Root Pruning Japanese Maples

tillygrowerMarch 12, 2012

I just finished my first proper repot of a JM into gritty mix. I've read Al's posts on proper root pruning, but I'm still not clear on whether I was supposed to leave some of the largest "anchoring" roots, or was I supposed to prune off all of these big roots, leaving only the fine roots behind.Is there a size/diameter "cutoff" of roots you are supposed to remove?

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peapod13(8 South Sound WA)

My take on this is that it depends on container size. I may be wrong and I hope Al will correct me, if necessary, before I lead you and others too far astray.

In very shallow/small pots (not necessarily bonsai pots but certainly bonsai pots are good examples) there is minimal soil and therefore minimal area for roots to grow. In these pots the tree will likely need all of the available soil area for nutrient uptake roots. As such it is common practice in bonsai to remove all larger roots and provide support for the tree by wiring the tree to the pot. By doing this the tree needs no support roots and the container arborists can dedicate all available roots space to nutrient transport roots by pruning all larger roots (larger being subjective).

In larger pots where the tree has enough soil, container arborist can allow both larger support roots and smaller nutrient transport roots. Assuming of course the soil has nutrients to transport.

In the 1-1-1 mix, the soil itself has almost no nutrients to give to the tree. Some nutrients will become available as the bark breaks down, but generally this soil is thought of as "sterile". The nutrients come almost entirely from the fertilizers we feed.

As to your question of the size of "larger vs smaller" roots, I either was told by Al, read on here somewhere or read in one of the books Al suggested that roots over 1/8" really provide little use in container culture.

Actually, I make a distinction between support and nutrient transport roots as though they were seperate types of roots but in reality only the very fine "hair" roots are capable of taking nutients and water from the soil. I think in time hair roots will become larger support roots, only transporting nutrients in their cambium layer and not actually capable of taking nutrients or water from the soil. Because the tree can get many more smaller diameter roots in the same space as one larger root and therefore more "cambium" or space to transport nutrients and grow the xylem and phloem, subsurface roots over 1/8" can be removed in container culture. Also removing larger roots (which may have many hair roots attached along it's length) will encourage the tree to send out many more smaller roots each terminating in hair roots.

Probably more indepth and too far astray in understanding from your original question.

Short answer, small pots fewer roots over 1/8" and may need outside help for supporting the tree (ie wiring from underneath like bonsai culture) larger pots can keep some roots over 1/8" for support but the tree can still only support itself against being blown over by the wind to the extent that the inertia of the weight of the pot is not over come by the energy of the wind blowing against the top of the tree.


    Bookmark   March 12, 2012 at 8:16PM
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tapla (mid-MI z5b-6a)

Good info, Blake.

Your goal is to have a few large roots attached to the trunk for water and nutrient transport, with the rest of the roots being very fine. The best way to achieve that end is to systematically prune the large roots back to just beyond a very small root that branches off of the main root, leaving the fine root as the new root 'leader', so to speak.

The first thing you prune off are roots growing upward or back toward the center of the root mass, and roots growing downward directly under the trunk. Eventually, you want your roots to spread evenly away from the trunk in a horizontal growth pattern before they start to branch. Then prune off roots growing directly downward, and truncate all the large roots near to the trunk, leaving a fine root to extend & colonize the soil.

I think the only part of root pruning that really requires much experience to make a good decision is how much rootage you can safely remove before you risk the plant shedding branches because it can't move enough water to keep a branch viable. Fortunately, there is something of a safety mechanism built into most deciduous trees, in that if you root prune before they begin the spring flush, they will only activate buds they can support, WHEN they can support them. This doesn't mean you can cut off all the roots and treat the stump as a cutting, but if you also prune the top, you can come pretty close to that in some plants - trident maple is one that comes to mind, but I wouldn't press things too much as a beginner.

One tip: If you have a 12" deep pot and the roots only occupy the top 3-4" of the soil, you might need to water every couple of days to make sure the soil is moist where the roots are. Let's make it 2 tips: Don't allow the roots to dry out while you're working on them. I usually work with a hose or a tub & dip or spray the roots very frequently until I get the plant repotted. It's those very fine roots that do all the work that hasten the plant's reestablishment.

I just (top) pruned about a dozen maples this weekend and today - still a little too early for me to be repotting. I have a really large trident maple forest that needs repotting. I'll tackle that this weekend - it's going to take a while as there are 20-25 trees in it. ;-)


    Bookmark   March 12, 2012 at 8:49PM
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peapod13(8 South Sound WA)

I don't envy your task Al, but would love to be a wall flower watching you at work to see what and how you prune. You have some beautiful trees from the pictures I've seen.


    Bookmark   March 12, 2012 at 9:58PM
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Thanks PeaPod and Al. I understand it better now. Al, your comment about pruning a large root until just after a small root branches from it helps a lot. For some reason, I couldn't get this concept from previous posts. Now that I understand this, I realize that I was probably overzealous in my root pruning of my first JM. I cut off some roots that were probably 3/4" in diameter. The tree hadn't yet opened its leaf buds, but sounds like I may have caused it some real problems. Yikes!

    Bookmark   March 12, 2012 at 10:34PM
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