2 cu. ft. of soil when harvested with the tree spade. In comparison, the root ball volume of an 18" fabric knit bag, 14" deep, is also approximately 2 cu. ft. (3). In nurseries more plants per acre are possible with fabric bags, as opposed to B&B, as spacing will be dictated by plant requirements as opposed to the size of the harvesting equipment (tractors and tree spades). More fabric bags trees, as opposed to B&B stock, can be loaded onto trucks for shipment. The smaller sized root balls of fabric bags are also lighter than comparable caliper B&B stock. Knit root control bags are now being employed by large commercial nurseries (11).

Bag installation
Producers have found that an auger mounted on the 3-point hitch of a tractor works well for digging holes for the fabric bags. A depth gauge mounted near the auger frame, as opposed near the tractor, can be set up to limit the depth of hole. The diameter of the fabric bag should be approximately an inch less than the diameter of the auger bit. It's important that the hole is not dug too deep. The augured hole should have flat bottom. In Georgia, with an experiment using Tex-R-Agroliners, it was more difficult to remove the bags if the bottom of the hole was rounded as the bags lodged into the depression (12). A plastic container, slightly less than the diameter of the fabric container, can be used to set the bag into the hole. A short section of the bag should appear above the soil line after the installation (13). If the bag is set too deep the plant could send roots over the top of the bag into the surrounding soil, thus negating the root trapping features of the fabric.

Harvesting fabric containers
It's generally recommended to harvest trees in root control bags anytime when the trees are dormant (3). Some species can be harvested after bud break in the spring, but this will depend upon the species, the weather during shipping and transplanting, and the care the tree receives after being planted. If the tree is going to be placed into a plastic container for sale during the summer, it should be put into an above ground container before spring bud break. Knit fabric bags should not be dug up during the heat of the summer as this can place too much stress on the trees after transplanting. The smaller volume of soil in a fabric bag simply won't have the water holding capacity to meet the needs of the developing foliage as it leaves out in the spring. Some growers also dig up trees and wrap the root system in burlap and place it in a wire basket as would be the case for a B&B tree (14)

When the tree is ready for harvest, workers should be able to easily dig up smaller knit fabric bags with shovels. The bags need to be handled carefully as the root systems won't be as strong as those from a plastic container. For larger bags a nylon sling can be attached to the base of the tree with the other end secured to the front end loader of a tractor or bobcat. The entire bag should come straight up out of the ground with little pull. Upon removal the tree can stored above ground until it is ready to be shipped out to a landscape contractor. During this time the root bag must be kept moist as the fabric bag is porous to air movement. It's best not too cover the roots with mulch (4) as this will make moving more difficult.

When the tree is ready to be planted the crew needs to be instructed to remove the fabric bag and water the tree thoroughly during the first summer of establishment. Failure to remove the fabric bag will result in a stunted tree as the root system will simply not be able to expand beyond the confines of the bag. Good field establishment rates have been observed if the trees have been properly cared for during both their time in the knit fabric bags and afterwards in the planting site (15).

From bag to pot
For sale to re-wholesalers or retailers the tree will need to be carefully removed from the fabric bag and then placed into a conventional plastic nursery container. It's often easier to simply lay the tree on its side to slit open the fabric bag (6). The market value of plant in sturdy, easily transported container will be higher (8) than one in a burlap bag or one that has had its root ball shrink wrapped in plastic. Plastic containers will survive transportation the best, and are familiar to buyers. For spring sales, fabric bag stock should be potted up while it is still dormant. It takes 4-6 weeks for the tree or shrub to leaf out and look healthy. For high-end retail nurseries, fabric bag trees can be paced into large tree boxes for later sale. The trees will need to be staked until the root system has anchored the top of the tree securely.

From bag to planting site
If producers are going to sell container trees to landscapers directly, which is not common, the installers will need to be instructed to handle the trees carefully. If there are large roots poking out through the fabric, hand pruners will be needed to cut them prior to slitting the fabric open. The trees will need to be carefully staked as they won't be able to anchor themselves until new roots have grown. They will have to be carefully watered or there will be leaf loss.

1. The pot-in-pot production system. 2003. Mark Holcomb, University of Tennessee.

2. The practicality of pot-in-pot. John Ruter, University of Georgia, Coastal Plain Experiment Station, American Nurserymen, January 1, 1997.

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

4. Root-Maker Products Company, LLC., Lacebark, Inc., Stillwater, Oklahoma.

5. Tex-R-Agroliners. NYP Corp. Elizabeth, New Jersey.

6. Fabric container grown trees. Ed Gilman, professor, Environmental Horticulture Department, University of Florida, Gainsville, FL.

7. Superoots Fielders. The Caledonian Tree Company, Scotland, United Kingdom. Distributed by Tree Amigos, LLC., Woodburn, OR

8. Focusing on fabrics. 2003. David Morgan, editor for American Nurseryman. May 15, 2003.

9. Root Control Incorporated, Oklahoma City, OK.

10. Vermeer tree spades, Pella, IA

11. Getting back to the roots. Matt Schlossberg. American Nurseryman, February 1, 2000.

12. Production of "Rotundiloba' sweetgum using Tex-R-Agroliners. 1998. Southern Nursery Association, Vol. 43, p. 59-61.

13. Meyer Tree Plantation, Inc. 2002. 10010 Gernimo Drive, Norman, Oklahoma.73026-5918. 405/329-0019.

14. Root control bags can ease most tree planting problems. Traxler's Farm, Avon, Ohio. 440/934-5188.

15. A comparative study between the stock grown by conventional nursery means and tree stock grown in Superoots Fielder. Simon Stokes, Myerscough College Arboriculture, Preston, Scotland.

First posted: December, 2004.



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