This is my response to a thread on BonsaiSite about ground growing in heavy clay soil in the heat of Texas:
Heavy clay soils are terrible for field growing. You would be better off growing in containers than digging holes in clay. I too have a ground growing area in very heavy clay and it requires some thought and possibly a lot of work to prepare the beds.
The first thing you have to do is establish a grade. You will be planting in beds ABOVE the surrounding grade. So, if your place is absolutely flat, you will have to import soil from a wide area. I'm lucky in that my area has a slight grade and the water drains naturally out of the beds. Digging holes for trees will just drown them or the tree will recognize the hole as a pot with heavy clay walls. A raised amended bed will allow you to get good growth in a shallow layer of amended soil. This layer should initially be six inches to a foot above the grade; it will settle over time, but that's ok. My beds are about four feet wide and initially were about ten inches deep (above the unamended grade).
Soil amendment is a must in heavy clay soils. This can involve a lot of money and work. You must amend by adding an equal quantity of good organic material (usually compost). So, if you rototill six inches of native soil, then you add six inches of compost and thoroughly mix it in. That can also be a challenge in clay soils. You can see that this is a LOT of compost for anything other than small beds. I spent a couple thousand dollars on compost amending an area approximately 100 x 100 feet. I also have a tractor tiller and row former, so at least making the beds was easy. If you are doing a large area and don't have equipment, think about paying someone to come in with equipment and doing it for you, it's usually cheaper than renting equipment and doing it yourself.
You have to give the trees enough space so you can maintain them and give them enough light and growing room unless you are doing a single row. I typically put them about 3 feet apart and keep 6 feet between the centers of the rows, and it's still a jungle after a few years.
I also live in a full blast heat and intense sun area. 100F temperatures are not uncommon, and sometimes even with dry winds. Growing maples in this setting is a challenge. It is possible to grow them in pots with protection, but even then they burn in summer. This doesn't matter in the growing out phase as long as they don't completely burn off the leaves. Maples in the ground, and other deciduous plants as well, hold up better in heat and sun than those in pots. The trick is to get them started. Small plants in the field are pretty much doomed. I don't plant anything smaller than an established one gallon for deciduous trees. These should be planted in winter while dormant. Another trick I use is to wrap them in Reemay and staple it shut. Reemay is a spun poly row cover that is white and very porous allowing lots of air and light to enter, but protecting them from drying winds and intense sunlight. The neighbors think I am crazy planting rows of trees and putting diapers on them. The Reemay pretty much disintegrates the following winter, or you can remove it then. This is the simplest and cheapest form of protection that I can think of.
Green broadleaf Japanese maples will be the most likely to succeed, but broadleaf red ones do well also, especially 'Bloodgood'. Forget the cutleaf cultivars; they make crummy bonsai anyhow despite their beauty as landscape plants. Green seedlings are the cheapest and toughest and you won't have any grafts to deal with. Probably 95% of the credible JM bonsai are green seedlings. White pines are going to be very difficult for you to grow, don't even think about putting them in beds, they sun scorch badly in direct sunlight over 100F. I can only grow them under 30% shadecloth. Black pines will do great in the ground, but need to be planted in winter to acclimatize to direct sunlight and exposure.
Brent
EvergreenGardenworks.com
see our blog at http://BonsaiNurseryman.typepad.com
Update on the surgery: Everything went well, but the doctor was over optimistic about the recovery period. It seems my body just wants to swell up after any invasion, so we are looking at a four to six week recovery period before the final surgery of taking down the ileostomy. This will probably be the last week in June or the first week in July, which makes it almost exactly one year from the first surgery.
