Trees in Containers II
The tree is more than first a seed, then a stem, then a living trunk, and then dead timber. The tree is a slow, enduring force straining to win the sky. ~Antoine de Saint-Exupery
This is a continuation of another thread that has topped out at 150 posts. You can find a link to the previous thread ant the helpful information it contaihns at the bottom of this post.
It's not much of a secret to many, that a good part of what I've learned about plants and plant-related science has come as an outgrowth of my pursuit of at least some degree of proficiency at bonsai. Please, make no mistake, the principles applied to containerized trees under bonsai culture can, and in most cases SHOULD be applied to all containerized trees grown for the long term. Because of the small volumes of soil and small containers these trees are grown in, you might look at bonsai as a form of container culture taken to another level. Before most of the plants I grow become bonsai, they often undergo many years of preparation and manipulation while still in the same size containers you are growing in, so while I am intimately familiar with growing plants in bonsai culture, it would have been impossible for me to arrive at that familiarity w/o an even more thorough understanding of growing woody plants in larger, pre-bonsai size containers like you grow in. This thread is a continuation of one I previously posted on the same topic.
I grow and manage a wide variety of temperate trees and shrubs, both deciduous and conifers, and 75 or more tropical/subtropical woody plants. I'd like to invite you to join the discussion with questions about your own containerized trees and/or your tree problems. I will try to answer your questions whenever I can.
The timing of certain procedures is closely related to energy management, which gets too little consideration by most growers tending trees in containers. Because repotting and root pruning seem to be most misunderstood on the list of what it takes to maintain trees that will continually grow at close to their genetic potential, I will include some observations about those procedures to open the discussion.
I have spent literally thousands of hours digging around in root-balls of trees (let's allow that trees means any woody plant material with tree-like roots) - tropical/subtropical trees, temperate trees collected from the wild and temperate nursery stock. The wild collected trees are a challenge, usually for their lack of roots close to the trunk, and have stories of their own. The nursery stock is probably the closest examples to what most of your trees are like below the soil line, so I'll offer my thoughts for you to consider or discard as you find fitting.
I've purchased many trees from nurseries that have been containerized for long periods. Our bonsai club, just this summer, invited a visiting artist to conduct a workshop on mugo pines. The nursery (a huge operation) where we have our meetings happened to have purchased several thousand of the mugos somewhere around 10 - 12 years ago and they had been potted-up into continually larger containers ever since. Why relate these uninteresting snippets? In the cases of material that has been progressively potted-up only, large perennial roots occupied nearly the entire volume of the container, plant vitality was in severe decline, and soil in the original root-ball had become so hard that in some cases a chisel was required to remove it.
In plants that are potted-up, rootage becomes entangled. As root diameters increase, portions of roots constrict flow of water and nutrients through other roots, much the same as in the case of girdling or encircling roots on trees grown in-ground. The ratio of fine, feeder roots to more lignified and perennial roots becomes skewed to favor the larger, and practically speaking, useless roots.
Initial symptoms of poor root conditions are progressive diminishing of branch extension and reduced vitality. As rootage becomes continually compressed and restricted, branch extension stops and individual branches might die as water/nutrient translocation is further compromised. Foliage quality may not (important to understand) indicate the tree is struggling until the condition is severe, but if you observe your trees carefully, you will find them increasingly unable to cope with stressful conditions - too much/little water, heat, sun, etc. Trees that are operating under conditions of stress that has progressed to strain, will usually be diagnosed in the end as suffering from attack by insects or other bio-agents while the underlying cause goes unnoticed.
I want to mention that I draw distinct delineation between simply potting up and repotting. Potting up temporarily offers room for fine rootage to grow and do the necessary work of water/nutrient uptake, but these new roots soon lignify, while rootage in the old root mass continues to grow and become increasingly restrictive. The larger and larger containers required for potting-up & the difficulty in handling them also makes us increasingly reluctant to undertake even potting-up, let alone undertake the task of repotting/root-pruning which grows increasingly difficult with each up-potting.
So we are clear on terminology, potting up simply involves moving the plant with its root mass and soil intact, or nearly so, to a larger container and filling in around the root/soil mass with additional soil. Repotting, on the other hand, includes the removal of all or part of the soil and the pruning of roots, with an eye to removing the largest roots, as well as those that would be considered defective. Examples are roots that are dead, those growing back toward the center of the root mass, encircling, girdling or j-hooked roots, and otherwise damaged roots.
I often explain the effects of repotting vs potting up like this:
Let's rate growth/vitality potential on a scale of 1-10, with 10 being the best. We're going to say that trees in containers can only achieve a growth/vitality rating of 9, due to the somewhat limiting effects of container culture. Lets also imagine that for every year a tree goes w/o repotting or potting up, its measure of growth/vitality slips by 1 number, That is to say you pot a tree and the first year it grows at a level of 9, the next year, an 8, the next year a 7. Lets also imagine we're going to go 3 years between repotting or potting up.
Here's what happens to the tree you repot/root prune:
year 1: 9
year 2: 8
year 3: 7
year 1: 9
year 2: 8
year 3: 7
year 1: 9
year 2: 8
year 3: 7
You can see that a full repotting and root pruning returns the plant to its full potential within the limits of other cultural influences for as long as you care to repot/root prune.
Looking now at how woody plants respond to only potting up:
year 1: 9
year 2: 8
year 3: 7
year 1: 8
year 2: 7
year 3: 6
year 1: 7
year 2: 6
year 3: 5
year 1: 6
year 2: 5
year 3: 4
year 1: 5
year 2: 4
year 3: 3
year 1: 4
year 2: 3
year 3: 2
year 1: 3
year 2: 2
year 3: 1
This is a fairly accurate illustration of the influence tight roots have on a woody plant's growth/vitality. You might think of it for a moment in the context of the longevity of bonsai trees vs the life expectancy of most trees grown as houseplants, the difference between 4 years and 400 years, lying primarily in how the roots are treated.
I haven't yet mentioned that the dissimilar characteristics of the old soil as compared to the new soil when potting-up are also a recipe for trouble. With a compacted soil in the old roots and a fresh batch of soil surrounding the roots of a freshly potted-up tree, it is nearly impossible to establish a watering regimen that doesn't keep the differing soils either too wet or too dry, both conditions occurring concurrently being the rule rather than the exception.
Most who read this would have great difficulty showing me a containerized tree that's more than 10 years old and as vigorous as it could be, unless it has been root-pruned at repotting time; yet I can show you hundreds of trees 20 years to 200 years old and older that are in perfect health. All have been root-pruned and given a fresh footing in in new soil at regular and frequent intervals.
Deciduous trees are some of the most forgiving of trees when it comes to root pruning. The process is quite simple and the long term benefits include best opportunities for plants to grow at or near their potential genetic vigor, and stronger plants that are able to resist the day to day perils that bring down weaker plants. Root-pruning is a procedure that might be considered borrowed from bonsai culture, but as noted above, bonsai culture is nothing more than highly refined container culture, and to restrict the practice of root-pruning to bonsai only, is an injustice to those of us who simply enjoy growing trees in containers.
Trees are much like human beings and enjoy each other's company. Only a few love to be alone. ~Jens Jensen
Now that I have made the case for why it is important to regularly perform full repots (not to be confused with potting-up) and prune the roots of your containerized trees regularly, I will offer some direction. Root-pruning is the systematic removal of the largest roots in the container with emphasis on removal of rootage growing directly under the trunk and at the perimeter of the root mass.
Root pruning can start immediately with year-old seedlings by removing the taproot just below the basal flare of dormant material, repotting, and treating the plant as a cutting. This will produce a plant with flat rootage that radiates outward from the base and that will be easy to care for in the future.
Young trees (under 10 yrs old) are nearly all dynamic mass and will tolerate root-pruning well. Most deciduous trees are extremely tolerant of root work. Acer buergerianum (trident maple) is routinely reduced to a main trunk with roots pruned all the way back to the basal flare and responds to the treatment with a fresh growth of fine, fibrous roots and a fresh flush of foliage each spring. The point here is, you don't need to be concerned about the pruning if you follow a few simple guidelines.
First, some generalities: undertake repotting of most deciduous material while the plant is quiescent (this is the period after the tree has met its chill requirement and has been released from dormancy, but has not begun to grow yet because of low soil temps). Most conifers are best repotted soon after the onset of spring growth. Most tropical and subtropical trees are best repotted in the month prior to their most robust growth period (summer). Citrus are probably best repotted in spring, but they can also be repotted successfully immediately after a push of top growth.
For most plants that have not been root-pruned before: With a pruning saw, saw off the bottom 1/3 of the root ball. With a hand-rake (like you use for scratching in the garden soil) and/or a wooden chopstick and/or the aid of water under high pressure from a garden hose, remove all the loose soil. Using a jet of water from the hose and the chopstick, remove the remaining soil - ALL of it. The exception here would be those plants that form dense mats of fine roots (citrus, bougainvillea, rhododendron ...). This should be done out of sun and wind to prevent the fine roots from drying. 5 minutes in the sun or wind can kill fine roots & set the tree back a week or more, so keep roots moist by misting very frequently or dipping the roots in a tub of water as you work. After the soil is removed, remove up to another 1/3 of the remaining mass of roots with a sharp pruning tool, taking the largest roots, and those roots growing directly under the trunk. Stop your pruning cuts just beyond where a smaller root branches toward the outside of the root you are pruning. Be sure to remove any J-hooked roots, encircling/girdling roots or others exhibiting abnormal growth.
Before you begin the pruning operation, be sure you have the soil & new container ready to go (drain screens in place, etc). The tree should fit loosely inside the walls of the container. Fill the container with soil to the desired ht, mounded in the center, & place tree on the mound. Add soil to cover roots & with a chopstick/skewer, or sharpened wood dowel, work soil into all voids in the roots, eliminating the air pockets and adding soil to the bottom of the basal root-flare. Temporarily securing the tree to the container with twine or small rope, even staking, against movement from wind or being jostled will fractionalize recovery time by helping to prevent breakage of newly-formed fine rootage. Place the tree in shade & out of wind until it leafs out and re-establishes in the container.
The first time you root-prune a tree will be the most difficult & will likely take up to an hour from start to finish, unless the tree is in larger than a 5 gallon container. When you're satisfied with the work, repot into a soil that you are certain will retain its structure until the next root-pruning/repot. Tree (genetic) vigor will dictate the length of time between repots. The slow growing, less vigorous species, and older trees will likely go 5 years between repots. For these slow growing trees, it is extremely important that soils retain aeration. For these trees, a soil of 2/3 inorganic parts and 1/3 organic (I prefer pine or fir bark) is a good choice. The more vigorous plants that will only go 2 years between repots can be planted in a soil with a higher organic component if you wish, but would still benefit from the 2/3 inorganic mix.
Most trees treated this way will fully recover within about 4 weeks after the repot By the end of 8 weeks, they will normally have caught & passed, in both development and in vitality, a similar root-bound plant that was only potted up
When root-pruning a quiescent plant, you needn't worry much about "balancing" top growth with rootage removed. The plant will tend to only "activate" the buds it can supply with water. It is, however, the optimum time to undertake any pruning you may wish to attend to.
This is how I treat most of my trees. Though I have many growing in bonsai pots, more of my plants are in nursery containers or terra-cotta and look very much like your trees, as they await the beginning of intensive training. With a little effort at developing a soil from what's available to you and some knowledge and application of root-pruning and repotting techniques, I'm absolutely sure that a good % of those nurturing trees in containers could look forward to results they can be very pleased with. This is the repotting technique described that allows bonsai trees to live for hundreds of years & be passed from generation to generation while other containerized trees that have not had their roots tended to, and have only been potted-up, are likely to be in severe decline, or compost, well before they're old enough to vote. ;o)
I hope you're bold enough to make it a part of your containerized tree maintenance, and I hope what I've written makes sense - it's well past prudent bedtime for me.
Knowing grass, I understand the meaning of persistence.
Knowing trees, I understand the meaning of perseverance.
Knowing bonsai I understand the meaning of patience. ~ Al