Lawn Grass versus Trees

I have some fruit tree saplings planted in my lawn. (yes I know, the lawn will go away eventually). The trees do okay for a while, but eventually they start looking... unhealthy. finally a few have died off. I guess it could be any number of factors, but I suspect it has to do with the grass encroaching on them. MY question is, what is the best way to keep grass away from the tree? should I rip it out and lay down mulch? I have a bunch of chicken-coop hay I could use. I just don't see it keeping the grass away. I could plant some brambles or shrubs, but are those going to compete with the delicate fruit trees? Should I have planned this better?

Is it better to sheet mulch it with some cardboard or to dig back the grass and then regular mulch it? I'm afraid I'll hurt the trees upper roots if I dig it. maybe it doesn't matter?

Adam, need a little more info:

What type of fruit trees are they?
When did you plant them?
Are they in full sun, afternoon sun, morning sun or dappled sun?
How often are they watered?

And finally if you could post some pictures that would be good too - but in the meantime just info on the above would be a good start ---- oh, and is this a new construction area?

I think the grass will be really rough on the trees and I'd definitely mulch. It should make a big difference in all sorts of ways.
I'll add another question to Jeanine's...can you find out/do you know if if it's a 'running' or 'tufting' grass? The runners have extremely aggressive mats of roots, generally shallow and horizontal.
If it's tufting, I find it extremely easy to kill just by smothering it with plenty of mulch. Running's a lot harder, and I'd lay thick, seriously overlapping cardboard. A running grass lawn will always be working its way back into the mulch, so prepare to be vigilant!
Whatever grass it is, I wouldn't dig it for many reasons, not least because if it's tufting, you don't need to, and if it's running, you'll just encourage it
Trees like a fungally-dominated environment; tree trimming mulch is an ideal high carbon (hence fungus-friendly) mulch.
A caveat: Be careful to leave an area clear of mulch around the trunks to avoid collar-rot. I haven't had any problems butting card right up against trees to kill grass, but maybe I've been lucky.
I'd add a few support species too, like comfrey, alliums, spring bulbs, clover...

Leila, thanks for the reminder. I have two new apples out by the pond that are planted all by their lonesome. I need to add some clover and maybe one of the new comfrey plants I just got. Mint too - the apples need some friends!

Thanks for the answers. The trees are jackfruit, carambola, and sugarapple. I also have an aglia (not a fruit tree, but smells good and butterflies like it. They get pretty decent morning and midday sun...enough that grass can grow. They're all young: a year or two at most, and not shading out the grass yet. The lawn was planted before I lived here, but it seems to be a running type--there is also some tufting types mixed in. I initially mulched around the trees, but of course once I stopped adding new mulch the grass just invaded, and now it's almost up to the trunks. Being that they are all tropical trees and it gets pretty close to freezing in the winters here--I guess the problem could be a temperature issue. I don't water them, but it rains most every day this time of year, so I believe they are not drying out. the earth is sandy, unfortunately, but that's a fact of life down here. I have a lot of compost--I might try stacking some high carbon around the tree--I wonder if straw is good for this? Fukuoka said tree branches were the best mulch for trees, I think. Maybe I'll try some of each.

I'd definitely try and use tree mulch. It won't blow around either

Leila Rich wrote:
Trees like a fungally-dominated environment; tree trimming mulch is an ideal high carbon (hence fungus-friendly) mulch.

The only way I know to stop running grass invading is to just keep pulling it out/back. The rhisomes dry out and die if left on top of mulch, with no soil contact. Don't put it in the compost, unless it's super hot (I wouldn't even then)
Florida sand, from what I know, just eats organic matter and it looks basically impossible to overdo it!
The old carpet and underlay are great grass-blockers. It has to be the ancient stuff with no artificial fibres.
I'm fine with using paper and fabric that will break down, but lots of people are not into it.

i would also do a trial dig out at the dripline, and see if you have some rootworms before you put down all that compost/food.

There are hammer in plastic boundry panels to keep the grass at the perimeter. might be worth it.