Let’s go forward, not two steps back

Figure 1: Once roots are circled, kinked or deformed, they will remain in that position for the life of the plant. The only solution to this problem is to grow seedlings in containers that prevent these problems.

An old adage goes that, “Without knowledge of history, the same errors will be made again and again.” I see this in a variety of ways; some papers in current journals show the same study in the same way with the same conclusions as was done years ago. And what I find most telling and offensive, typically the original work is not mentioned or cited in the literature review, confirming there was no review.

Similarly, the last few years there have been papers and presentations extolling the “advantages” of shaving the root ball of a container grown tree prior to planting. Recently, it came to my attention that a similar practice of growing tree seedlings in band-pots, then cutting off the congested roots at the bottom was being promoted. Band-pots are plastic versions of my original work in the early 1970’s growing seedlings in quart milk cartons made of wax coated paper with both bottom and top removed. Who knows, the next fad may be growing plants in recycled metal food cans since the rust holes provide air-pruning.

Consider the facts, history
1. Growing tree seedlings as well as many other species in smooth, round conventional plastic pots is undesirable to the long-term health of the plant. Plant autopsies clearly show once roots are deformed and congested, they remain forever in that position (Figure 1) [Plant Production in Containers II, Ch. 6, Containers, Problems and Progress]. So why do plastic pots with smooth internal walls persist? Convention and “that’s the way daddy did it?”

2. One of the undesirable ironies relative to growing plants in the unique environment of a conventional container is that an attractive plant top can be developed even though the root system is circling, congested and terrible. This is due to the luxury of water and nutrients continually bathing the few roots present. And because very few people consider the roots, since the top looks OK, it must be OK.

3. Prior to realizing item No. 2, my students and I grew attractive seedlings in bottomless milk cartons and thought they were great. Only after planting these seedlings out and finding them poorly anchored and subject to blow-over did we realize the problem was due to the undesirable root system [Klingaman, G. and J. King, 1981. Influence of container design on harvestability of field-grown oaks. New Horizons, 21-22, and Plant Production in Containers II, Ch. 2, 33-49.] This was the ultimate good news-bad news. The good news was the clear benefit of air-pruning of the tip of the taproot to stimulate root branching. The bad news was the fact that as secondary roots grew and contacted sides of the milk carton they were deflected downward and became a congested mass at the very bottom. In several experiments, we cut off the congested bottom roots only to find severe set-back of the seedling. Plus, the roots that developed at the face of the cut were also aimed downward, leaving little lateral support for the tree.
 

Figure 2: On the right, as roots appear when removed from RootMaker 18-cell tray. On the left, after mix was washed away. Figure 3: This bur oak was about 6 inches in stem diameter 12 inches above the soil surface and was six years old from the time the seed was planted. The tree spent the first three months in a RootMaker propagation container then was planted into the field in a knit fabric container and grown for 2½ years. At the end of the third growing season, stem diameter ranged from 2.25 to 2.75 inches and the trees were harvested, the knit fabric removed, and all were replanted on 10-foot centers and allowed to grow three more years. The largest root on any of the trees was one inch in diameter at the outer face of the 52-inch root ball.

4. The challenge was to find a way to utilize air-root-pruning of the taproot at the bottom, yet avoid the massive congestion of secondary roots at the bottom. The answer was to create a container with ledges and ribs to intercept and guide secondary roots into openings in the sidewall for further air-root-pruning (Figure 2). And it worked well [Plant Production in Containers II, Ch. 5, Container designs that work].

5. The key to growing plants in containers is to utilize containers designed to stop root circling and stimulate root branching at all stages from seed germination to landscape size specimens. Even with these advancements, transplanting must be done timely at all stages in the sequence to avoid roots running out of space for further development, thus slowing growth. Timing remains a key cultural factor.

6. When root development and root systems are at their optimum, root extension into the surrounding soil following transplanting can progress at a rate of one inch per day or more. As a result, by the end of the first growing season, the plant is well established and ready to perform admirably in the landscape the next year without further attention (Figure 3)[Plant Production in Containers II, Ch. 6, Containers; problems and progress].
 

7. Copper toxicity is real and highly undesirable relative to growing plants in containers and to the health and rate of root extension following transplanting [Furuta, T, et.al 1973. Chemical control of root growth in containers. Am Nsy page 13 and Cathey, H.M. 1970 Chemical pruning of plants. Am. Nsy Page 8 and Whitcomb, C.E. 2001. Improving containers; copper is not the answer NM Pro 76-78] because copper is elemental and is never broken down or destroyed.
 

Figure 4: Root development of elm seedlings following transplanting from a milk carton/bottomless plastic sleeve container (left) vs. the RootMaker container (right). Note the absence of horizontal support roots with the plastic sleeve.

8. In the early days of nursery production, weed control was by mechanical cultivation, bare root seedlings were the norm and open wounds in plant roots were abundant. Persistence of crown gall (Agrobacterium tumefaciens) and other opportunist pathogens looking for an open wound to colonize was a major problem. Walker reported in 1957 that “Infection takes place entirely, as far as is now known, through one or another type of wound” and “the deeper the wound the better the chances of infection” [Walker, John, 1957. Plant Pathology; McGraw-Hill, Ch. 4, Bacterial diseases]. Riker and Hildebrandt noted in 1953 that “bacteria enter tissues most commonly through wounds caused by insects or cultivation.” And, “Symptoms may not develop for several weeks, depending on temperature, humidity and growth of the host” [Plant Disease, the yearbook of agriculture. USDA, 1953]. Tatter in 1978 reported “Crown gall can usually be prevented by avoiding unnecessary wounding” and that “care should be exercised during cultivation to avoid wounding the stem or roots” [Tatter, T.A. Diseases of Shade Trees, Academic Press, Ch 3]. And Sinclair, Lyon and Johnson in 1987 noted that bacterial diseases are present around the world and “the cycle of infection is simple yet elegant.” “Bacteria in soil infect fresh wounds of any sort.” And “cells in wounds remain receptive for periods ranging from a few days during active plant growth to months while plants are dormant” [Diseases of Trees and Shrubs. Cornell U. Press 1987].
 

9. With the development of chemical weed control, container-grown seedlings and minimal root damage and wounding during the production and transplant process, problems with crown gall and other root invading pathogens have steadily declined.

Figure 5: Elm seedlings grown in a 3-gallon RootMaker vs. same volume smooth pot. Cutting the circling roots on the smooth pot is essential. However, to lose all of the active white roots on the RootMaker container would be absurd.

Two steps backward
10. Recently the practice of “shaving the sides of container grown plants” at time of transplanting or planting into the landscape has been advocated [Gilman, E.T, et.al. 2010. Rootball shaving improves root systems on seven tree species in containers. J. Env. Hort. 28:13-18]. The goal is to stop circling roots, but at the cost of creating lots of open wounds and problems for the plant both immediately in having few or no white active root tips to quickly extend out into the soil (Figure 4). Plus, the potential longer-term problems as bacterial and fungal diseases enter and colonize the many open wounds. This is a classic example of ignorance of history.

11. A further recommendation being touted is that tree seedlings should be grown in plastic band sleeve containers (i.e. the old bottomless milk cartons) then the congested wad of roots that develop at the bottom cut off prior to transplanting (Figure 5) [Gilman. E.F. and P. Anderson. 2006. Root pruning and transplant success for Cathedral live oaks. J. Env. Hort; 24:13-17]. This, too, is promoting problems both with open wounds and as new root growth is still mostly down. In addition this practice slows plant establishment and refuses to recognize that studies have already been done that show the error of that practice. Plus there are now practical alternative containers that air-root-prune both at the bottom (like the milk cartons) but also at an assortment of openings in the sidewall.

12. These errors fail to consider history and recognize the continued value of research done long ago. These errors may also go to the NIH factor (Not Invented Here). Egos are amazing phenomena, and when scientists refuse to do their homework and recognize and build on the contributions of predecessors, they are forced to waste time reinventing the wheel.

Carl E. Whitcomb, PhD, is founder of Lacebark Inc. in Stillwater, Okla.

Disclaimer
The views of this article are that of the author and not necessarily those of Nursery Management magazine or GIE Media Inc.