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.
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.
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
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).
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.
pot-in-pot production system. 2003. Mark Holcomb, University
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.
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,
11. Getting back to the roots. Matt Schlossberg. American Nurseryman,
February 1, 2000.
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,
14. Root control
bags can ease most tree planting problems. Traxler's Farm, Avon,
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.