By Leonard Phillips, ASLA Emeritus
Trees generally do poorly in city planting spaces for several reasons including poor maintenance after planting, public abuse of plants, and insufficient soil volume for root growth to sustain the tree following plant establishment. Another important, but less discussed reason why plants struggle in city plantings is the failure of nurseries to develop tree root systems suited to urban conditions.
When a seed germinates in the forest, a strong taproot plunges downward into the soil. The tip of the taproot has a strong dominance that suppresses secondary root branching. The objective of the taproot is to extend deeply to anchor the new plant and access moisture to avoid dehydration. Only after the taproot is secured and is providing water and nutrients and the new leaves are producing energy to a tree, does appreciable secondary branching begin to occur both above and below ground.
Conditions in a nursery
Growing conditions in the nursery are very different because adequate moisture and nutrients are provided. There is no need for a deep taproot. When the taproot is pruned at the proper time and position, horizontal secondary roots are produced and it is highly desirable to maintain these roots in the horizontal position. Trees grown with such procedures produce roots radially as well as a taproot downward. Such desirable root systems can be created consistently by air-root-pruning the tip of the taproot approximately four inches below the seed.
Some production nurseries achieve root pruning by using bottomless containers and constant shifting. As soon as the root structure has filled one bottomless container, the plant is shifted to the next larger size and the root pruning process is repeated. Forrest Keeling Nursery uses this idea and calls it the Root Production Method.
Other containers that also stimulate horizontal root development include the Accelerator from Nursery Supplies, as well as plastic containers from RootMaker. The Accelerator employs vertical corrugations to guide the root tips to air pruning slots located along the sidewall of the container. RootMaker plastic containers use ribs, ledges and interior container angles to direct roots to holes.
Fabric containers also promote root pruning and horizontal root development. Examples include the Smart Pot line of containers from High Caliper Growing Systems, as well as RootMaker’s Knit Fabric Container, which promotes root pruning and horizontal root development. RootMaker also offers an above-ground fabric container that features bi-layer fabric with a root-tip-trapping interior fabric laminated to a white exterior for water conservation and cooler temperatures.
Large strides have been made in container growth media, micronutrients, slow release N-P-K fertilizers, weed control, watering systems and more. Having a fibrous root system throughout the growth medium improves water use efficiency and absorption of nutrients, and reduces nutrient leaching. In addition, roots are more insulated from extremes of heat and cold, and plants establish more rapidly.
There are eight options to force a root system to branch:
1) Herbicide — One example has herbicide pellets sewn at intervals within a fabric. Root-tips that contact the herbicide die, causing branching to occur behind that point.
2) Copper — Copper is painted on the inside of a plastic container. The copper is toxic to roots, so root pruning occurs. Misuse of copper can be toxic to humans and the ground, so extra care should be taken when using these products.
3) Mechanical root pruning — Large trenching-type machines can chop roots of field-grown trees, which causes branching behind this cut. The trees are then dug, balled and burlapped. Some nurseries partially hand dig a few months before harvest to promote root branching. The open wounds of the cut, however, may allow pathogens to enter the plant.
4) Root suffocation — Roots growing in containers with water reservoirs in the bottom are pruned when the roots hit the open water.
5) Air-root pruning — Directing roots to air openings causes the root-tip to dehydrate and forces roots to branch behind this point.
6) Root constriction — Roots can be forced to branch behind a point of constriction while trying to grow through a fabric.
7) Root-tip-trapping — Roots can be forced to branch behind a point where the root-tip is snagged sufficiently enough to impede growth.
8) Light — Some fabric containers root prune by a combination of light and air.
Container production has come a long way since the days of heavy ceramic pots and tin cans. The introduction of polyethylene nursery pots changed everything. Whether production is above or below ground, and containers are hard or soft sided, root manipulation is the goal. Containers manipulate roots primarily through air, fabric strength or copper. Air-root-pruning pots direct roots toward holes, which stop root circling and promote branching. Fabric containers root prune by air, and have the added advantage of heat prevention from the sun. Copper-treated pots work in a similar manner but are falling out of favor due to toxicity from the copper treatment. With pot-in-pot production systems, crops are grown in sunken production pots which are typically nested in buried socket pots. This helps insulate root systems and prevent pots from tipping over.
Air-pruning Containers — the key to growing trees best suited to urban spaces is to maintain most roots in a horizontal position with continued branching. This can be accomplished by allowing air to prune the roots as they first reach the bottom and later the sides of the growing container. The dead root-tips have effectively been cauterized and are unlikely to be colonized by pathogens. Young seedlings respond by developing fibrous absorbing roots laterally in the container. Effective air-root-pruning containers guide root-tips into the numerous openings without having openings so large that rapid evaporation and salt accumulation become an issue. Repeated pruning on the bottom, as well as the container sides, helps the tree develop a vigorous and efficient root system.
Root-tip trapping involves fabric containers. Root pruning is accomplished as the root-tips contact the fuzzy inside fabric and can no longer extend. The trapped root-tip will typically stop elongating, become pudgy, and then lose hormonal control over root development, resulting in secondary branch roots. The advantage of this root-pruning technique is that the roots do not circle, no water is lost through the container sidewall, and roots are fully contained — even when the container is in the ground or snugly inside another container. When used above ground, the insulating value of such fabrics reduces root zone temperatures by about 20 degrees Fahrenheit. This allows root growth on the side of the container exposed to sun, and avoids the problems that arise when roots are killed by absorbed heat.
Root girdling or constriction pruning is effectively accomplished in field soil where roots are allowed to grow through a fabric container wall. Young root-tips extend freely through the fabric, but as soon as they increase in diameter ever so slightly they become girdled. Because the roots are young and actively growing, secondary root branching occurs in a similar manner to air-root pruning or root-tip trapping. In other words, the choking causes the root to lose its apical dominance and lateral branching or pruning occurs inside the bag. In the ground the roots function outside the container wall and absorb water and nutrients which are transferred to the leaves via the central xylem tissues. However, the constriction prevents the downward movement of sugars from the leaves from going beyond the inside wall of the fabric. This accumulation of sugars causes nodules to form that have many root buds. When the tree is harvested and the fabric removed, robust root growth appears from the nodules.
Most water and nutrient absorption occurs at or just behind the white root-tip. When root-tips grow out from the base of the plant and are stimulated to branch as a result of air-root-pruning or root-tip-trapping when they reach the sidewall, branching typically occurs about four inches back along the young root. The “four-inch rule” appears to apply to all species when roots are young and reactive. If this large number of roots hits the side of a smooth, conventional container and begins to circle, little is gained. On the other hand, if this large number of roots contacts the sidewall and is again either air-root-pruned or pruned by root-tip-trapping, the resulting mass of roots infiltrates the entire mass of growth medium for maximum absorption of water and nutrients to support top growth.
With their roots established, tender seedlings must be hardened off before being potted and moved outside. During the hardening process, trees continue to develop roots to fill their larger containers. Top growth is halted until there are enough roots to support new growth. Depending on the species, growing conditions, and other factors, five to as many as seven flushes of growth may occur in a single growing season.
Advantages in urban landscapes
Trees with fibrous root systems and active white root-tips at the time of planting have a much greater likelihood of success in restricted urban spaces. The fibrous root system can immediately begin to exploit any favorable soil volume that exists. However, the volume of soil suitable for root exploration becomes the limiting factor and tree decline in restricted spaces begins just as occurs in a container. Research indicates that trees grown with fibrous, shallow root systems have the capacity to function well in restricted urban spaces for a much longer period of time than conventional field-grown trees.
With smooth-walled conventional containers, roots become concentrated and congested against the inner sidewall with little root exploration of the central volume of the container. Once that inner sidewall area is exploited, root exploration and nutrient mining is restricted and the tree begins to stagnate and decline. The new types of containers eliminate this problem.
Furthermore, although watering may be required more in the air-pruning containers for the first few months, compared to smooth-walled containers, after the plant has become established in the container, water needs decrease. Because of full aeration, a heavier soil mix or medium can be used in these containers than the soil used in a solid-wall container. This allows growers to select a mix that has greater water retention capacity.
The above methods of root pruning have been in commercial use for more than two decades. Therefore we know the plants develop healthy root structures after planting in the landscape. However, disadvantages include the following:
* Smaller ball size — root pruning often produces a larger plant in a smaller-than-expected ball size. The ANLA has set standards regarding ball sizes for plants grown in fabric containers. The smaller ball size is easier to handle, but landscapers who are accustomed to handling really big root balls will have to get used to it. They must be watered and staked as needed.
* Fabric removal — The fabric must be removed at planting time on trees grown in root-girdling, root-trapping, root-constricting, and every other type of fabric container. When handled properly, fabric removal is easy, but it must be done.
With timely air-root pruning of seedlings and provisions for continued horizontal root development, trees can be consistently produced with highly efficient fibrous root systems that extend in all directions following transplanting. Not only do trees grown this way require little, if any, staking, they establish more quickly following transplanting, and more efficiently utilize the volume of soil available, because of their many small roots. The greater the support of roots for the top, the faster both top and roots grow and the healthier the tree.
Leonard Phillips can be reached via e-mail at firstname.lastname@example.org.
Author’s Note: The mention of container suppliers in this article does not constitute an endorsement of these products. They are mentioned only for providing the reader with useful information.
* Personal communications with Kurt Reiger and Keith Warren, 2007
* High Caliper Growing — Root Control, Inc., http://www.treebag.com/index.htm, 2007
* Whitcomb, Carl E., “A New Look at Root Growth in City Spaces,” Landscape Superintendent and Maintenance Professional, June 2007
* Whitcomb, Carl E., “The Obsolete Taproot,” LCN, August 2004