How To Cut A Taproot?

Hey There,

So, I've purchased some land on a mesa in southern Oregon, and after the purchase I am given to understand that the soil is only about 18" deep before hitting bedrock. Now, all of the other potential concerns aside, I would like to start an orchard & homestead on my land, but I'm concerned about how deep the taproot will grow on any trees I attempt to grow on the property.

I've developed a structure I'm planning to share here later that I've dubbed the "Hugel Stem," which should add a good 5-6 feet of dirt where the trees are grown, but as the trees get older I'm given to understand the taproots will dig down as far as 20-30 feet, depending on the tree. Obviously, in this situation, this will lead to the trees doing quite well for a number of years before eventually growing sick and dying as the taproot meets the barrier of bedrock.

What I'd like to do is cut the taproot after the trees have reached an acceptable height, but not so early on as to make them as small as a bonsai. I want them to be able to bear fruit, after all. And plenty of it.

So... does anyone know if there's a special technique that can be used on trees to cut the taproot without killing the tree?

Most trees don't have "taproots" like a carrot; they put out lateral roots and then send down vertical roots from those. In general, plants will do what they need to do to adapt. Here's what the Morton Arboretum says about it:

10. Myth: All trees have a taproot.
Fact: Trees that germinate with a taproot eventually lose the taproot as the secondary roots grow and become more dominant. For most trees, the taproots that had initially established themselves are nonexistent when the tree reaches maturity.

We just had a stand of pine trees uprooted by a storm. All the roots formed a dense mat in the top 18 inches of soil. No tap roots in sight.

Here's a pretty good discussion of the topic:
http://www.deeproot.com/blog/blog-entries/how-deep-do-tree-roots-really-grow

In short: root growth is determined by a combination of factors, including tree species, soil type, soil compaction, and soil drainage.

Robert Kourik has a book devoted to roots at http://www.robertkourik.com/books/roots1.html . According to his site, those 18 inches of soil you have host 90% of the roots anyway:

Did you know?

• About 90% of a tree’s roots are to be found in the top 18 inches of the soil.

• At the end of its first year’s growth, an apple tree can incorporate as many as 17,000,000 root hairs with a total length of well over a mile!

• The glorious magnolia (Magnolia grandiflora), can grow roots 3.77 times wider than the dripline.

I also recall that Mark Shepard, in Restoration Agriculture, cited the Hundred Horse Tree, a 4,000 year old chestnut, that grows on bare rock on the eastern flank of Mt. Etna in Italy. It has the largest girth of any tree ever recorded, according to him. The Canadian Shield covers about half of Canada, and there are plenty of large trees growing there. Absence of a taproot doens't guarantee failure at all.

Tap roots can find ways into the bedrock and develop a strong foothold for the tree. In "The Designers Manual " Bill Mollison has a quirky idea that might help you :

"One way to plant an apple tree in very hard ground is to detonate a small plug of gelignite a foot or two below the surface , the roots will follow the shatter pattern , and further elaborate it ."

Sounds like a fun weekend !