Intact Rootball Vs. Rootbound
by Brent Walston
edited by Robert Potts
A continued discussion of healthy roots by Brent Walston, taken from postings on Bonsaisite Forums.
What's an intact rootball?
An intact rootball is when you can knock the nursery can or pot off the root ball and it won't fall apart. This is sort like the advice of bending the branch to the point where it is just about to break. How in the world do you know without doing it? There are several tricks. You can wiggle the stem. If it stem moves in the pot, don't try unpotting it. If the stem seems pretty solid, try the next test. Try to pick the plant up by the stem. If the surface starts to give before you can pick it up, it is too soon. If you can pick the plant and pot up by the stem, do the next test. With a surface just under to pot to catch it, knock the pot off the roots. Don't pull it off. The proper procedure is to hold the stem in one hand and give the rim of the pot a sharp rap with the palm of your other hand. If the pot drops off cleanly and the rootball doesn't fall apart, you can pull it and inspect it. If the pot falls to the surface and the root ball collapses back into the pot, it's too soon. That's why you want something just under the bottom of the pot.
Why inspect the rootball?
People don't inspect rootballs nearly enough. Hardly ever do I read a post asking for help where the pot has been knocked of and the roots inspected, even though this is the most revealing test you can perform. I do it all the time for healthy and sick plants just to see what is going on. A healthy growing plant will have a nice intact rootball with lots of lovely white growing root tips.
At what point do plants stop growing in a pot? It's a bit involved, but the simple answer is that new growth stops or slows when they become rootbound. So what's rootbound? Rootbound is when there is no effective space for new roots to occupy. Roots effectively occupy the entire volume of space between the soil particles. One of the first symptoms of being rootbound is, in fact, that plant growth slows despite favorable environmental conditions (light, water, fertilizer, etc). The second symptom is that rootbound plants begin having difficulty taking up fertilizer. This is undoubtedly related to the inability to form new root tissue. You see this as a chlorosis despite the fact that they have been properly fertilized.
People often confuse leaves with new growth. New growth is the process of continually opening the terminal bud of a stem (shoot formation). Plants can be, and frequently are, alive and relatively healthy with absolutely no new growth. This happens when plants are severely rootbound, there is a lack of fertilizer, or after a trauma such as barerooting. The existing buds will open, leaves will form, but no shoots will develop. This condition will persist until conditions change. I have seen many plants survive year after year without shoot growth. New growth each year consists of a succession of opening terminal and axillary buds in the spring without any shoots to form an internode. If you look closely at the stems there are just a pile of leaf bundle scars piled up on one another. Talk about close internodes!
Rootbound plants need to be rootpruned and shifted at the nearest appropriate opportunity. This usually means winter, because unlike shifting an intact rootball, rootbound plants must be rootpruned to initiate proper root growth.
It is difficult to tell when a plant is rootbound just by observing the roots. I think it is better to determine 'rootbound' by both the symptoms of growth (or lack thereof) and the physical density of the roots. For our purposes (bonsai), trees should be rootpruned and repotted long before they reach rootbound conditions. This doesn't happen overnight. There is a long gradual procession of slowing growth over time, usually several years before all new growth stops. It is clearly evident what is happening if you stop to look.
Root growth patterns are species dependent
Some species quickly occupy the soil mass uniformly (Buxus). And yes, there are species that love to occupy the bottom of the pot with roots, but not the top, Cedrus and Quercus come to mind. But given enough time, both of these genera manage to occupy the entire soil mass, albeit over many years. While doing this, shoot growth is present, but obviously slowed. Growth of this kind presents a problem when root pruning and repotting because you often don't really know where the root crown is. It is very easy to buy a rootbound nursery plant, slice off the bottom portion of the roots with a saw or axe, and then find out you just cut off the bottom of the trunk. You have to proceed slowly and carefully when root pruning rootbound trees. It is difficult and arduous.
In this, as well as most of the other articles at this website, I have tried to point out that container growing is a system. Rarely can you change one condition without changing others or changing the growth dynamics. At first it may seem like a daunting task to understand the interrelationships, but it is a necessary to learn these processes to be able to successfully manipulate plant growth.
Overpotting and Root Pruning
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