If I understand you will be trenching between the house and the tree? That is bad since the tree no doubt has a lot of roots reaching into the cool soil under the house. but, it is what it is.
Obviously, try to trench as far from the base of the tree as possible.
The general rule is when you reduce roots, then also reduce the branches. So, prune a little to give the workmen room to work. Paint any exposed bark THE SAME DAY, BETTER WITHIN ONE HOUR to prevent sunburn. do not put of the painting until even the next day. Sunburn of citrus bark happens just as fast as it does on us when we have no tan to start with. ouch
Try to give a good deep watering about 2 days before, so that the tree has as little water stress as possible.
last of all, don't keep the trench open any longer than you have to. they should back fill the trench the same day or at the VERY latest the next day. Water the back fill as soon as the dirt is back in the trench.
oh, and one more thing. When they start to dig the part near the tree, take a file and sharpen the edge of the shovel. It will make the digging easier and give a cleaner cut to the root. Use a limb saw or a SHARP ax on larger roots to avoid mashing and tearing them.

thanks... hadn't thought of pruning... good idea... they will be using a small trenching machine... like a mower, but trenching... won't have any control over sharpness and my hunch, based on what I've seen so far, is the trench is likely to be open for days... I don't know... at the moment I'm just sick about the whole thing.

Am in a small complex... actually it's www.swzc.org... and I've got the only back yard anyone is using... and I use it a lot... of course, they won't need to go through the other back yards...

argghhh... if they do leave the trench open is there anything I can do to help the tree?

I totally disagree about the pruning. Here's why:

Your tree is going to go into a drastic reaction mode once those roots are severed. They (plants in general) are programmed to send the majority of energy (which is MANUFACTURED IN THE LEAVES) to that part of the plant in most need. In other words, the tree will try to repair the damage by providing the energy to make more roots! This process will start right away!

IF you prune the top also, then two things happen: the tree loses all of those energy factories, and it will also want to try to send resources UP (away from root replacement) to make more leaves. Yikes, a double whammy! It is essential for the health and recovery of the tree that you keep the foliage in tact.

It's a myth that the foliage must be pruned to compensate for root loss, and I hope that you now understand why. Top pruning is not recommended during transplanting, either.

The advice about recutting the mangled roots after the trenching is finished is excellent. Roots will regenerate quite rapidly if the cuts aren't torn and mashed.

Make sense?

As for the chances for this plant....it doesn't sound good to me, but it's always worth a try. You might want to bring in a bunch of mulch for temperary protection of the remaining root system. Mulch can help prevent soil compaction and might encourage faster root development closer to the tree. Water, water, water.

hmmm, well we do disagree Rhizo.
I do agree that pruning the top would be bad. To be clear, I'm not suggesting pruning the top. Prune the side where they will be working as well as taking out a little bit of the lowest branches. the tree will have less roots and will not be able to draw enough water to keep the whole tree going. Any way, I have seen it work so I stand by my recommendation.
note that professional nurseries, when they dig trees for sale, prune them severely precisely because they have cut away so much root.
Transplanting from nursery container to the ground is a different story. The roots in that case are usually only 6 to 12 weeks old (unless it's root bound which is a whole other problem) With new potted nursery stock, the new roots and new flush growth are in balance and neither should be cut or pruned unless the stems are long, in which case they should be cut back to about 6 to 8 inches to encourage branching. This is what commercial nurseries recomend and what growers do, and I have had excellent success with it myself.
Since they will be using a trenches, you may need to trim the damaged ends a bit yourself.
Regarding back fill, try talking to the crew, maybe they will work with you, maybe not, but it can't hurt to ask.
Finally, make sure the tree still gets it's regular water in the root zone that has not been dug up. The biggest loss of trees in this sort of situation is where the water gets turned off or the irrigation basin is effected and the tree does not get water. The remaining roots must get the water they are accustom to.
good luck!

Anne, I got your email but my reply was blocked and didn't go through.

Again...removing foliage to 'compensate for root loss' is a practice that is no longer recommended. That's not to say, unfortunately, that there are still plenty of people out there that haven't kept up with technology. It takes a long time to beat those myths down into the ground. The top and the root system do not need to 'be in balance'.

Remember that pruning, whether it be to the above ground portion of a plant or below, is a growth STIMULATING procedure. Just think what kind of stress that causes to a plant that has had its root system mangled!

The thought processes behind the 'balancing the top to compensate for root loss' used to be accepted as a normal best management practice. However, in the past 20 years or so, we've simply learned more about how plants work. I mean, how they really and truly respond and function. Once you understand a little bit about plant physiology, then it begins to make sense.

Believe me, when I first started out in this field, I was taught and followed plenty of practices that have been totally turned around today. I, too, pruned many a tree due to root loss and never lost any.

But after many years of observation and study, I know that they establish SO much faster when they aren't pruned. And with far fewer stress related side affects.

Here is a link that might be useful: Interesting information

k, riz, I'm willing to learn, do you have any studies or facts to back up what you are saying? Why do professional nurseries prune almost all the branches and leaves off when they dig trees?
What you say goes against my experience, but as I say, I'm willing to learn if you have some facts to back it up.
Your last statement seems to be about transplanting from pot to ground. That is a very different situation than an established tree. RE from pot to ground, I have done it both ways and when I have a new tree that is leggy from its first flush after digging, I prune it hard to 6 inch branches to encourage more branching. That works very well. That is what Dolan Young of Young's nursery recommends and he grows and sells between 100,000 and 300,000 trees a year for commercial growers. I have followed his advice and my trees have done very well. Trees that I did not prune at planting or soon after did not do nearly as well.
But again, that is new trees from pot to ground, not an established tree.
At any rate, I think we can agree that making sure the tree does get the same water that it was accustom to is very important.
I to have learned to do things differently than I did when I started. I switched to all mulching in 1979. I never bought into the stone fruit pruning that said leave a little bit of the branch, but I always cut to the shoulder which is now the accepted practice. I used to back fill new transplants with amendments but now I fill with native soil. Just to name a few changes I've made over the years.
You mention understanding plant physiology as though I didn't. hmmmmm. Well your point there was that pruning stimulates growth. Well the same is going to be true of the trees roots. They will be stimulated to grow and once the soil is replaced they will do just that. So, a little pruning of the top as well will balance the water take up demand in the short run. Soon after, as the new roots grow, so will the new top growth.
Well, this thread is in danger of turning into an argument, and I don't care to argue so I apologize if it seems that way. I just differ in my opinion and experience. If you have some university studies or specific comparison date, that would be worth looking at.

Hi, softmentor. This will never turn into an argument, lol. I'm not expressing my opinion at all....nothing but solid facts. ;-) And I'm not trying to convince you of anything, believe me.

The same rules of growth and development, carbon allocation and stress response are true whether we are talking about transplanted trees or those damaged in a construction process. You are one hundred percent right that root pruning (either on purpose or by accident) will stimulate an enormous surge of new root growth (didn't I already say that?). And that is the only response that we want from the tree. Top pruning will not only slow down that response by the removal of the hormonal pathway, but will re-allocate the energy sources to the roots and direct them to replace shoots and foliage. Whammy.

I'll include a couple more (be sure to re-read the one I posted earlier) links to skim through. You can do your own research using such key phrases as "pruning to compensate for root loss" (or similar).

http://www.forestry.state.al.us/publication/TF_publications/pruning/pruning_a_new_look.htm

You've said one thing that really mystifies me, though. I've never, ever seen a nursery grown tree or shrub that was pruned prior to or just after digging. I've done considerable training of nursery owners/employees and have felt obliged to mention that old practice as a myth, but have never seen it done. If a shipment of trees had come to me that way, I would have sent them straight back to the nursery and so should you.

Here is a link that might be useful: Click here