Exactly, take out the one you don't want... but remove the stump or use stumpkiller, or it'll start growing again.

10 feet is nothing in the life of those trees - if they are left the trunks will be touching each other and pushing for space before many more years have passed...One tree allowed to achieve its full shape will be more pleasing in the end than four or five crowded together.

'Houston, we have a problem'
At 12 inches apart, you cannot remove the stump without compromising the tree you are trying to spare and I would advise NOT to use stump killer in this instance. The roots are intertwined and trees exchange food & water underground and so they will the stump killer too, killing both trees. Just cut the sacrificial tree down as low as possible, then use a stump grinder to finish the job. You can rent one real cheap or hire an arborist.

Here is an article you will find extremely interesting: http://blog.drwile.com/?p=14646

Any application of stumpkiller to a cut down tree stump will only kill that actual tree - there is no process of osmosis or translocation that will transfer the stumpkiller to other, separate tree roots nearby, so long as the stumpkiller isn't poured by the gallon onto surrounding soil. In point of fact, grinding out stumps and roots is more likely to cause damage to other trees if not done with care,but grinding them out is still the best option to prevent honey fungus infection.

There actually is a process of translocation but not through osmosis, it's through the roots. Just read the article. Trees are living things and they correlate in remarkable ways.

Excerpt: That means the mycorrhiza transferred the photosynthesis products of the exposed trees to the roots of the unexposed trees! In addition, the researchers found that these photosynthesis products even ended up in trees that were of a different species than the exposed trees!

Stump grinder is just for the stump. Don't worry about the roots, they will naturally decompose without upsetting the neighboring tree.

Link to this article please, Bathgate, I would like to read it; having used stumpkiller on all sorts of woody trunked shrubs and trees growing in proximity with others down the years, I've never yet known any surrounding tree or shrub die as well, so I'm intrigued.

The link is right in front of you. Simply scroll up.

Here's another one, different source, same conclusion;

http://ideastations.org/science-matters/question-your-world/question-your-world-how-do-trees-help-each-other-out

Oh yea, sorry, missed it... hmm, well having read both links, I ain't convinced, its a great pity there's no link to the actual scientific results in dwile's blog... but extrapolating that, because an altered form of carbon had a 4% uptake in other trees, a poison will be transferred from one tree to another via mycorrhizae is not a reasonable theory to make from the information there. In my extensive experience, there is no uptake of stumpkiller by surrounding tree roots. I once used it on cut down stumps every other shrub in a 30 foot row of Prunus laurocerasus, because too many had been planted initially some years earlier, with no ill effect on the remaining plants. But we can agree to disagree, if you still think its a risk.

If I may add my thoughts:
Mycorrhizae do indeed assimilate products of photosynthesis using enzymes specifically for natural carbon compounds produced during this process. They then absorb these smaller molecules and then convert them into carbon compounds that they use for their own biology. In close association they also 'give' to roots of other plants.
However there is no evidence as far as I have found that shows mycorrhizae are able to absorb stump killer [too large a molecule to pass through the fungal membranes] and transfer it to neighbouring roots. They don't posses the enzymes to digest it.

I have been watching the research on this for many years now:systemic weed killers are also a case in point.

That's very interesting Seaburngirl. Amazing how a stand of trees will function as a single organism in many ways. I also read that a mother tree knows which are her own seedlings and will transfer nutrients and water to them through an underground network. There's so much more I don't understand, but this makes me stop and think.

I have successfully used SBK on weed shrubs entwined with landscape shrubs, without harming the one I wanted to keep. I will admit that they were small, compared to their bigger neighbors.

Seaburngirl - I assume that also means that systemic herbicides cannot be transferred via the mycorrhizal route? Would it kill the mycorrhizae growing in association with a particular root?

That's an interesting idea Paul. usually the majority of trees tend to inhibit the growth of their own seedlings too close to them as they will compete with the parent tree for light/water and nutrients. The concept is called alleopathy and lots of plants show this, though some do produce chemicals to encourage the growth of plants such as clovers.

Yes Bamboo that's is my understanding too. Fungicides work differently to herbicides as their biochemistry is different.

Not really, because the roots are also grafted together or fused together.....read the article.

I did read the article Paul but as bamboo says lacks clear science evidence or ref to research. But the concept of alleopathy is genuine and once established commensalism does happen. But trees usually disperse their seed some way away from them to avoid competition.

I don't know the full science behind it either, there's much to be discovered, but the underground network is extensive and complex. I can drive from New York to LA, or I can go to Boston, or the Big EZ. It's just a matter of what roads I choose.

I studied Botany & Zoology at uni so I understand the science behind plant growth etc That's why it's disappointing there aren't the links to the original research so I can read it to a greater depth. I still have contact with the botany dept and regularly attend seminars to keep my 'finger on the pulse' so to speak.

Please may I ask a related question? I have mint roots which have grown along a crevice which now needs filing with mortar. I'm trying to remove all the mint from that area. I would like to kill the roots before doing the mortaring and am worried about whatever I use leaching out to planting at the end of the crevice. Any advice about what would be safe to use would be very much appreciated, with a apologies for muscling in on the question -seemed a good idea when the experts are all assembled!

I know you are smart Seaburngirl. I just added a little humor to lighten the mood. I like the idea of driving down to the Big EZ aka New Orleans. I don't think we'll ever fully understand how it all works.

sadly my geography is appalling so the destinations didn't make any sense :o) So sorry I missed the humour I didn't know what/where the Big EZ was Paul. My uk geography is not much better. Unless I have been there or need to go there I haven't a clue. I dropped geography aged 13 :p Hated the teacher.

Stera if you use a glyphosate weedkiller and perhaps use a paint brush to add it carefully to the plant stems then it wont contaminate neighbouring plants. These weed killers are absorbed by the plant and translocated down to the roots. In the past I have used a fine crochet hook to try to pull out stems. The lime from the mortar is just as likely to leach out. but unless you have extremely sensitive acid loving plants it wont be a problem.

FYI, Sbg, "the Big Easy" is New Orleans, Louisiana's nickname. What makes it so easy is beyond me--especially after Hurricane Katrina! Maybe just the fact that, like where I live, shoveling snow is an unknown! :)

Nothing to do with liquor / gambling and prohibition then? well I live and learn.
why is new York the big apple? don't understand that either :o)

It seems more like a big onion

I've been interested to read your conversations above. I've recently been reading about this theory regarding trees communicating and supporting one another. It's fascinating. A good example is of the Acacia Trees in Africa which put out a bitter taste in response to being nibbled. Then all the other trees that haven't yet been nibbled start doing it as well, so the Giraffes have to go some distance before they can feed on the same kind of trees. Wonderful isn't it? Makes you think...if they can do that...what else do they get up to? I'm no scientist of course, I don't have the brain wiring for it. But I am confident that the boffins will get to the truth of it in good time. In the meantime, it makes me think twice about being violent to trees in any way! :) When I moved here I wanted a multi-stem Birch, but the nursery advised planting three, one foot apart. So in a few years I will know whether they can grow happily so close. A few years ago I saw an amazing sight in a woodland....a great big tree with a smaller conifer growing right out of it...must have seeded in between two roots. And the way it was growing it looked just like the large tree was protecting the conifer. Wish I'd taken a photo! Love Trees :)

Ho's that for salesmanship Karen?, getting you to buy 3, when you only wanted 1, very clever, Derek.

Lol Derek! So cynical! ;) No, it was Bluebell Nursery. They are excellent. I wanted multi stem and they only had very young saplings, so it was the only way. :)

I saw that documentary about the Acacia Trees - remarkable! They also spoke about how plants respond to different pollinators - blooming at night and different times to accommodate certain bats and moths.

I'll let them off then Karen, {on this occasion} Derek.

Whoa, awesome dialogue! Lots to think about. For my application, I think the visual benefits are appealing. Gonna chop down the inferior tree!

That's what George Washington would have done.