The Four-Inch Rule: Air Pruning from Germination & Rootmaker

[cornfed]

As I began learning more about bonsai, I stumbled across a concept you guys are all very familiar with, but was new to me: Air-Pruning. I learned about it while researching a local tree nursery that uses "The Rootmaker System" by reading the literature on their website. Pictured below is an example of what they write. This nursery is not a bonsai nursery at all, but I suspect they may be inadvertently growing perfect stock to experiment with... at least better than what is available at the local yard & houseplant nursery.

A primer on the system narrated by Dr. Whitcomb​

I'm new to this. Any trees I acquire are going to be nursery stock, cuttings or air-layered from other trees. They won't be ready for a bonsai pot, and the roots/trunk are the first thing I will have to work on. I am betting that these pots will be better suited to that end than other options before me, so I bought some from them directly, cut them in half and 3D-printed a bottom for the second half to create my own BOGO.

I read further about Dr. Whitcomb, and discovered his system was created after he observed what he calls the four-inch rule. He apparently discovered it while observing plants that were germinated in milk-carton tubes of different heights and went from there. Whitcomb has written a number of books, including one called Plant Production in Containers II that I would love to get my hands on.

Anyone that has ever pruned shrubs or trees to make a hedge has experienced the ‘4-inch rule’. It has long been known that when a twig or branch is cut, branching occurs, but did you notice where? Typically from the point of the pruning cut and extending back about 4 inches. Allow the numerous branches produced as a result of the first pruning to grow out 4 to 6 inches then prune again and more branching occurs. Branching occurs as a result of removal of the terminal bud, and the hormone that suppresses branching (also known as apical dominance). What has been known only in recent years is the fact that the 4-inch rule applies to roots as well. This is especially dramatic with young roots that are the below ground counterparts to the twigs pruned to create a hedge. As roots grow older they become less responsive to pruning just as occurs with larger limbs and branches.

Dr. Whitcomb on the Four-Inch Rule​

And well... I just think that is interesting and worth sharing. I know he is speaking in the context of helping nurseries grow better trees for transplant into landscapes, but there has to be some information in here that is useful to us in bonsai. It sounds like Telperion Farms used Rootmaker in their process. Knowing that, and after reading Whitcomb's research on how the four-inch rule is most applicable to seedlings, I think I am going to start some seeds in his special propagation trays and just enjoy the whole dang process of growing this tree, as slow as it may be. The local club can help me manage numbers if I have success.

But on the other hand, it seems like just repotting a nursery plant into a Rootmaker Pot does not provide the full benefit of the system, which begins at germination. But that is exactly what I did, so.... I created this topic to share the information I learned and to document my own experiments with the containers to train young trees for pre-bonsai. Does the pot still provide benefit to older trees and cuttings? I am guessing yes by the mountain of evidence that is all of you who use colanders and pond baskets... so I am hopeful.

Air pruning, pond baskets, rootmakers & colanders are all well-worn subjects on this board. I know I am not alone in the forum experimenting with these pots. But I have never seen anybody bring up this Four-Inch Rule. (I did a search).

Questions I will have to explore:

• What exactly are my goals for the tree I am putting in the pot?
• Do I use a different substrate than what I would put in a traditional training pot?
• What material properties should the substrate have and with what mix will I achieve it?
• How often will I need to water that substrate?
• How much water is lost to extra evaporation? What could ease that loss?
• What will I do when the roots fill the container? Larger size rootmaker? More traditional bonsai training pot? Put it in the ground over a tile? Bag?

Anybody who has been down this path, I welcome your experience here.

 

[Shibui]

That 4 inch rule seems to be similar to what I have seen and now tell anyone that will listen. My experience has been far less than 4 inch though. When roots are cut the vast majority of roots grow from the cut ends, much like new shoots from a pruned Chinese elm. I have lots of photos of roots doing this from the field grown trident maples I grow here.
Cutting roots way out rarely results in new feeder roots close to the trunk. The roots must be cut shorter than the final length you are aiming at because roots always grow longer just as the top of the tree always grows upward after chopping.

Air pruning pots are great for developing lots of fine roots but will those roots be where you want them for the final bonsai pot? For the roots to fit a bonsai pot the air pruning pot needs to be much smaller than most I have seen used, at least in the initial stages. I find that manual root pruning can give the same, or better, results.

Good luck with the research. I look forward to seeing more.

 

[cornfed]

Yes, I saw a similar sentiment in a past thread about these type of pots. Why air-prune when you manually prune the roots when you repot? It's a worthwhile question.

I think what Whitcomb is saying is very specific, and doesn't apply to cut roots the same way (where like you say, new roots shoot from the cut). Looking back, I might've picked the wrong section to quote from that paper. Those two videos embedded above are the best way to understand for anyone who wants to.

With roots, the white tip is most responsive and when air-root-pruning occurs at the proper location, secondary roots typically begin to form quickly and within 3 to 5 days the 4-inch rule is obvious. By contrast, in nature the tip of a taproot extends downward until conditions become unfavorable (rock, hard subsoil, lack of oxygen, water table, etc.). Only when the tip of the taproot stops growing or dies does secondary branching occur, but by then the tissues just beneath the soil have matured and few branch roots are produced on most species. As a result, only a fraction of the secondary roots form in nature compared to when the tip of the young taproot is air-pruned in a container at a point about four inches below the seed (Figure 1). Horizontal roots respond to the 4-inch rule as well. However, the 4-inch rule does not apply to roots that extend out to the sidewall of smooth conventional containers and circle.

And

Therefore, to develop the most fibrous root system depth of the propagation container should be about 4 inches deep and no more than 8 inches wide. To make a propagation container 8 inches wide consumes far too much space, therefore a container 4 inches deep and 2 “to 4” wide is the practical optimum.

Below is from a second paper he has written on the subject.

When a seed germinates in nature, the fundamental objective is to extend the primary or taproot as deep and as fast as possible in order to secure the young plant in place and access moisture, critical to survival. Every seed of every species has this same objective. With some species, the dominance of the taproot is short lived. With trees and some other species, taproot dominance may continue for a number of years. As long as the tip of the taproot remains active, development of secondary branch root production is suppressed. However, in the long term anchorage of a tree and gathering of water and nutrients to support energy production in leaves ---the secondary branch root system in the top 10 to 12 inches of soil is far more important.

Most of this is generally known and accepted. But what is still mostly unknown or ignored is the practical and economical way to skip the multi-year dominance of a taproot and go directly to the much more supportive, multi-branched secondary root system (Figure 1). The key is air-pruning the taproot while it is still soft and actively growing. If you miss this opportunity, you cannot go back. When older roots are cut using antique ways, an open wound is created, ideal for entrance of pathogens. In addition, older tissue is much less responsive to pruning and secondary root production. When an older root is pruned, new roots develop mostly around the face of the pruning cut. By contrast, when a very young root is air-pruned, no open would is created. The dehydrated root tip is much like a cauterized wound and unsuitable for pathogens. When a soft and active root tip is air-pruned, very quickly, typically in only a day or two, secondary and more horizontal branch roots begin to grow from the short vertical axis of the taproot. And, importantly, these secondary roots develop around the entire circumference of the taproot (Figure 2).

So my takeaway is... yes, putting a nursery tree or a cutting in an air-pruning pot like a Rootmaker will benefit the tree's roots the same way that has been documented that a colander or pond basket will benefit the tree's roots. Namely it will prevent circling roots.

BUT to get the MAXIMUM ADVANTAGE (of his specific system at least), the first several days of air-pruning after germination are critical. Which leads to my wanting to germinate some seeds. I mean, if you can be reasonably ensured of a radial distribution of roots because of the stress-free methods you use early in a tree's development, wouldn't that be great? Better flair? Better Taper? It's worth a shot...

When I visit the nursery this week that uses his system, I will have more to report back. Some photos as well.

 

[Mikecheck123]

Yes, I saw a similar sentiment in a past thread about these type of pots. Why air-prune when you manually prune the roots when you repot? It's a worthwhile question.

I think what Whitcomb is saying is very specific, and doesn't apply to cut roots the same way (where like you say, new roots shoot from the cut). Looking back, I might've picked the wrong section to quote from that paper. Those two videos embedded above are the best way to understand for anyone who wants to.



And



Below is from a second paper he has written on the subject.





So my takeaway is... yes, putting a nursery tree or a cutting in an air-pruning pot like a Rootmaker will benefit the tree's roots the same way that has been documented that a colander or pond basket will benefit the tree's roots. Namely it will prevent circling roots.

BUT to get the MAXIMUM ADVANTAGE (of his specific system at least), the first several days of air-pruning after germination are critical. Which leads to my wanting to germinate some seeds. I mean, if you can be reasonably ensured of a radial distribution of roots because of the stress-free methods you use early in a tree's development, wouldn't that be great? Better flair? Better Taper? It's worth a shot...

When I visit the nursery this week that uses his system, I will have more to report back. Some photos as well.

I'd take all of that advice with a grain of salt, since it sounds like someone is trying to sell the ROOTMAKER (TM) GROW SYSTEM (R).

The science of root development is not a mystery and all nurseries and tree farms in the United States that make a profit at all know that you need strong roots for strong trees, and that strong roots are grown incrementally.

The "four-inch rule" seems overly simplistic, though. Half the fun of bonsai is learning how different species behave in different ways.

 

[cornfed]

it sounds like someone is trying to sell the ROOTMAKER (TM) GROW SYSTEM (R).

I was worried about this.

But root development was a mystery to me. I'm trying to learn more about it.

 

[hemmy]

But I have never seen anybody bring up this Four-Inch Rule. (I did a search)

Ha! Timely coincidence, I mentioned it a couple hours before your post. But I didn’t remember it as a Dr. Whitcomb rule.

https://www.bonsainut.com/threads/3...uvronensis”-in-5-gallon-pot.49036/post-847235
https://www.bonsainut.com/threads/3...uvronensis”-in-5-gallon-pot.49036/post-847235

 

[cornfed]

Crazy, I followed that thread lightly too. That tree is a gem.

Yeah, I just learned about all this recently, so what might be common knowledge around here is new to me. Are there other rules? Like I said, I'd like to read his book, but maybe not for $60. Still, I like the scientific approach and am interested in the horticulture of it all. Who knows, maybe I just end up being the guy who enjoys getting pre-bonsai ready for other club members? I'm new to this.

 

[hemmy]

I was worried about this.

But root development was a mystery to me. I'm trying to learn more about it.

I think if you look across the nursery industry there have been a lot of adopters of root pruning pots. The benefits appear to be real for top growth and most importantly transplanting landscape trees.

There are many references to Rootmaker and other pots on this site. I’ve been using them for about a decade for development. They aren’t magical for bonsai, you still have to do the rootwork to set up for great nebari. I’ll sometimes still put a tree on a small tile in the pot to help with the root spread. But I do see good growth results and great ramified root masses.

The real advantage for development growth occurs when you can shift from the seedling stage through the multi-gallon sizes. I haven’t been very disciplined at maximizing the up-potting in season.

 

[hemmy]

Are there other rules?

I’m sure the nursery industry has a lots of “rules”. There is obviously some crossover, but what we do to our trees is so far from the goals of the nursery industry.

Also, roots will still circle if left in RM pots too long. I don’t see any big different in moisture losses between the injection molded RM pots and regular nursery cans. I just use a variable mix of pumice, bark, and sometimes peat for development.

My biggest advice is do the root work early and often. It slows them down somewhat, but otherwise your are spending more time in the future on root grafting, air layering, etc.

 

[cornfed]

Thanks for the info! That's exactly what I'm trying to figure out, where can I fit this tool into the process. It sounds like it is most beneficial from germination. But I think I would like to try to germinate some seeds.

 

[cmeg1]

I really do not grow much beyond starters.I do like the idea of a self pruning type container.Seems there is two kinds of this.Air-pruning & entrapment pruning where the tips get tangled and self prune or branch.
I totally like this in early stage for sure!!!
I have even considered it in an actual bonsai pot.Just simply line the pot with the entrapment pruning type....only if it was sold in rolls I think its awesome really.

I employ air pruning alot........recently started propogating everything in stonewool......the mini blocks air-prune out the bottom.......The gro wool can simply be laid in mesh 10x20 flats and do a great job of air pruning out the bottom.

Small seedlings can be really condusive to this air-layer procedure.Trays of stonewool seem great for this as I can just add another layer of stonewool around the girdles/ringbark.
I may experiment this year with the gro wool in very shallow rootpruning pots since it stays moist for so long and can sustain at an1” depth with fresh seedling air-layers like this zelkova here.....make the starts of a pancake nebari extremely early in the process.

Will be a fun season......I dig this stonewool hydro media

 

[cornfed]

Can't argue with those photos! It looks like your system works well.

I had never heard of stonewool before... I might have to give that a look. How long do they stay in those cubes before the roots fill out and you have to pot up, and what do you put them in next?

 

[Svenge]

I have even considered it in an actual bonsai pot.Just simply line the pot with the entrapment pruning type....only if it was sold in rolls I think its awesome really.

Grow bags are made of geotextile fabric, which you can get in large rolls. Needle-punched/needle-felted non-woven fabric is the felted kind that would work for root entrapment.

 

[cornfed]

Pulled one of my Yamadori Mulberries out of it's 1-gallon rootmaker yesterday to put in a training pot.

This bad boy was just a stick when I put it in. Not one fine root, just a thick tap root.

I was pretty pleased with the results. It spent 13 months in the Rootmaker. Here it is today before a trim. Well, yesterday...

 

[sorce]

Hey this is dope! Can't believe I missed this, I found that video for the first time the other day!

I reckon if real trees get a 4in rule, we should use a more appropriate 1/2-2in rule!

I hope y'all are cutting them coney pots into 3rds!

Shallow Magic!

Sorce

 

[cornfed]

Thanks Sorce!

This mulberry was in one of the injection molded pots and couldn't be cut in half.



But I do have a couple of the Root builder pots.



I cut them in half and 3D printed a bottom for the second. Cut the cost to basically $2.80 per pot. It'll make a 3-inch tall pot and a 4-inch tall pot.