[NAFEX] Allelopathy or Just Tough Competition?

Alan Haigh alandhaigh at gmail.com
Thu Dec 1 05:52:00 EST 2011


I have consistently found the presence of black walnut roots to be stunting
to most common fruit<http://forums2.gardenweb.com/forums/load/fruit/msg1119045214851.html>trees
and especially peaches. It is well known that juglones, a compound
exuded by black walnut roots and fallen leaves,  is toxic to many plants,
but I had an interesting experience today when transplanting trees at two
different sites that makes me wonder if that is the only way in which black
walnuts are stunting.


At the first site there is a large black walnut near a small orchard I
installed three years ago. I was adding a mulberry near it today. Although
the orchard is a fair distance from this tree, its roots are incredibly
thick where I planted the fruit trees before and where I planted the
mulberry today- much thicker than the kinds of root systems that are sent
out by other, similar sized forest trees- species like ash, maple, oak and
tulip. The fruit trees have not established as quickly as usual- they are
growing, and I am confident they will eventually thrive, but it is taking a
while.

At the second site there are no black walnuts, but there are two very
large, cold- hardy pecan trees whose roots tightly weave through the soil
in the same manner as the black walnuts at the first site. It is a fairly
large home orchard with most fruit trees beyond the range of these roots,
but in the area where they do dominate it has been a kind of Bermuda
triangle for peach<http://forums2.gardenweb.com/forums/load/fruit/msg1119045214851.html>trees
over the last several years.


 At first, after planting, the trees are fine, but by the third year they
would quickly go into decline and out of productivity, barely surviving. In
the process of planting I would create wide holes where competing roots
were cut back a significant distance, which seemed to prevent the peach
trees from being stunted at first.


 Today I transplanted one peach tree from that site to see if I could get
it to flourish where those black roots aren't present. This tree had barely
grown since I put it there 3 or 4 growing seasons ago- or so I thought. At
least the top hadn't grown much. When I began the process of bare root
transplanting it I was extremely surprised at how massive the roots had
become without coinciding vigorous top growth.


 It took me four times as long to transplant as I expected and I have never
seen such a great looking root system attached to such a pathetic tree. The
annual shoots were not more than 3" long and there weren't enough of them.

I have read that juglone can destroy hair roots but these are pecans and
not black walnuts and pecans are not reported to generate levels of juglone
adequate to stunt other trees, to my knowledge.


 At any rate, I find it quite amazing that a tree can be thoroughly stunted
on top while growing a very substantial root system. Clearly that root
system was not functioning well and I wonder if the stunting was the result
of toxins or just an extraordinarily competitive species grabbing most
available water and nutrients from the soil at the expense of less
competitive trees. If it is the latter and pecans and black walnuts have
similar root systems, maybe there is more at play than allelopathy when
black walnuts stunt other plants.


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