At 08:32 AM 6/16/03 -0400, Nina Shishkoff wrote:
Lynn,
Unrestrained top growth should do the trick for all trees that I have worked
with. Can't think of one that did not work that way.
Jerry Meislik

Hmmmm. That's a tough one. Obviously, the more foliage, the more
transpiration, and the easier to overwhelm a weak root system.
However, if you put the tree in a high-humidity tent, which would
minimize transpiration, would greater foliage improve root growth?
There would be more energy available for growth, but as I explained a
couple weeks ago, sugar is moved in plants using a "source-sink"
mechanism, so sugar won't move to where a potential root *ought* to
be, only to a new, actively growing root. Besides, some research
shows that high sugar concentration inhibits lateral root development.

And how do you get root initiation? By the action of hormones.
Auxin, produced by apical meristems of shoots stimulates lateral root
growth. So if you prune the top growth, you will temporarily be
reducing auxin, although as soon as back-budding occurs, and all the
new branches begin to grow, the amount of auxin will be increased.

So a tree that buds back quickly and easily would probably produce
new roots rapidly, too. However, if a tree was in real trouble, it
might be better to leave the strong apical meristems in place. I
don't know; I'm afraid I'd be stuck with trial and error.

Nina, et al

I have spent years closely studying the response of plants to pruning. Your
instincts are correct, leave meristem growth to quickly rebuild a new root
system. I have written several articles about this effect at my website,
one of them is Pruning Bareroot Seedlings.

I have even done comparison studies on establishing Acer palmatum var
atropurpureum plugs into my 2 3/4 inch pots. I divided 100 plants, half
were top pruned (this was early summer) and the other half were not pruned
at all upon shifting them from plugs to pots. They were placed side by side
in the shade. By the end of summer, the unpruned ones were completely root
established and pushing new growth, the top pruned ones were just sulking.
By the way, at that time I thought the opposite would happen! That was
probably eight or ten years ago. Since then, I have paid very close
attention to balancing growth and transpiration so that I could maintain
maximum terminal growth.

That is the key, maintaining water balance. In winter, it's easy,
especially for deciduous plants. You can start enormous plants with
practically no root system. We do it all the time with bareroot fruit and
ornamental landscape trees. Bonsai and other container plants are no
exception. The very long period of low or no transpiration coupled with
cool moist conditions allows you to leave maximum meristem growth. Pruning
only delays the response and pushes you farther into hot weather. I have
got this process down to where I lose very few plants now. I get two or
three year old bareroot seedlings and cut off 2/3 or more of the roots in
order to get them in my 2 3/4 inch pots. the tops are sometimes two feet
tall. By about this time, they are beginning to push new growth and have
nearly filled the pots with roots.

Summer is more difficult because the transpiration is very high and it must
be limited in some fashion. I find that removing foliage is superior to
actual pruning, which removes the meristem and delays the growth response.
Bud break following leaf removal is quite fast and results in a large area
of leaf surface restored in a very short time. Knowing how much foliage to
remove and how much protection to give a plant in summer just takes
experience, there is no way around it. I am still actively repotting small
and one gallon plants that are in collapsed soil mix with very poor root
systems. Fortunately, the weak roots have already limited the amount of
foliage so little to no pruning is required when they are placed under
shade cloth and given some extra watering when it is very hot.

One area that is still testing my powers is conifers. These guys just don't
like summer repotting. I have just about learned their limits but I still
get caught. This year I am losing about a half dozen nice white pines
because I repotted too late and didn't give them enough protection. I knew
I was pushing the envelope, but I was afraid they would not survive anyhow
without repotting. I still think I could have saved them if I had the
proper facilities, but the new nursery isn't that far along yet. What they
need is grafting house conditions, where both temperature and humidity can
be controlled summer and winter, usually cooler than regular greenhouse
conditions.

If you want to generate new roots, general heading back of plants is not
the way to limit transpiration. Prune methodically, remove unwanted
branches, and that isn't enough remove foliage, but maintain those terminal
buds. You can correct any aesthetic problems later, after the plant is
established.

Brent in Northern California
Evergreen Gardenworks USDA Zone 8 Sunset Zone 14

http://www.EvergreenGardenworks.com

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