This is an old topic, but I did not want to start a new topic as all of the old information is relevant to my questions(s). I am planting cherry trees in 12 inch raised beds that are made from 4 X 4 pressure treated lumber. Is 4X4 by large enough? They will be on Gisela 6 Rootstock.
I found that 4X8 works better. There are only so many roots in a 4X4 and on wet years the only roots that may survive would be on the raised beds. I don’t grow cherries, and I am in a 55+ rainfall area. Peaches so far have done well on the 4X8 raised beds.
Not clear whether you are asking if the 4x4 lumber size is ‘large enough’, or a 4x4 bed size is ‘large enough’.
The lumber is 2 inch by 12 inch pressure treated pine cut into 4 foot lengths so it is a 4ft x 4ft bed. My question is that large enough for the root system? Thanks for asking, I wasn’t clear.
My dwarf almonds are in 2.5x2.5 raised beds (wood is 2x6) and I’d say they are doing very well - for growing in the eastern rain forests. 
It’s like growing in a big sunken pot.
I agree with Anne, That should provide enough aerated soil to assure survival and productivity. It is a lot more soil than what people use when growing fruit trees in pots (potting soil represents about half the volume of real soil in terms of capacity of sustaining plant volume). How restrictive that amount of soil will be depends on how terrible the drainage is below (as well as the qualities of the raised soil itself)- if the trees stop growing when you still want more size, you can always expand the beds.
The only problem I can see is the tree could grow too big in a few years. Then, if you have a wet year, all the roots outside of the raised bed would die and there might not be enough roots for that big tree to survive. If you keep the tree small, the 4X4 should work fine. I don’t know what size the root stock will allow the tree to get.
I did hear of a gardener in my area that planted his peach trees on berms. In the winter the area between the berms would stand water. The trees did fine the first 3 or 4 years but then the trees grew too big and they died in the summer due to lack of roots.
Wouldn’t it be the same as if the tree was planted on a slope? Roots grow down? My logic may be wrong. I hope it is not because I am planting 4 trees in 4 ft x 4 ft boxes
This may be a leap too far. Maybe the berms were not tall enough to create any adequately drained soil during an extended wet spell- or maybe something else killed the trees. People keep peach trees alive in containers for many years, and I’ve managed to do this in extremely wet locations without enlarging mounds as the trees get larger. There are many variables, and one experience at one site in one soil is not very conclusive. However, inadequate raised soil would more likely kill trees gradually than suddenly, IMO.
Dave Wilson Nursery has a video on youtube about raised beds. I asked a question in the comments section about roots and raised beds and this was the response:
Me-
What happens when the roots hit the side of the raised bed, doesn’t it harm the tree?? What size beds are you using? 3 Ft x 3 Ft or 4 Ft x 4 Ft
Reply
Dave Wilson Nursery
Dave Wilson Nursery2 weeks ago
We have used all different sized beds in trial planting’s. From 2’x2’ to 8’x12’ From 4" high to 24" high. Using wood, rock, block and railroad ties. Some with no border at all just planting on a mound. When the roots hit the side of a solid bed they go down into native soil. No harm still maintaining that raised area that keeps the root crown oxygenated and protects crown from becoming waterlogged and stagnant. A raised bed will give you success in otherwise wet, heavy and clay soil types.
This is the video -
It is useful that they are doing research useful specifically to home growers. Hardly anyone else is.
Would love to hear and see pics of the progress of this. I am planning to install the 3 beds I’ve built this winter this coming spring.
My soil is good down to 3-4-5 inches in most places… then red sticky rock filled clay. The rock varies from baseball to 10 pound boulder size.
I plant almost everything in raised borderless beds… i just break up a wide strip of ground 3 4 5 inch deep (avoiding the rock and clay as much as possible) and then rake the good soil up to make a borderless raised bed.
Everything grows well in those… fruit trees, bushes, cane fruit, veggies, strawberries,etc.
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