Being formerly with the Extension Service, I would recommend you contact your local agent if he or she is knowledgeable about fruit. Best to get a trained set of eyes out to your site so you won't overlook future problems. You don't want to make giant pot out of the hole you are going to make in the granite and not have it draining quickly. At least do a test to see what the drainage will be before you put in the trees.

As far as root sizes go, that will depend on the rootstocks the apple and peaches are grafted on. Generally speaking, roots extend out to the dripline on mature standard trees. That size hole will be too big for dwarfing rootstocks. I'd go with semidwarf to avoid staking and still get benefit from early production.

Roots normally extend far beyond the dripline on mature standard trees. The vast majority of the roots will be within the top couple of feet of soil. Digging deeper than about three feet might improve drainage (depending on your native soil profile), but would not need to be done just to provide a place for the roots to grow.

I totally agree with Jerry about the potential problems associated with a poorly draining bowl cut into the native soil/rock. In some situations, a large berm might be useful. Someone more familiar with the soil profile of your area (your local extension agent, experienced growers in your area, a professional at your local highly reputable nursery, etc) should be able to give you better advise.

If the hardpan is that bad, why would you want to plant fruit trees in it?
Putting good soil in a basin of hardpan only makes a mud puddle.

That may turn out to be the situation in Dave's case, but there's no way for us to know without being there or having more information. Just because the soil is "hard" doesn't mean it won't drain. That's why Jerry recommended at least testing for drainage before planting.

Planting in hardpan is my only option. I have no other place for them.
As for drainage, I dug a 2' sq hole and filled it with water.
It took a whole week to drain.
There's my situation. I know not a good one.
Thanks for the help,
Dave

Decomposed granite here (a moist climate) would drain well, which makes me wonder what you actually have.

Is it excavatable by ripping with a backhoe or trencher? That soft?

If the ground is reasonably sloped, bringing the hole to be backfilled out to the downhill side so it can drain laterally that way might work. Or simply bust up the rock well over a big area and plant in that. Those would probably work at least OK here even on impermeable rock (given available irrigation), but I don't know about there.

Finally, true hardpans often have bottoms within reach of excavation equipment. Digging the planting hole all the way through them allows drainage thereafter (a tactic perhaps better recognized for septic-tank drainfields).

Just wanted to mention that poorly drained soil can be an indication to go with a raised bed, with non-native, well drained soil in the raised bed.

I have no problem with raised beds for veggies, but for apple trees?
Holy cow. We have the same problem. How big, and what material?
And the cost alone would drive me into the poor house.
Dave

What's the difference between the cost of soil for a berm on top of the impeded drainage layer and filling giant holes dug out of it? I'd think less soil would be used for the berm, plus you wouldn't have the expense of digging holes out of the existing material and disposing of the material so generated.

You don't necessarily need a huge depth of soil for trees, especially ones grafted onto fully dwarfing rootstocks. You are likely to need some kind of support system for these as the comparatively feeble roots that dwarf the tops also aren't very good at holding them upright over time.

Install drainage if needed as determined by percolation test ...

All planting holes must be dug at least three times as wide as the root ball or wider but no deeper than the root ball to avoid settling. Where drainage is marginal, all plants should be planted shallow or in mounds or berms ...

If the existing soil is very poor, remove and replace with good field soil or place at least six inches of good field soil on the surface. However, you should match soil types as backfilling with a good sandy loam in a heavy clay will serve as a collection point for water and the roots will suffocate ...

--Whitcomb, Establishment and Maintenance of Landscape Plants (1987 (1991), Lacebark Inc., Stillwater)

Here is a link that might be useful: Books Available

I once took a took a trip with my extension specialist boss to an area near Homestead FL. While there, it was amazing to see all of the tropical trees grown in grid patterns in what looked like ditch witch trenches. The specialist we were meeting explained that the marl soil was basically an ancient sea bed, it looked like solid limestone to me. He explained that in order to plant an orchard, they use a ditch witch to dig a grid of trenches one foot deep and plant the trees at the intersections. the trenching rubble gets pushed back in the trenches after planting and the roots grow only in the trenches.

After one hurricane blew over many trees, the trenches were cleared of rubble, tree roots placed in them and the rubble placed back in the trenches. I don't recall but they were probably growing hydroponically.

We were there to look at some tomato experiments, talk about bizarre! First they pulverized the marl into about 1" sized aggregate and windrowed it with plastic mulch on top. Next, they drove rebar into the ground (rock/marl) with a pneumatic hammer and then planted the beds. Drip tape in the beds were used for fertigation. Believe it or not it worked well, who'd have thunk it.

My point in all of this pushindirt; strange ideas have been tried and succeeded, don't give up too easily!

Roots grow in aerated water between soil particles, rather than in the particles themselves. That's how it's possible to grow tomato plants hydroponically and so on. In climax tropical rain forests most kinds of flowering plants grow up in the air, on the branches and trunks of trees, where they can get more light than on the shadowy forest floor. Where conditions are moist enough (and the plants have a means of coping with the nutrient situations present, such as bromeliads with their "vases"), no contact with mineral soil required.

I was going to mention the South Florida example as well. Actually the marl is a whitish or reddish silt soil. The ripped rock is porous Miami Limestone. Both are used for fruit, but more so the rock, and the marl more so for vegetables. Possibly you saw thin marl over rock where they were both dealt with (which would be a more favorable situation).

They may use trenchers now on the rock, but they used to push or pull rock-plow chisels with catapiller tractors (if you pushed, the treads immediatly further crushed the rock). This may not be quite the same with granite in that the Miami Limestone is already quite porous and can hold more water than say granite gravel. Still, decomposed granite may also be moderately porous and ameanable to this: simply ripping and crushing and back into the trenches as "soil."

Pushin dirt,

I've had extensive problems with waterlogged soil. Some good advice posted here. I highly recommend doing mounds, or a terrace. The bigger, the better. Also rip a trench to daylight, for drainage, if you can. Peaches simply will not tolerate water-logged soil. You'll be wasting your money to try it. Even if you can get them started, it's likely after a few years, you'll get a good wet spring, and your peach trees will die. Apples are more tolerant of waterlogged soil, if they're on standard roots.

Some other options are plums. Plums will take considerably more waterlogging than peaches. And more than apples. In fact, it's fairly difficult to kill plums from too much water. Another option is to order peaches on a plum rootstock, like St. Julian. That will give you some extra water-logging tolerance for peaches.

Hey Gonebananas
That trip was in 1991 and my memory is a little hazy about all the details. Glad you helped fill in the blanks. I do recall the tomato "soil" was reddish but definitely rock-like, the ground around the orchards was leaf covered but maybe very light where it was visible. Florida agriculture is really astounding and eye opening. Living there and working in the vegetable crop extension service was a great experience. I don't miss the lovebugs though.

I just wanted to chime in on this conversation. I live in SW MO. I have maybe 4 to 6 in of topsoil. I have planted apple trees and done well. For me I added a fair ammount of peat moss to the holes. I know some people dont like peat moss. IM not big on it either for the most part. HOwever it does work well for this. It absorbs so much water. Another alternative is coco husks. This is a little more expensive but probably the best route. My holes would drain in a couple days, not a week when I started. I hope this helps